The dynamics of cooperative bacterial virulence in the field

Laboratory experiments have shown that the fitness of micro-organisms can depend on cooperation between cells. While this insight has revolutionized our understanding of microbial life, results from artificial microcosms have not been validated in complex natural populations. Here, we investigate the sociality of essential virulence factors (crystal toxins) in the pathogen Bacillus thuringiensis using diamondback moth larvae (Plutella xylostella) as hosts. We show that: crystal toxins are a novel form of cooperative public good; and, in a manipulative field experiment, observed stable high relatedness and both frequency and density dependent selection on toxin production. Conditions favoring social virulence can therefore persist under natural population dynamic conditions, and social interactions (rapid cheat invasion) may account for the rarity of disease outbreaks caused by B. thuringiensis.