Whole-genome-based phylogeny of Bacillus cytotoxicus reveals different clades within the species and provides clues on ecology and evolution.

Bacillus cytotoxicus is a member of the Bacillus cereus group linked to fatal cases of diarrheal disease. Information on B. cytotoxicus is very limited; in particular comprehensive genomic data is lacking. Thus, we applied a genomic approach to characterize B. cytotoxicus and decipher its population structure. To this end, complete genomes of ten B. cytotoxicus were sequenced and compared to the four publicly available full B. cytotoxicus genomes and genomes of other B. cereus group members. Average nucleotide identity, core genome, and pan genome clustering resulted in clear distinction of B. cytotoxicus strains from other strains of the B. cereus group. Genomic content analyses showed that a hydroxyphenylalanine operon is present in B. cytotoxicus, but absent in all other members of the B. cereus group. It enables degradation of aromatic compounds to succinate and pyruvate and was likely acquired from another Bacillus species. It allows for utilization of tyrosine and might have given a B. cytotoxicus ancestor an evolutionary advantage resulting in species differentiation. Plasmid content showed that B. cytotoxicus is flexible in exchanging genes, allowing for quick adaptation to the environment. Genome-based phylogenetic analyses divided the B. cytotoxicus strains into four clades that also differed in virulence gene content.