1 Introduction

Predatory bacteria are becoming increasingly recognized as the apex predators of microbial communities, honed by evolution into proficient killers of other microbes.1-3 Some micropredators, such as Herpetosiphon spp. and myxobacteria, are able to kill a broad range of prey, including Gram‐negative bacteria, Gram‐positive bacteria, and fungi.4-6 Such broad range predation involves the secretion of toxic materials (hydrolytic enzymes and secondary metabolites) into the extracellular milieu, which can, therefore, be considered public goods—a shared resource produced communally.7-9 This mechanism of predation is therefore often referred to as “wolf‐pack” predation, as it apparently requires high densities of predatory cells, and is social—i.e., all predators contribute to the same public goods.10

Nevertheless, the molecular mechanisms of social predation are relatively poorly understood in myxobacteria and other microbial wolf‐pack hunters. It is known that myxobacteria secrete enzymes with antibacterial activity,11 and that the outer membrane vesicles (OMVs) they secrete are also intrinsically antimicrobial.12 However, even though they have low‐complexity proteomes,13, 14 we largely do not know which OMV cargo proteins are responsible for killing a particular prey, nor (except in a few specific cases) can we explain why a particular prey is more or less susceptible to communal attack by myxobacteria.15-18

In contrast, we know considerably more about the molecular mechanisms and evolutionary forces at play during fruiting body formation, a cooperative process that myxobacteria engage in when prey is scarce.19

1.1 Myxobacteria Respond Socially to Starvation Nutrient limitation triggers populations of the model myxobacterium Myxococcus xanthus (M. xanthus) to engage in multicellular development, which requires cellular differentiation as well as a minimum population size. Upon starvation, around 100 000 cells aggregate into a raised mound (fruiting bodies), within which about 10% of the cells sporulate.19 It is thought that fruiting body sporulation ensures that when prey becomes available again, a predation‐efficient population of germinants will emerge, rather than a predation‐inefficient single germinant.20 During development, cells differentiate into discrete cell types. Peripheral rods around the base of the fruit seem to have a role in “scouting” for nutrient availability, whereas most cells entering the fruit are destined to autolyze, providing energy for surviving cells to complete sporulation and differentiate into spores.19 Such complex multicellular behavior requires sophisticated regulation, with coordinated intracellular and intercellular signaling. Myxobacteria consequently have a huge regulatory potential, through a multitude of two‐component system signaling pathways and Ser/Thr kinases.21-23 During nutrient limitation, each cell senses its own starvation and communicates it to other cells in the population.24 Intercellular signaling is mediated by the “A‐signal,” which is a secreted mixture of peptides, proteases, and amino acids. The A‐signal is a “quorum signal,” as the amount secreted reflects the number of starving cells present,24 and development only proceeds if sufficient cells (a quorum) are available to form a fruiting body. Later stages of development require the coordination of motility (by regulating the frequency of reversals in the gliding direction) and sporulation, and this is orchestrated by the “C‐signal.” The C‐signal is a cell surface‐associated signal that stimulates signaling in other cells upon cell–cell contact.25, 26 Thus, the C‐signal responds to cell density, which progressively increases during development as cells aggregate. Only at the high cell densities found within a fruiting body is enough C‐signaling possible for sporulation to commence.26 Development is thus cooperative, and although 90% of cells forming a fruiting body are destined to undergo programmed cell death (autolysis), they thereby provide the energy and nutrients required by the remaining 10% to sporulate.27 However, this makes fruiting body formation vulnerable to “cheating” genotypes. For example, a genotype that undergoes autolysis at a frequency <90% will be disproportionately represented among the spores within a fruit and in the resulting population of germinants. The fitness advantage of the cheat depends on its relative frequency in the population.28 When a cheat constitutes a substantial proportion of the population, the number of cells undergoing autolysis can be reduced to a point where none of the surviving cells in the population are able to sporulate. Cheating genotypes emerge spontaneously, are abundant in natural populations, and can drive entire populations to extinction.29 Thus, cheaters represent a burden on populations that social organisms need to mitigate.30