Abstract
BackgroundThe Bacillus cereus sensu lato group consists of six species (B. anthracis, B. cereus, B. mycoides, B. pseudomycoides, B. thuringiensis, and B. weihenstephanensis). While classical microbial taxonomy proposed these organisms as distinct species, newer molecular phylogenies and comparative genome sequencing suggests that these organisms should be classified as a single species (thus, we will refer to these organisms collectively as the Bc species-group). How do we account for the underlying similarity of these phenotypically diverse microbes? It has been established for some time that the most rapidly evolving and evolutionarily flexible portions of the bacterial genome are regulatory sequences and transcriptional networks. Other studies have suggested that the sigma factor gene family of these organisms has diverged and expanded significantly relative to their ancestors; sigma factors are those portions of the bacterial transcriptional apparatus that control RNA polymerase recognition for promoter selection. Thus, examining sigma factor divergence in these organisms would concurrently examine both regulatory sequences and transcriptional networks important for divergence. We began this examination by comparison to the sigma factor gene set of B. subtilis.ResultsPhylogenetic analysis of the Bc species-group utilizing 157 single-copy genes of the family Bacillaceae suggests that several taxonomic revisions of the genus Bacillus should be considered. Within the Bc species-group there is little indication that the currently recognized species form related sub-groupings, suggesting that they are members of the same species. The sigma factor gene family encoded by the Bc species-group appears to be the result of a dynamic gene-duplication and gene-loss process that in previous analyses underestimated the true heterogeneity of the sigma factor content in the Bc species-group.ConclusionsExpansion of the sigma factor gene family appears to have preferentially occurred within the extracytoplasmic function (ECF) sigma factor genes, while the primary alternative (PA) sigma factor genes are, in general, highly conserved with those found in B. subtilis. Divergence of the sigma-controlled transcriptional regulons among various members of the Bc species-group likely has a major role in explaining the diversity of phenotypic characteristics seen in members of the Bc species-group.
Highlights
The Bacillus cereus sensu lato group consists of six species (B. anthracis, B. cereus, B. mycoides, B. pseudomycoides, B. thuringiensis, and B. weihenstephanensis)
To place these studies in context, we began by constructing a phylogeny of the Bacillaceae using whole-genome single copy genes. This phylogeny suggested that the current taxonomic affiliation of many members of the Bacillaceae should be reconsidered. Using this phylogeny as a basis, we examined the phylogenetic relationships of the sigma factors encoded by members of the B. cereus (Bc) speciesgroup
The initial divergence within the Bacillaceae was between Exiguobacterium, an aerobic, asporogenous, and irregularly shaped Gram-positive bacterium recently linked to bacteraemia [42], and the bulk of the family
Summary
The Bacillus cereus sensu lato group consists of six species (B. anthracis, B. cereus, B. mycoides, B. pseudomycoides, B. thuringiensis, and B. weihenstephanensis). The genus Bacillus consists of a heterogeneous group of Gram-positive heterotrophic aerobic or facultative anaerobic bacilli with the ability to form environmentally resistant, metabolically inert spores [1] These soil-borne organisms are ubiquitous throughout the world, and occupy surprisingly diverse environments [2,3]. Within this large genus, the B. cereus sensu lato group consists of six species [B. anthracis (Ba), B. cereus (Bc), B. mycoides, B. pseudomycoides, B. thuringiensis (Bt), and B. weihenstephanensis], based on classical microbial taxonomy [4]. Newer molecular phylogenies and comparative genome sequencing suggests that these organisms should be classified as a single species [5] On the surface, this conclusion seems difficult to reconcile with the varied biological characteristics of these organisms. We will refer to these organisms as the Bc species-group, to reflect the extremely close phylogenetic relationships between these organisms
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