A Comprehensive Review on the Roles of Metals Mediating Insect–Microbial Pathogen Interactions

Abstract

Insects and microbial pathogens are ubiquitous and play significant roles in various biological processes, while microbial pathogens are microscopic organisms that can cause diseases in multiple hosts. Insects and microbial pathogens engage in diverse interactions, leveraging each other’s presence. Metals are crucial in shaping these interactions between insects and microbial pathogens. However, metals such as Fe, Cu, Zn, Co, Mo, and Ni are integral to various physiological processes in insects, including immune function and resistance against pathogens. Insects have evolved multiple mechanisms to take up, transport, and regulate metal concentrations to fight against pathogenic microbes and act as a vector to transport microbial pathogens to plants and cause various plant diseases. Hence, it is paramount to inhibit insect–microbe interaction to control pathogen transfer from one plant to another or carry pathogens from other sources. This review aims to succinate the role of metals in the interactions between insects and microbial pathogens. It summarizes the significance of metals in the physiology, immune response, and competition for metals between insects, microbial pathogens, and plants. The scope of this review covers these imperative metals and their acquisition, storage, and regulation mechanisms in insect and microbial pathogens. The paper will discuss various scientific studies and sources, including molecular and biochemical studies and genetic and genomic analysis.

1. Introduction

Insects and microbial pathogens carry out many essential biological functions because they are present in varying amounts in nature [1]. Insects are a versatile group of organisms occupying various ecological niches and are requisite for many reasons, including pollination, decomposition, and pest control. However, some insects are pests that pose significant economic and health risks to humans, animals, and plants.

Microbial pathogens, however, are microscopic organisms that can cause disease in various hosts, including insects, humans, plants, and animals [2]. Insects and microbial pathogens interact in a variety of ways. Some insect-specific microbial pathogens have evolved to exploit their insect hosts’ unique physiological and behavioral properties [3]. Other pathogens have a broader host range and can infect multiple species of insects. Insects, in turn, have evolved various mechanisms such as physical barriers, immune responses, and behavioral adaptation to defend themselves against microbial pathogens [4]. The interactions between insects and microbial pathogens involve various physiological and biochemical processes [5]. Metals are a critical factor that plays a crucial role in these interactions. Moreover, the study investigated that metals are pertinent to many physiological processes in insects, plants, and microbial pathogens, including digestion, respiration, and immunity [6]. However, metals can also limit the growth and virulence of microbial pathogens and act as a defense mechanism for insects against infection [7]. Understanding the role of metals in the interactions between insects and microbial pathogens can provide insights into the mechanisms of disease transmission and the development of new strategies to control insect pests and microbial pathogens.

Extensive documentation highlights that metals are essential for various biological processes and important for facilitating interactions among plants, animals, and their environment [8]. Metals involve physiological and biochemical processes, including energy production, enzyme activation, and cellular signaling [9,10]. They also play an indispensable role as a cofactor in many proteins and enzymes, such as hemoglobin, myoglobin, and cytochrome c, involved in respiration and other metabolic pathways [11]. However, various studies have identified that metals play a crucial role in immunological reactions and act as cofactors for enzymes accountable for producing reactive oxygen species (ROS) and other antimicrobial compounds [12]. These metals have toxic effects on living organisms when present in higher amounts. Metals toxicity can disrupt the structure and functions of proteins and enzymes by binding to their functional groups nonspecifically [13]. Toxic metals in higher concentrations produce higher ROS, which has been observed to produce harmful effects on cells and tissues [14]. This review aims to draw together a range of scientific studies and sources, including molecular and biochemical studies and genetic and genomic analysis, on the role of metals mediating insect–microbial pathogen interactions, as well as provide an overview to highlight the importance of metals in biological systems and the potential applications of this knowledge for pest and disease control.

2. Role of Metals in Insects

2.1. Iron (Fe)

Iron (Fe) plays several vital roles in insects. It is an essential part of hemoglobin and acts as a cofactor in several enzymes involved in various metabolic pathways, such as respiration and energy production [15].

Fe is also crucial for insects’ proper development and growth, as it synthesizes proteins and DNA [16]. The mechanism of Fe absorption in insects encompasses acidification of the midgut, interaction with Fe-binding proteins like transferrin, direct uptake of heme, and storage of excess Fe as ferritin [17]. Acidification of the midgut creates an acidic environment that enhances Fe solubility, allowing for its absorption by gut epithelial cells. Insects also utilize transferrin receptors on gut cells to facilitate the endocytosis of Fe-transferrin complexes [18]. Blood-feeding insects can directly absorb heme from host blood, which is transported across the gut epithelium for various physiological processes. Excess Fe is stored in ferritin, acting as a Fe reservoir that can be utilized when Fe availability is limited. These mechanisms ensure efficient Fe acquisition and utilization in insects for essential physiological functions. It can also improve the insect immune system by producing ROS to inhibit the growth of microbial pathogens [19].

Moreover, the study showed that a protein known as ferroportin helps transport and regulate Fe in various types of cells, whether absorbed from the diet via intestinal enterocytes, recycled by macrophages, or stored in hepatocytes. These proteins, such as transferrin, cross the cell membrane to reach the plasma Fe carrier protein [20]. The precise topology and mechanism of Fe transport through ferroportin are not well-understood, and these are considered to be significant unresolved questions in Fe biology. Ferroportin is abundant in specific cells known for Fe export, such as duodenal enterocytes, splenic and hepatic macrophages, and to a lesser extent, hepatocytes [21]. It is also found in the lung, renal tubules, and erythrocyte precursors in the bone marrow [22], although its function in these locations is unclear. Ferroportin transports Fe into the bloodstream on the basal side of enterocytes, and divalent metal transporters expressed on the luminal side of enterocytes regulate Fe absorption in the gut. In the transferrin cycle, the divalent metal transporter actively transfers Fe into enterocytes and is also expressed in macrophages and endosomes [23]. The release of free Fe from the transferrin-receptor complex causes a pH-mediated conformational shift in the endosomes, where it is then transported to the cytoplasm via a divalent metal transporter [24]. Serum transferrin, a member of the transferrin superfamily of proteins, including ovotransferrin and lactoferrin, transports Fe throughout the body. This export protein enables Fe efflux from macrophages and Fe acquisition by enterocytes [25]. Heptaglobin binds hemoglobin to the Heme that is secured by hemopexin. Circulating Hp can effectively manage moderate hemolysis, saturating at 1.5 g/L free Hb [26]. Hepatocytes and macrophages have receptors that uniquely recognize the Hp/Hb complex, as shown in Figure 1.

Fe is an essential nutrient for the survival and growth of microbial pathogens, as it is required for several critical physiological processes [27]. However, Fe is not readily available in the host environment, as it is tightly bound to host proteins such as transferrin and lactoferrin to prevent microbial growth [28]. Therefore, microbial pathogens have evolved multiple strategies to harvest Fe from the host. One such strategy is the production of siderophores, small molecules that chelate Fe and facilitate its acquisition from cells [29]. Many bacterial pathogens produce siderophores; some can even intercept siderophores from other microbes. Another strategy is the expression of high-affinity Fe transporters, which actively enable the pathogen to take up Fe from host proteins. However, the Gram-positive pathogen Staphylococcus aureus expresses the ScaABC transporter, specific for transferrin-bound Fe [30]. Siderophores are increasingly recognized for their contribution to virulence beyond simple Fe chelation. They also act as signals that elicit a strong host defense, promoting mitophagy, hypoxic responses, and cytokine production [31,32]. Therefore, microbial pathogens must overcome these obstacles to acquire sufficient Fe for survival and virulence. Scrutinizing the mechanisms of Fe acquisition in pathogens is crucial for developing novel antimicrobial strategies [33]. However, targeting siderophores or Fe transporters could limit the growth and virulence of pathogens. For pathogenic bacteria, obtaining sufficient Fe during and after infection inside the host is one of the main barriers. At the host–pathogen interface, an analytical structure describes the flow of signaling and the struggle for shared resources between the host and pathogen, and host–pathogen competition for this valuable transition metal take place, as shown in Figure 2. In addition, Fe-based therapies, such as Fe chelation therapy, have also been studied as potential treatments for infectious diseases [34]. However, the efficacy of such therapies has yet to be determined, as Fe is integral to host physiology and immune function.

It has been widely recognized that microbial pathogens depend on acquiring and utilizing Fe from their host for survival. However, the host immune system’s mechanisms for retaining Fe limit this process [35]. Several Fe acquisition mechanisms have evolved in microbial pathogens to overcome this limitation, including the production of siderophores, Fe transporters, heme acquisition systems, and Fe-regulated surface proteins. Siderophores are small Fe-chelating molecules secreted by pathogens that bind and transport Fe into the cell [36]. Fe transporters and heme acquisition systems help pathogens acquire Fe from host proteins, while Fe-regulated surface proteins capture Fe from host transferrin and lactoferrin. Fe-responsive regulators such as Fur tightly regulate Fe acquisition in pathogens, which regulate the expression of genes involved in Fe acquisition and metabolism [37,38]. Fe acquisition mechanisms such as siderophores and Fe transporters in bacterial infections are crucial virulence factors that allow bacteria to overcome host Fe retention mechanisms and establish conditions [39]. Fe acquisition mechanisms such as high-affinity Fe transporters and Fe-responsive regulators are pertinent to fungal virulence and pathogenicity in fungal infections. In parasitic infections, Fe is involved in several essential processes, such as energy production, DNA synthesis, and oxidative metabolism, and is prominent for the parasite’s survival and growth in the host [40]. The role of Fe in interactions between insects and microbial pathogens is complex and diverse, with both essentiality and toxicity playing integral parts.

2.2. Interaction of Plants with Insects and Microbes via Fe

There is substantial proof that Fe is an essential nutrient for plants and many microbes and necessary for their growth and development. The availability of Fe can influence plant–insect and plant–microbe interactions [41], and Fe-rich soils can increase the abundance and activity of certain beneficial microbial species, such as mycorrhizal fungi. These rhizobacteria can help plants become more herbivore-tolerant and improve their defenses against herbivores [42,43]. Moreover, these beneficial microbial species may also prevent and reduce the populations of certain insect pests [44]. In addition, certain insect species can give plants access to Fe-rich food sources of nutrition by bringing Fe-rich soil particles to the root zone as shown in

Table 1. Interaction of plants with insects and microbes through Fe.

Fe	Role in Plants	Role in Insects	Role in Microbes
Absorption	Plants take up Fe through their roots and transport it to different plant parts for use in metabolic processes [51].	Insects acquire Fe from their diet, including plant material [52].	Microbes acquire Fe from their environment for growth and metabolism [53].
Defense	Fe produces defensive compounds in plants, such as phytochelatin and lignin [54].	Insects can use Fe-binding proteins to sequester Fe from their diet and prevent it from being used by pathogens [55].	Some microbes can produce Fe-chelating compounds called siderophores, competing with plants and insects for Fe [56].
Signaling	Fe is involved in signaling pathways in plants, e.g., the regulation of gene expression and responses to stress [57].	Fe can act as a signaling molecule in insects, affecting behavior and development [58].	Fe availability can also affect microbial gene expression and behavior [59].
Regulation	Plants regulate Fe acquisition and transport through a complex system of genes and proteins [60].	Insects require a certain amount of Fe for proper growth and development, but excess Fe can be toxic [61].	Microbes have evolved mechanisms to sense and respond to Fe availability in their environment [62].

. However, Fe-deficient soils can negatively impact plant-microbial interactions, and specific plant pathogens can exploit the lack of Fe to infect and damage plants [45].

The plant Fe deficiency response is regulated at the transcriptional and post-translational levels. Hormones like auxin, ethylene, nitric oxide, cytokinin, and gibberellic acid play vital roles in this process [46]. In Arabidopsis, ethylene and gibberellic acid enhance Fe uptake by increasing FRO2 and IRT1 expression. Ethylene and auxin promote nitric oxide accumulation, stabilizing FIT and improving Fe uptake. Auxin also stimulates lateral root formation for increased Fe absorption [47].

Conversely, cytokinin inhibits root growth and suppresses Fe deficiency response genes: salicylic acid and jasmonic acid, two major defense hormones, influence plant Fe acquisition [48]. Salicylic acid positively affects Fe uptake gene expression in Arabidopsis through auxin and ethylene signaling [49]. On the other hand, jasmonic acid negatively regulates Fe acquisition by downregulating Fe uptake genes independently of FIT [50]. Ethylene and auxin hormones are crucial in the plant immune signaling network. This connection between Fe availability and immunity highlights their potential role in Fe uptake responses in plant roots.

2.3. Zinc (Zn) and Copper (Cu)

Like humans, insects rely on dietary intake of trace metals like Zn and Cu for proper physiological functioning. These specified metals are responsible for multiform insect processes, including DNA synthesis, oxidation reactions, cuticle biosynthesis, and acting as essential cofactors for numerous enzymes [63]. Their presence is indispensable for insects’ everyday functioning and overall well-being at the molecular and biochemical levels. In insect studies, two families of Zn transporters: ten dZip and seven dZnT proteins, analogous to human Zip (SLC39) and ZnT (SLC30) families [64]. These transporters play roles in Zn influx and efflux, with specific expression in the midgut (dZip1) and Malpighian tubules (dZnT35C), contributing to Zn absorption and excretion. Zn is distributed throughout the gastrointestinal tract, with higher accumulation in the posterior midgut, crop, and Malpighian tubules [65]. dZip1 and dZip2 import Zn from the lumen into the enterocyte, while dZnt1 and ZnT77C release imported Zn into circulation from the basolateral membrane. The silencing of dZnt1, specifically in the gut, increases lethality under Zn-deficient conditions, highlighting its crucial role in Zn absorption [66]. Zn repletion leads to the suppression of dZip1 and dZip2 mRNA expression and the protein expression of dZnt1 [67].

The expression of dZnt1 and dZnT35C, a potential ZnT2 homolog, is regulated by dMTF1. FOI, an ortholog of dZip6 and dZip10, is essential for cell migration and gonad morphogenesis by controlling DE-cadherin expression at the posttranscriptional level [66]. A Catsup mutant with a defective dZip7 exhibits high levels of catecholamines and shows signs of semi-dominant lethality. The mutant also displays defects in membrane protein trafficking and increased ER stress [68]. On the other hand, Zn is embarked on regulating gene expression in insects, where it acts as a cofactor for several transcription factors that control gene expression [69]. Zn is also involved in regulating insect development, which plays a crucial role in insect cuticle formation and the regulation of molting.

The most prominent finding to emerge is that Cu is a crucial part of the innate immune system of insects, where it acts as a cofactor for the enzyme phenoloxidase. Phenoloxidase plays a significant role in the insect immune response by catalyzing the oxidation of phenolic compounds to quinones, which are toxic to microorganisms [70,71]. The quinones also contribute to the formation of melanin, which is pivotal for encapsulating pathogens via insect defense cells.

Phenol + O₂ + Cu²⁺ → Quinone + H₂O + Cu⁺

Quinone + Quinone + Cu⁺ → Melanin + Cu²⁺

In this reaction, the copper becomes an ion (Cu²⁺), acting as a cofactor for the phenoloxidase enzyme and facilitating the transfer of electrons during the oxidation of phenols. The reaction cannot proceed without the Cu ion, and melanization cannot occur, making the insect vulnerable to foreign invaders.

Furthermore, Cu also regulates the insect’s antioxidant defense system, which protects the insect’s cells from oxidative damage caused by the ROS produced during the immune response [72]. However, it has been reported that dMTF-1 is a crucial regulator of essential metal homeostasis, controlling gene expression in metal pathways [73]. DmATP7 is vital for Cu uptake and efflux in insects, particularly during larval development. DmATP7 term depends on functional dMTF-1, while its background expression is maintained in dMTF-1 knockout flies [74]. dMTF-1 also regulates the Cu importer protein Ctr1B in response to Cu-specific stress, facilitating increased Cu uptake. Knockout flies lacking dMTF-1 exhibit decreased survival and prolonged development due to impaired metal regulation [75]. dMTF-1 is also crucial for transcription factors; an insect tightly regulates these metals. It controls the expression of ZnT and Zn exporter proteins involved in the uptake and efflux of Zn while also regulating Cu-related genes. dMTF-1’s concentration gradient between the cytosol and nucleus governs the regulation of Zn exporters [76]. Zn toxicity induces dMTF-1 upregulation and translocation into the nucleus, where it binds to the MRE upstream of ZnT, promoting ZnT transcription and Zn exporter production [77]. Although the regulation of Zn importers via dMTF-1 in insects is not yet established, it is possible that some Zn importers may also be influenced by dMTF-1. This regulatory mechanism involving dMTF-1 ensures the maintenance of Zn and Cu homeostasis, essential for RNA and DNA metabolism. On the other hand, Zn is required for the proper functioning of several immune-related enzymes, including alkaline phosphatase and carbonic anhydrase. These enzymes play an integral role in the insect’s immune response by regulating the insect’s tissue pH and modulating the immune cells’ activity [78]. Zn is also immersed in regulating the expression of several immune-related genes in insects, including genes encoding antimicrobial peptides, which are essential in the insect’s defense against microbial pathogens [79].

A number of studies have disclosed that Zn and Cu are crucial cofactors for various enzymes involved in multiple biochemical reactions in insects. Zn is required to function in enzymes integrated into DNA synthesis, RNA transcription, and protein synthesis, as well as repair enzymes properly [80]. Zn ions (Zn²⁺) are coordinated to the amino acid residues of DNA polymerases, such as DNA polymerase III. This coordination stabilizes the enzyme’s binding to the DNA template and allows for the accurate replication of genetic information [81,82]. Zn ions interact with the negatively charged phosphate backbone of the DNA molecule and form coordination complexes that stabilize the enzyme-DNA complex. This coordination also helps to properly position the deoxynucleoside triphosphate (dNTP) substrates for incorporation into the growing DNA chain, resulting in the actual complexity of the genetic information [83]. In addition to its role in DNA polymerase activity, Zn is also involved in the movement of other enzymes involved in DNA synthesis and repair, such as DNA ligases and topoisomerases. Zn ions cooperate in binding these enzymes to DNA substrates, allowing for the efficient repair of DNA strand breaks and the accurate replication of genetic information [84].

Moreover, the host immune system tightly regulates the availability of these metals in the host environment, which can limit their accessibility to invading pathogens. In microbial pathogens, Zn can also control the expression of virulence factors and the formation of biofilms, which can enhance their ability to colonize and infect the host [85]. However, the host immune system can absorb Zn through various mechanisms, such as the production of metal-binding proteins, to limit its availability to invading pathogens [86]. Cu is an essential cofactor for many enzymes involved in cellular processes, including respiration, response to oxidative stress, and Fe acquisition, as well as control the virulence factor and biofilm formation in pathogens. However, excess Cu can be toxic to cells by generating ROS, which can damage DNA, proteins, and lipids [87], as shown in Figure 3. Hence, microbial pathogens have evolved various mechanisms to deal with excess Cu, such as producing Cu-binding proteins and activating detoxification systems. Overall, the roles of Zn and Cu in microbial pathogens are complex and tightly regulated by the host immune system [86].

2.4. Interaction of Plants with Insects and Microbes through Zn and Cu Metals

Plants interact with insects and microbes by competing with them to gain metals such as Zn and Cu. Within the realm of enzyme activity, Zn is essential for the proper function of DNA/RNA polymerase enzymes, ribosomes, and superoxide dismutase (SOD) [88]. It exhibits specific significance in plants, present in carbonic anhydrase and stromal processing peptides, thereby contributing to photosynthesis [89]. Furthermore, Zn contributes to protein structure, with approximately 4% of Arabidopsis proteins containing Zn finger domains, emphasizing its functional importance in plant physiology [90]. Cu is imperative for active functioning critical enzymes such as cytochrome oxidases, ascorbate oxidase, superoxide dismutase (SOD), and polyphenol oxidase [91,92]. In plants, Cu is also required for the receptor signaling of the hormone ethylene, which plays a crucial role in plant development and disease resistance [93]. These metals are needed to form chlorophyll, which is pivotal for photosynthesis, and for auxin production. This hormone stimulates cell division, elongation, tissue differentiation, and tropism (responses to environmental stimuli) [94]. In addition, these metals can be used by insects and microbes as energy sources and for the biotransformation of compounds, such as nitrogen and sulfur. In addition, Zn and Cu can be used by plants to ward off herbivores, as they are toxic to certain insects [95]. However, Zn toxicity occurs in agricultural soils treated with sewage sludge, in urban and suburban soils enhanced via anthropogenic inputs of Zn, especially in soils with low pH, and in soils affected by mining and smelting activities [96]. Mechanisms for creating either low or high Zn scenarios in plant and animal systems are essential for Zn-based disease and pest control.

Eukaryotic cells have an impressive ability to regulate the levels of Zn within their interiors. Despite Zn being commonly present in lower concentrations at the interiors of the cell, it is present in higher ranges outside the eukaryotic cells [97]. A diverse range of proteins, such as ZIP (ZRT- and IRT-like proteins), ZNT (Zn transporter), and metallothioneins (MTs) that sequester Zn, are involved in regulating Zn equilibrium in plants. ZNT proteins transport Zn within cells, averting cytotoxicity by sequestering it within vacuoles, while ZIP proteins facilitate the absorption of Zn from the soil into plant root cells [98]. MTs act as Zn chelators, storing excess Zn in a harmless form, protecting against Zn deficiency. This cooperative system ensures Zn’s controlled distribution, storage, and acquisition. ZIP transporters respond to low Zn levels by upregulation, whereas ZNT transporters and MTs become active when Zn levels are high, guaranteeing proper physiological development and plant function [14,99].

In contrast, ZNT proteins decrease intracellular Zn levels by promoting Zn release from the cell or its uptake into intracellular vesicles. The sequestration of Zn is primarily controlled by Zn-dependent mechanisms that regulate the transcription, translation, and intracellular trafficking of these transporters [100]. Indeed, many studies show that the expression levels of Zn transporters in plant tumors have been found to correlate with the severity of malignancy, indicating that disruptions in intracellular Zn homeostasis can contribute to cancer progression. In various types of cancer, specific Zn importers are upregulated, potentially enabling tumor cells to evade programmed cell death (apoptosis) and activate survival mechanisms through autophagy [101,102]. ZIPs and ZNTs are among the critical Zn transporters involved in these processes, as shown in Figure 4. In addition, microbes can use these metals to make antibiotics that plants can use to protect against infection [103].

Zn and Cu can be found in soil and taken up by plants, and are also present in the bodies of insects and microbes [104]. Zn also helps protect plants from diseases caused by fungi, bacteria, and viruses by producing phytohormones, such as salicylic acid, and activating the plant’s defense mechanisms [105]. Zn can also induce the expression of defense-related genes, such as those involved in synthesizing phytoalexins, which are antimicrobial compounds that may help plants ward off disease [106]. In addition, Zn can stimulate the production of secondary metabolites, such as flavonoids, with antimicrobial properties [107,108]. Zn also helps maintain the structural integrity of plant tissues, which can prevent pathogen invasion.

Cu is significant for enzymes to function properly and to protect plants from environmental factors such as cold, heat, and drought [109]. One of the main functions of Cu in plants is as a cofactor for enzymes implicated in various metabolic pathways, including photosynthesis, respiration, and lignin synthesis. It serves as a component of the primary electron donor in the photosystem 1 of plants [110]. Due to its ability to readily gain and lose electrons, Cu acts as a cofactor for oxidase, mono, and di-oxygenase enzymes such as amine oxidases, ammonia monooxidase, ceruloplasmin, and lysyl oxidase. Additionally, Cu is involved in the function of enzymes responsible for eliminating superoxide radicals, including superoxide dismutase and ascorbate oxidase [111]. It can also improve plants’ resistance to these stresses by increasing antioxidant activity and reducing oxidative damage. Additionally, Cu is pertinent in maintaining membrane integrity and stability, which can help prevent water loss during drought and cold stress [112].

Insects and microbes are vital to plants’ health, as they help to provide plants with essential nutrients and water [113]. Microbes also help break down soil nutrients, making them available to plants. Insects can also provide prominent nutrients to the plant, such as nitrogen, phosphorus, and potassium, and other beneficial compounds such as Ca, Mg, and S, as well as the vitamins and hormones necessary for the proper functioning of enzymes and other metabolic processes in plants [114,115]. In addition to providing essential nutrients, insects can transfer other beneficial compounds to plants. However, some insects can transmit plant-growth-promoting hormones such as gibberellins and auxins to stimulate plant growth and development [116]. Insects can also transfer vitamins and antioxidants such as vitamins C and E, which may help protect plants from oxidative damage [117].

There is substantial proof that Zn and Cu can benefit insects and microbes. Zn is essential for the growth and development of insects and can also help protect them from disease (

Table 2. Condensing the role of metals in the defensive mechanisms of insects.

Insects	Metals	Defensive Mechanisms	References
Ironclad beetle	Fe	Hardens exoskeleton for protection against predators.	[120]
Bombardier Beetle	Cu	Copper enzymes facilitate the expulsion of toxic sprays.	[121]
Leafhopper	Zn	Protection of exoskeleton for defense against predators.	[122]
Silverfish	Ag	Antimicrobial defense, silver scales provide camouflage and protection.	[123]
Sweat Bees	Hg	It uses mercury to deter parasites and pathogens.	[124]
Fireflies	Co	Cobalt ions play a role in the production of bioluminescence.	[125]
Nickel-Eating Moth	Ni	Nickel detoxification enables feeding on nickel-rich plants.	[126]
Selenium-Superior Fly	Se	Selenium detoxification offers resistance to toxic plants.	[127]

). Other metals like Cu are also imperative for the functioning of enzymes in insects and help protect them from environmental stressors [118]. Zn and Cu are paramount for the health of plant-growth-promoting rhizobacteria as they help break down organic matter and make nutrients available to plants. It activates defense mechanisms, induces expression of defense genes, increases the activity of enzymes involved in ROS production, stimulates the production of secondary metabolites, and helps maintain the structural integrity of plant tissues [119]. Therefore, to probe the mechanisms by which metals increases plant disease resistance can help us to develop sustainable crop protection and production strategies in the face of changing environmental conditions.

2.5. Metals Other Than Fe, Cu, and Zn

Metals other than Fe, Cu, and Zn, like manganese (Mn), nickel (Ni), cobalt (Co), and molybdenum (Mo), are also essential trace elements that play prominent roles in the growth and development of plants, insects, and microbes [128]. Metal ions, mainly through the Haber–Weiss reaction, are pivotal for oxidative modifications of free amino acids and proteins [129]. Commonly oxidized amino acid residues include histidine, arginine, lysine, proline, methionine, and cysteine. These site-specific modifications occur at metal binding sites within proteins [130]. One significant consequence of oxygen-free radical-induced protein damage is their susceptibility to protease degradation [131].

Additionally, protein oxidation can release its binding metals, such as Fe²⁺ from [4Fe-4S] clusters found in certain dehydratases like aconitases [132]. On the other hand, metal (e.g. Mn, Ni, Co, etc.) binding to the cell nucleus leads to genotoxic damage, including DNA base modifications, DNA–protein cross-linkages, DNA strand breaks, rearrangements, and depurination [133]. Reactive oxygen species generated via metal-mediated production induce pro-mutagenic adducts, such as 8-oxoG (8-oxo guanine), which can cause C to T transversion mutations without DNA repair [134]. Metal-induced carcinogenicity and acute toxicity involve oxidative damage, DNA methylation aberration, and chromatin condensation [135].

Manganese (Mn) is involved in synthesizing chitin, a component of the insect’s exoskeleton. In addition, it also plays a significant role in the development of reproductive organs and is necessary for the appropriate maturation of eggs in certain insect species [136]. Ni is implicated in the metabolism of carbohydrates, amino acids, and lipids and is necessary for synthesizing enzymes that are pivotal for insect growth and development [137]. Co is essential for the metabolism of carbohydrates, amino acids, and lipids and is involved in the synthesis of hemoglobin, which is vital for insect oxygen transport [138]. However, Mo is an essential nutrient utilized as a prosthetic group in oxidoreductases. Its molybdoenzymes, identified in Drosophila, play crucial roles in metabolism, including the breakdown of acetaldehyde and purines [139].

Furthermore, these metals also play a pivotal role in the physiology and pathogenesis of the microbial pathogens, in which delicate mechanisms have evolved to acquire and regulate their levels from the host environment [140]. Mn is vital for the growth and survival of bacterial pathogens and contributes to biological processes like oxidative stress management and DNA protection [141]. Mn is also enlisted in expressing virulence factors such as adhesins and capsules in bacterial pathogens [142]. Moreover, it can affect the stability and folding of proteins involved in the virulence factor synthesis, such as capsule polysaccharides [143]. Subsequently, Ni is considered the most critical cofactor for several enzymes involved in energy metabolism and nitrogen fixation and is required for the growth and survival of many bacterial pathogens. Bacterial pathogens have undergone evolutionary adaptations in their acquisition mechanisms, enabling them to effectively regulate nickel (Ni) levels from the host environment [144]. Ni is also a critical component of some virulence factors such as urease in bacterial pathogens [145]. Co is a component of vitamin B12, which is pivotal for the growth and survival of many bacterial pathogens [146,147], while it can also regulate virulence factors, such as siderophores, in the bacterial pathogens [86]. Moreover, the bacterial virulence factors such as adhesins and capsules also require an optimum concentration of Mo for their expression [148], and Mo is also an essential cofactor for the function of various enzymes involved in redox reactions, including nitrate reductase, formate dehydrogenase, and aldehyde oxidase [149].

2 Mo(VI) + 3 NADH + 9 H⁺ + 2 NO₃⁻ → 2 Mo(IV) + 3 NAD⁺ + 6 H₂O + 2 NO₂⁻

Mo(VI) + NADH + H⁺ + HCOOH → Mo(IV) + NAD⁺ + H₂O + CO₂

Mo(VI) + H₂O + RCHO → Mo(IV) + 2 H⁺ + RCOOH

In redox reactions, Mo acts as a catalytic virtuoso, facilitating the transfer of electrons and protons between the substrates, esteemed cofactors (such as NADH), and the ultimate products. The presence of Mo is crucial for the proper functioning of these enzymes and their involvement in redox processes.

3. Role of Metals in Microbial Physiology and Virulence

Trace metals are crucial for developing physiology and virulence in microbial pathogens, as they rely on these metals to facilitate essential biological processes and augment their pathogenic potential. For example, Zn is required for the fungal pathogen Candida albicans to express virulence factors such as adhesins and invasins. Moreover, its acquisition mechanisms have also evolved in C. albicans, allowing the fungus to acquire Zn from the host environment through manipulating proteins like Zn transporters and Zn-regulated transcription factors [150]. Cu is paramount for several biological processes in microbial pathogens, such as oxidative stress resistance, energy metabolism, and DNA synthesis, for example, the bacterial pathogen Pseudomonas aeruginosa requires Cu for siderophores and exotoxin expressions [151]. P. aeruginosa can also acquire Cu by producing Cu transporters and Cu-regulated transcription factors [152].

Furthermore, in Streptococcus pneumoniae, Mn has been identified as necessary to grow and produce a cytolytic toxin known as pneumolysin, which facilitates pneumococcal infections [153,154]. Besides pneumolysin production, Mn has a significant aspect in gene regulation in responsible for stress resistance and DNA protection in this microbe. While in S. aureus, both adhesins expression and biofilm formation, which carries out energy metabolism and oxidative stress resistance, are required to develop an infection in the host [155], where Mn is needed for gene regulation [156]. Developing these virulence factors, such as adhesins, capsules, and other surface-associated proteins, generally helps in the bacterial invasion, colonization, and evasion from the host immune responses [154].

Subsequently, copious bacterial pathogens like Helicobacter pylori and Klebsiella pneumonia required Ni for their growth and physiological development. For example, in these bacteria, Ni acts as a cofactor in urease enzymes to stabilize its structure, helps in bacterial resistance in an acidic environment such as the stomach, and helps in bacterial colonization in the host, protecting them from host immune responses [157]. Urease activity is crucial for H. pylori colonization in the stomach as it neutralizes gastric acids and allows the bacteria to survive in hostile environments [158], as shown in

Table 3. Summarizing specific microbes, virulent factors, and metals associated with their roles.

Microbes	Virulence Factor	Role of Trace Metals	References
Candida albicans	Phospholipase enzymes	Ca: Activates and stabilizes phospholipase enzymes, also secretes candida lysin.	[165]
Pseudomonas aeruginosa	Pyocyanin	Fe: Crucial for pyocyanin synthesis, a virulent pigment.	[166,167]
Streptococcus pneumoniae	Pneumolysin	Zn: Activates pneumolysin, causing host cell damage.	[168]
Helicobacter pylori	Urease	Ni: Essential for urease activity, aiding survival in an acidic stomach.	[169]
Escherichia coli	Shiga toxin	Fe, Zn, Cu, and Mg: Essential for toxin production and stability.	[170,171]
Staphylococcus aureus	Coagulase	Ca: Activates coagulase and aids in blood clotting.	[172]
Vibrio cholerae	Cholera toxin	Zn: Stabilizes cholera toxin structure for practical function.	[173]

. Ni is also immersed in regulating genes involved in energy metabolism and DNA repair in H. pylori [159].

Recent research studies have shown Co’s decisive role in the physiology and virulence of several bacterial pathogens. For example, in P. aeruginosa, Co is required to express virulence factors, including pyoverdine, a siderophore that facilitates the uptake of Fe by the bacterium. Pyoverdine is also integrated into biofilm formation, a key mechanism for P. aeruginosa’s survival and pathogenesis [160,161]. Co is also incorporated in regulating genes involved in energy metabolism and resistance to oxidative stress in P. aeruginosa [162]. In addition, Co is essential for the activity of Co-dependent enzymes, such as nitrile hydratase, involved in the metabolism of nitriles and cobalamin (vitamin B12)-dependent enzymes critical for various cellular processes, including DNA synthesis and methionine metabolism [163]. Co also plays an imperative role in antibiotic resistance mechanisms in several pathogenic bacteria such as S. aureus, Escherichia coli, and K. pneumoniae. Metalloenzymes such as β-lactamases and aminoglycoside-modifying enzymes need Co for their proper function. These enzymes can degrade the toxic nature of antimicrobial agents, which inhibit their growth [164]. The importance of metals in bacterial pathogens’ virulence and antibiotic resistance underscores the potential of metal-related therapies to combat bacterial infections.

4. Interactions between Metals and Insects/Microbial Pathogens

Trace metals as micronutrients are required by all living organisms for their survival and physicochemical processes. As discussed above, microorganisms, plants, and animals compete to obtain these micronutrients to fulfill their needs. Likewise, insects and microbial pathogens also interact with other to acquire trace metals in the required concentrations. Therefore, these metals play an essential role in the association between living things. The need for these micronutrients and their physiological functions in microorganisms have been discussed in Section 3. A recent study cites that insects have evolved several mechanisms to acquire, transport, and regulate metal levels to combat microbial infections [174]. For instance, Fe is critical for the immune response of insects against microbial infections. Insects can sequester Fe from hemolymph (insect blood) and store it in tissues to limit the availability of Fe to invading pathogens [175]. Insects can also produce antibacterial peptides that require Fe for their activity, underlying the importance of Fe for the immune response [80]. In addition, Fe can also affect the composition of insect-associated microbial communities. They also have been shown to harbor various microorganisms, including bacteria, fungi, and viruses, which can have both beneficial and harmful effects on their hosts [176]. However, the unavailability of Fe can affect the growth and survival of these microorganisms and potentially alter their interactions with the insect host.

Another trace metal, such as Cu, is also essential for the immune response of insects against microbial infections [177]. Insects can produce Cu-binding proteins and transporters to remove Cu from the environment and limit its availability to invading pathogens. Cu regulates several immune genes in insects, including those encoding antimicrobial peptides, homeostasis, and proteins in response to oxidative stress that can occur during the immune response and other physiological processes [178]. In addition, Cu has been shown to upregulate the expression of genes encoding antimicrobial peptides (AMPs), which are critical effectors of the insect’s innate immune system. AMPs are small cationic peptides that can kill various microbial pathogens by disrupting their cell membranes [179].

Interestingly, one of the well-known trace minerals, Zn, is also required for the growth, development, and immune function of insects and microbial pathogens. Zn may have positive and negative effects on these organisms, depending on the concentration and method of application. Insects can produce Zn-binding proteins and transporters to regulate Zn levels and limit their availability to invading pathogens [180]. Glavinic et al. showed that supplementing honeybees with Zn can increase their resistance to specific pathogens such as Nosema ceranae [181]. However, high Zn concentrations can also be toxic to insects, diversely affecting their growth and survival. Zn can also positively affect the growth and virulence of certain bacteria, such as P. aeruginosa and S. pneumonia [182]. However, it can inhibit the growth of certain bacteria, such as Salmonella typhimurium and E. coli. Microbial pathogens have also evolved mechanisms to acquire and regulate metal levels to induce insect infection. For example, some bacterial pathogens can produce siderophores to remove Fe from the insect host. In contrast, some fungal pathogens can produce Zn-binding proteins and transporters to regulate Zn levels for their growth and survival [183]. Overall, the interactions between metals and insects/microbial pathogens are complex and diverse, and further research is needed to fully understand the underlying mechanisms and implications for host–pathogen interactions.

5. Mechanisms of Competition between Insects and Microbial Pathogens for Essential Metals

The competition for metals such as Fe, Cu, and Zn is critical to interacting with insects and microbial pathogens [174,184]. Both insects and pathogens require these metals for their growth and survival, and they have evolved multiple mechanisms to acquire and regulate metal levels. One of the primary mechanisms of competition for metals is the production of metal-binding proteins and transporters [185]. Insects can produce metal-binding proteins and transporters to regulate metal levels and limit their availability to invading pathogens. For example, insects can produce metallothionein proteins that bind to metals such as Zn and Cd, restricting their availability to invading pathogens [186].

Similarly, insects can produce ferritin proteins that sequester Fe, limiting their availability to invading pathogens. Microbial pathogens can also produce metal-binding proteins and transporters to remove metals from the insect host, as some bacterial pathogens can produce siderophores that bind to Fe and transport it into the bacterial cell [187]. Analogously, some fungal pathogens can produce Zn-binding proteins and transporters to grab Zn from the insect host [188].

Insects and microbial pathogens both compete for metals by regulating genes for metal uptake and metabolism. For example, insects may upregulate metal acquisition and metabolism genes in response to metal limitation, while microbial pathogens may also upregulate metal uptake and metabolism genes in response to metal availability [86,189]. Collectively, these micronutrients and elements have complex interactions with insects and microbial pathogens. While they are integral for proper insect development and immune function, they may also have antimicrobial properties that can help protect insects from pathogens.

6. Metals That Limit Microbial Growth and Virulence

Metals can limit microbial growth and virulence by affecting several biological processes, such as the reaction to oxidative stress, energy metabolism, and DNA synthesis [190]. Here are some exemplifications: Fe confinement can limit the growth and virulence of many bacterial pathogens. For example, in the bacterial pathogen P. aeruginosa, Fe limitation can reduce the expression of virulence factors such as siderophores and exotoxins and limit the growth of the bacterium. Fe limitation can also activate the expression of genes involved in the oxidative stress response, which can further restrict the growth and purulence of the bacterium [191,192]. The confinement of Cu can exert growth-limiting and virulence-reducing effects on various microbial pathogens. For example, in the fungal pathogen C. albicans, the unavailability of Cu can reduce the expression of virulence factors such as adhesins and invasins and limit fungal growth [150]. The limitation of Cu can induce the activation of gene expression related to the oxidative stress response, thereby restricting fungal development and diminishing virulence [193].

The proliferation and pathogenicity of diverse microbial pathogens can be compromised without an optimum concentration of Zn. In the case of the bacterial pathogen E. coli, Zn limitation can reduce the expression of virulence factors such as adhesins and fimbriae and activate the expression of genes involved in response to oxidative stress, which limits the sprout of the bacterium [194,195]. The absence of Mn has been demonstrated to significantly affect microbial physiological properties, emphasizing the importance of Mn in microbial systems. Mn in a low concentration can lead to the downregulation of genes encoding virulence factors, while increased Mn availability can lead to their upregulation [196]. In addition, another trace metal, such as Mo, can affect bacterial physiology in several ways. For example, the concentration of Mo beyond the optimum requirement can lead to changes in bacterial metabolism, including a shift toward anaerobic respiration and altered expression of genes involved in energy metabolism [197]. There is ample room for further progress in determining how metals limit microbial growth and virulence, which is necessary for developing new strategies to combat microbial infections. Targeting metal acquisition and metabolism pathways in microbial pathogens may be a promising approach for developing novel antimicrobial therapies. Metal supplementation can also be a beneficial adjunctive therapy to enhance the host’s immune response against microbial infections [86,198].

7. Microorganisms Require Trace Metals for Their Pathogenicity Enhancements

Various pathogenic microorganisms need different trace elements for their physiological functions to improve their pathogenicity mechanisms; for example, Fe is required by some microbes that help in enhancing the expression of virulent genes. In Yersinia pestis, Fe boosts the face of type III secretory system (T3SS) and type IV secretory system (T4SS). Similarly, Fe promotes these systems in Legionella pneumophila, enabling the bacterium to transport toxins into the host cells [199]. T3SS is a complex molecular machinery that spans the bacterial cell envelope, allowing the bacterium to transport effector proteins into host cells. These effector proteins can trigger host cell signaling pathways, disrupting cellular processes and ultimately promoting the survival and growth of the pathogen in the host [200]. In addition, Fe can stimulate the expression of sundry other virulence factors in different microbial pathogens. Likewise, Fe can promote the production of adhesins and toxins in pathogens such as S. aureus and E. coli. This association between Fe and the pathogens allows them to enhance their virulence and better exploit host systems [201].

In contrast, Cu is considered to be a natural antimicrobial agent that has been deployed to control microbial growth for centuries. Microbial pathogens have evolved multiple resistant mechanisms to counteract the toxic effects of Cu and utilize it for their survival and vigilance [202]. Some microbial pathogens leverage the regulation of Cu transport and efflux systems to negotiate with Cu. Microorganisms can also rely on Cu as a core nutrient, prompting the development of specific transport mechanisms to absorb Cu from their ambiances. Additionally, particular pathogens possess efflux pumps capable of expelling excess Cu, thereby fostering against potential toxicity [203]. In addition, some pathogens produce antioxidant enzymes, such as superoxide dismutase and catalase, that can neutralize the ROS generated by Cu. ROS can damage cellular components, including DNA, proteins, and lipids, and lead to cell death. Therefore, these pathogens can counteract ROS activity by utilizing these enzymes to neutralize ROS and protect the cell [204]. It can also enhance the response to oxidative stress in several microbial pathogens, including P. aeruginosa and Salmonella enterica [205].

Moreover, Zn can liaise with microbial pathogens by promoting the expression of virulent genes and enhancing their ability to evade host immune cells, e.g., the bacterial pathogen S. pneumoniae fosters the face of the pneumolysin toxin in the presence of Zn, which is the main toxin in establishing pneumococcal infections [154,206]. Alternatively, Zn has been reported to promote the expression of other pathogenic determinants, such as adhesion molecules and invasive factors, in various microbial pathogens, including C. albicans [207] and S. aureus [208]. It can also evade the host’s immune response to favor microbial infections. Mo can act as a cofactor for various enzymes, including nitrogenases, nitrate reductases, and sulfite oxidases involved in nitrogen fixation, nitrogen assimilation, and sulfur metabolism [209]. The pathogenic bacterium P. aeruginosa is a highly pathogenic and persistent microbe that utilizes Mo for its physiological functions. Mo is essential for expressing multiple pathogenic elements, including the siderophore, ferricrocin, pyoverdine, the enzyme catalase–peroxidase, and protease LasB, as evidenced by numerous investigations [210]. Another study examined the role of Mo in the pathogenicity of the fungus Aspergillus fumigatus [211]. These mechanisms through which trace metals and microbial pathogens work together are complex and diverse, and understanding these interactions is critical for developing effective strategies to control and prevent microbial infections.

8. Pathogens Use Metals to Evade the Host’s Immune Response

Pathogens exhibit creative strategies to escape the host immune response, often employing metals to change their physiology. Most bacterial pathogens depend on Fe for their growth and survival [212]. These cunning invaders utilize intricate Fe capture mechanisms to extract it from the host, thereby restricting its availability to immune cells, thus undermining the host’s defenses [39,213]. Several pathogens can also produce Fe-binding proteins, such as transferrin-binding and lactoferrin-binding proteins, to sequester Fe from the host [214]. It can limit the availability of Fe to host immune cells and impair their function. As reported by various scientific analyses, Cu is involved in the oxidative stress response of host immune cells, such as neutrophils and macrophages. Pathogens can employ Cu efflux pumps to diminish Cu levels within host immune cells, impairing their oxidative capacity [86,215]. Some pathogens can also produce Cu-binding proteins such as CopA and CopB in P. aeruginosa to sequester Cu from the host and restrict its availability to host immune cells [216,217].

Multivalent metals like Zn are entangled in multiple immune responses, including the regulation of cytokine production and the activation of immune cells. Pathogens can utilize Zn efflux pumps to decrease Zn levels in host immune cells and impair their function [218]. Specific pathogens, like E. coli, can generate Zn-binding proteins (e.g., ZnuA and ZinT). These proteins help the pathogens to seize and limit the availability of Zn from the host, consequently impeding the function of immune cells [219]. Specific bacterial pathogens, like H. pylori, responsible for gastric ulcers and cancer, employ Ni to evade their host’s immune response. Researchers found that the pathogen can take Ni from the host environment and use it to produce enzymes called ureases that can break down urea and release ammonia [220]. This ammonia can neutralize the stomach’s acidic environment and create a more favorable environment for the colonization of the pathogen. In addition, ureases produced by H. pylori can interact with the host’s immune cells and impair their function, allowing the pathogen to escape the host’s immune responses [221]. In addition, another bacterial pathogen, S. aureus, also utilizes Ni to resist the host immune response. S. aureus can take Ni from the host environment and use it to produce a surface protein called IsdA, which reduces microbial cellular hydrophobicity and decreases bacterial cellular hydrophobicity, posing them to resist the antibacterial human skin fatty acids and peptides [222]. Investigating the utilization of these miscellaneous metals via pathogens sheds light on how they evade the host’s immune response. This knowledge is crucial for devising novel approaches to combat microbial infections. In summary, this review underscores the significance of metals as vital facilitators in insect–microbe interactions, paving the way for exciting prospects in future research endeavors within this domain.

9. Implications and Applications

9.1. Development of Novel Insecticides and Antimicrobial Therapies

In scientific innovation, metals are vital in developing new insecticides and antimicrobial treatments. Researchers utilize metals like Cu and Zn to create robust solutions against pests and diseases [223]. Cu disrupts fungal invaders, protecting precious crops from their insatiable appetite [224]. Furthermore, Zn exposes its elemental ability, engaging in a microbial duel, vanquishing the bacteria, fungi, and viruses that threaten our delicate balance [225]. Novel insecticides and antimicrobial therapies are crucial for agriculture and public health. However, resistance to traditional chemicals is a growing concern [226]. Developing novel insecticides and antimicrobial treatments targeting specific biological pathways or mechanisms can help address these challenges. However, the safety and efficacy of these novel compounds must be thoroughly evaluated to ensure they do not pose unintended risks to the environment or human health [227]. However, this development will improve the innovation of novel insecticides and antimicrobial therapies.

9.2. Promoting Agricultural Sustainability

Several insect pests and microbial pathogens are major crop threats, resulting in significant economic losses [228]. Understanding the role of metals in the interaction between insect and microbial pathogens can lead to the development of sustainable and environmentally friendly pest and disease control strategies that target metal uptake and metabolic pathways in these organisms [229]. However, excessive use of metals in agricultural systems can negatively impact insect and microbial populations. Contradicting reports about high concentrations of Cu and Zn can lead to the selection of metal-resistant bacteria and disruption of microbial communities that play essential roles in nutrient cycling and soil health. Insects can also develop resistance to metals, reducing the effectiveness of metal-based pesticides and making plants vulnerable to insect damage [230]. Therefore, promoting agricultural sustainability requires balancing the benefits and risks of using metals in agriculture. It can be achieved by developing alternative pest management strategies, such as biological control and integrated pest management, that minimize the use of metals and promote the natural regulation of insect and microbial populations [231]. Figure 5 illustrates the intricate interactions between soil, plant, and microbes and how fumigants or drought stress can kill beneficial bacteria in the soil. Furthermore, exploring interactions between metals, insects, and microbial pathogens can provide insights into the mechanisms of insect immunity and microbial pathogenesis, leading to the development of novel and sustainable crop protection strategies.

9.3. Potential Applications for Pest and Pathogen Control

The role of metals in insect and microbial pathogen interactions has received significant attention due to their potential applications in pest and pathogen control. Metal ions, such as Cu, Zn, and Fe, play an essential role in insect physiology, including growth, development, and reproduction [232]. In addition, some metals exhibit antimicrobial properties that can help control insect microbial pathogens. One pest management method is biological control, which involves using natural enemies to control pests. Several biological control agents, such as entomopathogenic fungi and bacteria, have been effective against pests and pathogens [233]. Metals play a crucial role in these biological control agents’ survival, virulence, and interplay with pests and pathogens.

Metal homeostasis and redox signaling mechanisms are pivotal in the responses of insects and microbial pathogens to environmental stressors, including exposure to toxic metals [232]. Many models about nanoparticles, biomimicry, and bioremediation are emerging research areas that hold promise for developing new approaches to pest and pathogen control [234]. These approaches rely on metals’ unique properties and interactions with biological systems to develop effective control measures.

10. Critical Observation and Further Research

Metals play a crucial role in microbial–insect interactions, but their precise mechanisms and specific functions are largely unknown. Further research is needed to better understand the interactions between metals, insects, and microbial pathogens to develop strategies to combat insect-borne diseases. In addition, research is required to explore the full potential of metals in pest and disease control and their effectiveness and safety in these applications. Insects are an essential food source for humans and animals but are responsible for spreading various diseases. Interactions between insects and microbial pathogens can be complex, involving multiple species and multiple routes of transmission. Therefore, research is also needed to better understand the mechanisms of information and the potential impact of these interactions on human and animal health. Insects are often the primary hosts of microbial pathogens, making them more susceptible to the transmission of pathogens. Effective control of insect-borne diseases requires an integrated approach that includes insect control, vector surveillance, and strategies to reduce human–animal contact. Climate change will likely significantly impact insect populations and the spread of microbial diseases. Therefore, future research is needed to understand the impact of climate change and other environmental factors on the interactions between insects and microbial pathogens.

11. Conclusions

In summary, metals are crucial in insects and microbial pathogens. In insects, metals such as Cu, Fe, Zn, Mn, Ni, Co, and Mo are essential for vital biological processes such as respiration, immune response, and enzymatic reactions. In microbial pathogens, metals such as Fe and Mn are critical in enhancing their virulence factors, as they are involved in oxidative stress defense, metabolism, and gene regulation. However, the availability and regulation of metals in insects and microbial pathogens can significantly affect their health and survival. Therefore, exploring the role of metals in these organisms can provide valuable insights into their biology and potential targets for developing new treatments and control strategies. Research into the role of metals in insect and microbial pathogens can have significant implications for developing new approaches to combat these organisms.

However, since metals are critical to the survival and virulence of microbial pathogens, targeting their metal acquisition and regulatory mechanisms could potentially lead to the development of novel antimicrobial agents. In addition, understanding how insects take up and regulate metals could lead to the development of new insecticides that interfere with their vital biological processes. The reason for studying the interactions between metals, insects, and microbial pathogens can shed light on how these organisms respond to environmental stressors such as changes in metal availability. This knowledge can be used to develop new strategies to control insect pests and microbial pathogens under different environmental conditions. Overall, understanding the role of metals in insect and microbial pathogens can lead to developing new, more effective strategies to control these organisms, which could have significant implications for agriculture, public health, and environmental management.