Bacterial predation is a ubiquitous and fundamental biological process, which influences the community composition of microbial ecosystems. Among the best characterised bacterial predators are the myxobacteria, which include the model organism <i>Myxococcus xanthus</i>. Predation by <i>M. xanthus</i> involves the secretion of antibiotic metabolites and hydrolytic enzymes, which results in the lysis of prey organisms and release of prey nutrients into the extracellular milieu. Due to the generalist nature of this predatory mechanism, <i>M. xanthus</i> has a broad prey range, being able to kill and consume Gram-negative/positive bacteria and fungi. Potential prey organisms have evolved a range of behaviours which protect themselves from attack by predators. In recent years, several investigations have studied the molecular responses of a broad variety of prey organisms to <i>M. xanthus</i> predation. It seems that the diverse mechanisms employed by prey belong to a much smaller number of general “predation resistance” strategies. In this mini-review, we present the current state of knowledge regarding <i>M. xanthus</i> predation, and how prey organisms resist predation. As previous molecular studies of prey susceptibility have focussed on individual genes/metabolites, we have also undertaken a genome-wide screen for genes of <i>Pseudomonas aeruginosa</i> which contribute to its ability to resist predation. <i>P. aeruginosa</i> is a World Health Organisation priority 1 antibiotic-resistant pathogen. It is metabolically versatile and has an array of pathogenic mechanisms, leading to its prevalence as an opportunistic pathogen. Using a library of nearly 5,500 defined transposon insertion mutants, we screened for “prey genes”, which when mutated allowed increased predation by a fluorescent strain of <i>M. xanthus</i>. A set of candidate “prey proteins” were identified, which shared common functional roles and whose nature suggested that predation resistance by <i>P. aeruginosa</i> requires an effective metal/oxidative stress system, an intact motility system, and mechanisms for de-toxifying antimicrobial peptides.

While protein translocation in Gram-negative bacteria is well understood, our knowledge about the translocation of other high-molecular-weight substances is limited. Nozzle-like structures that secrete exopolymeric substances during gliding motility have previously been observed in the outer membranes of cyanobacteria and myxobacteria. Here, we show that these nozzles are composed of the secretins PilQ/GspD, the outer membrane component of the type II and III secretion systems, the type IV pilus apparatus, and filamentous phage extrusion machinery. Our results show for the first time that secretins may be used for secretion of non-proteinaceous polymers in some bacteria, considerably expanding the repertoire of substrates of these multifunctional outer membrane gates. Moreover, we show that gspD is an essential gene in Myxococcus xanthus, which, when depleted, renders this bacterium defective in slime secretion and gliding motility.SignificanceMany bacteria exhibit gliding motility, movement across surfaces. This motility has been correlated with the deposit of slime trails in their wake. To date, the mechanism of slime secretion has not been understood, and no cell envelope-structures have been identified that are involved in slime secretion during gliding motility. Here, we show that cyanobacteria and myxobacteria use the secretins PilQ/GspD, the outer membrane channels of the T2SS, for slime secretion, which demonstrates a novel cargo transport capacity of these multifunctional outer membrane gates.