Evolutionary ecology of microbial wars: within-host competition and (incidental) virulence

Abstract

Invading an occupied niche is a formidable ecological challenge, and one of particular human importance in the context of food-borne microbial pathogens. We discuss distinct categories of invader-triggered environmental change that facilitate invasion by emptying their niche of competitors. Evidence is reviewed that gut bacteria use such strategies to manipulate their environment (via bacteriocins, temperate phage viruses or immuno-manipulation) at the expense of their competitors are reviewed. The possible virulence implications of microbial warfare among multiple co-infecting strains are diverse. Killing competitors can reduce virulence by reducing overall microbial densities, or increase virulence if for example the allelopathic mechanism involves immuno-manipulation. Finally, we place microbial anti-competitor strategies in a social evolution framework, highlighting how costly anti-competitor strategies can be understood as examples of microbial spite. We conclude by discussing other invasive species that have also developed such proactive strategies of invasion.

Keywords: disease biology; evolutionary theory; microbial biology; social evolution; virulence.

Figure 1

Schematic diagrams of (a) chemical, (b) viral and (c) immuno-manipulative weapons of mass microbial destruction. Squares represent microbial densities of competing weapons-carriers (C) and susceptibles (S). Circles represent the density of ‘weapon’, the agent of environmental change (B for bacteriocin, or other chemical toxin, V for temperate phage virus, and T for immuno-provocative toxin). The cylinder represents a multicellular host, the subject of immuno-manipulation by T. Pointed arrows represent a positive relationship (e.g. C increase the density of B), flat arrows represent a negative relationship (e.g. B decrease the density of S).