Molecular Microbiology Research 2024, Vol.14, No.5, 208-217 http://microbescipublisher.com/index.php/mmr 209 2 Types of Microbial Predators 2.1 Bacterial predators Bacterial predators are a diverse group of microorganisms that prey on other bacteria, often through complex and specialized mechanisms. One prominent example is the deltaproteobacteria group, which includes Bdellovibrio and Bacteriovorax species. These Gram-negative bacteria are known for their ability to prey on other Gram-negative bacteria, making them potential biocontrol agents against foodborne and plant pathogens such as Escherichia coli, Salmonella spp., and Pseudomonas spp.. Bdellovibrio bacteriovorus, in particular, has been highlighted for its potential as a "living antibiotic" due to its ability to lyse pathogenic bacteria (McNeely et al., 2017; Herencias et al., 2020). Another group of bacterial predators includes the myxobacteria, such as Myxococcus xanthus. These bacteria employ a generalist predatory mechanism involving the secretion of antibiotic metabolites and hydrolytic enzymes, which can lyse a wide range of prey organisms, including both Gram-negative and Gram-positive bacteria as well as fungi (Sydney et al., 2021). The facultative predatory Actinomycetota spp. also exhibit diverse predation strategies, including epibiotic and wolfpack attacks, which involve the production of secondary metabolites to lyse prey cells (Ibrahimi et al., 2023). 2.2 Fungal predators Fungal predators, on the other hand, often employ different strategies to control their prey. For instance, certain soil-dwelling fungi, such as Mortierella species, form symbiotic relationships with toxin-producing bacteria that live within their hyphae. These bacterial endosymbionts produce anthelmintic metabolites that protect the fungi from nematode attacks, thereby enhancing their survival and efficacy as biocontrol agents (Figure 1) (Büttner et al., 2021). The study demonstrated through experiments that fungi without symbiotic bacteria were more vulnerable to attacks when co-cultured with nematodes, resulting in a significant increase in nematode numbers. In contrast, fungi with symbiotic bacteria were able to effectively reduce nematode populations. Further experiments showed that the defensive ability of the fungi could be restored by supplementing with nematode-repelling compounds, indicating that these compounds play a crucial role in the fungi’s defense against predators. Another example of fungal predators includes entomopathogenic fungi like Metarhizium anisopliae and Beauveria bassiana. These fungi are known for their ability to infect and kill insect pests, and their combined use with soil-dwelling predators has shown improved efficacy in controlling pests such as the western flower thrips. Pseudomonas brassicacearum DF41, a biocontrol agent against fungal pathogens, can resist predation by nematodes through the production of toxic metabolites and biofilm formation, which blocks the nematode's feeding structures (Nandi et al., 2016). 3 Mechanisms of Predation 3.1 Direct predation mechanisms 3.1.1 Attachment and invasion Microbial predators employ various strategies to attach to and invade their prey. Bdellovibrio bacteriovorus, for instance, attaches to the exterior of Gram-negative bacteria and invades the periplasmic space, where it replicates and eventually lyses the host cell. This attachment and invasion process is facilitated by specific enzymes, such as lytic transglycosylases, which cleave the prey's peptidoglycan, allowing the predator to enter and establish a niche within the prey cell (Banks et al., 2023). 3.1.2 Intracellular digestion Once inside the prey, microbial predators digest the host's cellular contents to obtain nutrients. Bdellovibrio bacteriovorus, for example, digests the prey's cellular components within the periplasmic space, utilizing a variety
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