Divergence in environmental adaptation between terrestrial clades of the Bacillus cereus group.

Bacteria in the Bacillus cereus group encompasses diverse niches and include causative agents of anthrax and food poisoning, specialized invertebrate pathogens and psychrotolerant species. Understanding whether natural selection has led to divergence between phylogenetic clades can help understanding the processes shaping speciation, and has important implications for understanding the distribution and biological safety of this group. We tested whether the three most common terrestrial species in this group (B. cereus, Bacillus thuringiensis and Bacillus mycoides) are ecologically specialized in terms of resource use, thermal adaptation and fitness in different environmental conditions. All isolates tested grew vigorously in protein rich media and insect cadavers, but exploitation of soil or plant derived nutrients was similarly weak for all species. Psychrotolerance rather than clade predicted growth patterns in artificial media, while psychrotolerant isolates could outcompete B. thuringiensis in insect cadavers at low temperature. Nevertheless, for B. thuringiensis and B. mycoides, clade and taxonomic species were important predictors of growth and relative fitness in live insect infections. The common ecological niche in these terrestrial B. cereus species is the ability to exploit protein rich resources such as cadavers. However, selection has led to different phylogenetic groups developing different strategies for accessing this resource. Thus, clades, as well as traditional taxonomic phenotypes, predict biologically important traits.