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

Abstract

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.

Adaptation of Bacillus thuringiensis to Plant Colonization Affects Differentiation and Toxicity

“…Mixtures of B. thuringiensis crystal proteins and spores can effectively control varieties of insects such as larvae of caterpillars, beetles, and nematodes ( 18 ). Several previous studies support the potent and specialized invertebrate pathogenicity of B. thuringiensis ( 16 , 19 – 21 ). While the animal-associated environment of B. thuringiensis is well studied, other niches such as plant roots and soil are relatively less explored.…”

“…Monnerat and colleagues demonstrated that when B. thuringiensis is inoculated into the soil, it could translocate throughout the plants and afterwards colonize the phylloplane stably ( 26 ). Recent evidence suggested that B. thuringiensis occurs in the plant rhizosphere that might play an important niche for the bacterium to access its animal hosts ( 20 , 21 ). Similarly, B. cereus has also been isolated from the rhizosphere ( 27 , 28 ).…”

Signatures of selection in core and accessory genomes indicate different ecological drivers of diversification among Bacillus cereus clades

“…These features are characteristic of adaptation to low temperatures, where metabolic functions are downregulated in response to cold (Barria et al, 2013; López‐Maury et al, 2008; Tribelli & López, 2018). This supports other studies indicating that strains within Clade 3 are psychrotolerant specialists (Lechner et al, 1998; Liu et al, 2018; Manktelow et al, 2021) and demonstrates how the methodology used here can identify important genes with specific variants within ecologically distinct groups. Different patterns of enrichment amongst conserved clade‐specific core genes also suggest that the clades are ecologically distinct, and purifying selection may maintain new species by purging novel variation caused by mutation and HGT (Cohan, 2016, 2017).…”

“…To accomplish this, we tested bacterial genomes from an economically important and well‐studied model group for signatures of selection. The Bc group contains many different strains, all thought to be well‐adapted to exploit protein‐rich food such as cadavers (Manktelow et al, 2021; Rasigade et al, 2018). Despite high levels of genetic similarity, the clade structure of the group is distinct and robust to multiple phylogenetic methods.…”

Adaptation ofBacillus thuringiensisto plant colonization affects differentiation and toxicity

“…Monnerat and colleagues demonstrated that when B. thuringiensis is inoculated into the soil, it could translocate throughout the plants and afterwards colonize the phylloplane stably (26). Recent evidences suggested that B. thuringiensis occurs in the plant rhizosphere that might play an important niche for the bacterium to access its animal hosts (20, 21). Similarly, B. cereus has been also isolated from the rhizosphere (27, 28).…”