Abstract
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.
Highlights
Bacillus cytotoxicus was first described in 2013 as a thermotolerant member of the Bacillus cereus group[1]
B. cytotoxicus has an approximate 85% average nucleotide identity (ANI) when compared to other B. cereus group members, with the highest value of 86.9% with B. pseudomycoides
Similar to B. cytotoxicus, B. pseudomycoides had an ANI of approximately 86% when compared to other B. cereus group members
Summary
Bacillus cytotoxicus was first described in 2013 as a thermotolerant member of the Bacillus cereus group[1]. Typing and 16S rRNA sequence comparisons showed that NVH 391-98T belonged to the novel species B. cytotoxicus[1]. This species characteristically harbors the cytK-1 variant of the gene encoding cytotoxin K, which is highly toxic to human intestinal Caco-2 and Vero cells[25]. B. cytotoxicus was described as a novel species in 2013 based on five strains, four of which were linked to foodborne disease[1]. It has been known for some time that cytotoxicity of B. cytotoxicus strains varies, with strain NVH 883-00 having been reported to be non-cytotoxic[26]. The generated data significantly extends the very limited body of knowledge on B. cytotoxicus, in particular allowing for novel insights into the evolution and differentiation of this species
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