Predators promote defence of rhizosphere bacterial populations by selective feeding on non-toxic cheaters

Abstract

Soil pseudomonads increase their competitiveness by producing toxic secondary metabolites, which inhibit competitors and repel predators. Toxin production is regulated by cell-cell signalling and efficiently protects the bacterial population. However, cell communication is unstable, and natural populations often contain signal blind mutants displaying an altered phenotype defective in exoproduct synthesis. Such mutants are weak competitors, and we hypothesized that their fitness depends on natural communities on the exoproducts of wild-type bacteria, especially defence toxins. We established mixed populations of wild-type and signal blind, non-toxic gacS-deficient mutants of Pseudomonas fluorescens CHA0 in batch and rhizosphere systems. Bacteria were grazed by representatives of the most important bacterial predators in soil, nematodes (Caenorhabditis elegans) and protozoa (Acanthamoeba castellanii). The gacS mutants showed a negative frequency-dependent fitness and could reach up to one-third of the population, suggesting that they rely on the exoproducts of the wild-type bacteria. Both predators preferentially consumed the mutant strain, but populations with a low mutant load were resistant to predation, allowing the mutant to remain competitive at low relative density. The results suggest that signal blind Pseudomonas increase their fitness by exploiting the toxins produced by wild-type bacteria, and that predation promotes the production of bacterial defence compounds by selectively eliminating non-toxic mutants. Therefore, predators not only regulate population dynamics of soil bacteria but also structure the genetic and phenotypic constitution of bacterial communities.

Evolutionary history predicts the stability of cooperation in microbial communities

“…This mutant is typical for spontaneous mutations deactivating secondary metabolism in pseudomonads 28 and lacks extracellular proteases required for growing on albumin 3,30 . It grows better than the wild type when cooperation is not required 4 , but preliminary experiments showed that it is incapable of growing on QSM medium alone. In order to investigate its growth inhibition by the cooperators, we grow this mutant under nonlimiting nutrient conditions (Lysogeny Broth (LB)) supplemented with small amounts of supernatant from communities grown in QSM.…”

“…Invasion by unrelated defectors occurs from outside the population, whereas signal-blind defector mutants emerge by cell division inside the population of the cooperating ancestor. Based on our results, we propose that inhibition of closely related defectors was due to a combination of policing, in which cooperators invest into harming of defectors 34 , and disruption of QS via pleiotropic effect reducing resistance to various biotic and abiotic stressors 4,35 . The increased sensitivity of defectors to genetically related cooperators points to a suppression of close kin refusing to cooperate.…”

“…Several bacteria secrete diffusible compounds that function as public goods benefiting all present organisms independently of their investment. Public goods include iron-scavenging siderophores 2 , extracellular enzymes 3 or secondary metabolites harming competitors and predators 4 . Such cooperative traits improve growth rate, competitiveness and virulence of pathogens.…”

Phage selection for bacterial cheats leads to population decline

“…Such costs may differ between individuals adopting different social behaviours, and include energy or time committed to defence or resistance (e.g. [22,24]), or costs associated with trade-offs involved in evolved resistance to enemies [19,20]. Despite their ubiquitousness in nature and demonstrated importance in population ecology and evolutionary biology (e.g.…”

“…[16,19,20], but see [21]), or constitute a cost that potentially impacts other cooperative behaviours (e.g. resource access and sharing, quorum sensing [19,22,23]). Such costs may differ between individuals adopting different social behaviours, and include energy or time committed to defence or resistance (e.g.…”