Bacillus cytotoxicus Genomics: Chromosomal Diversity and Plasmidome Versatility

Abstract

Bacillus cytotoxicus is the thermotolerant representative of the Bacillus cereus group. This group, also known as B. cereus sensu lato, comprises both beneficial and pathogenic members and includes psychrotolerant and thermotolerant species. Bacillus cytotoxicus was originally recovered from a fatal outbreak in France in 1998. This species forms a remote cluster from the B. cereus group members and reliably contains the cytk-1 gene, coding for a cytotoxic variant of cytotoxin K. Although this species was originally thought to be homogenous, intra-species diversity has been recently described with four clades, six random amplified polymorphic DNA (RAPD) patterns, and 11 plasmids profiles. This study aimed to get new insights into the genomic diversity of B. cytotoxicus and to decipher the underlying chromosomal and plasmidial variations among six representative isolates through whole genome sequencing (WGS). Among the six sequenced strains, four fitted the previously described genomic clades A and D, while the remaining two constituted new distinct branches. As for the plasmid content of these strains, three large plasmids were putatively conjugative and three small ones potentially mobilizable, harboring coding genes for putative leaderless bacteriocins. Mobile genetic elements, such as prophages, Insertion Sequences (IS), and Bacillus cereus repeats (bcr) greatly contributed to the B. cytotoxicus diversity. As for IS elements and bcr, IS3 and bcr1 were the most abundant elements and, along with the group II intron B.c.I8, were found in all analyzed B. cytotoxicus strains. When compared to other B. cytotoxicus strains, the type-strain NVH 391-98 displayed a relatively low number of IS. Our results shed new light on the contribution of mobile genetic elements to the genome plasticity of B. cytotoxicus and their potential role in horizontal gene transfer.

Conjugation across Bacillus cereus and kin: A review

“…pIS56-63 sequence exhibits strong similarities with pHT73 as it carries a cry gene, multiple MGEs and conjugation-related genes, even though experimental data assessing its conjugative capacities are still missing (Figure 3). Another plasmid harbors a conjugative locus related to that of pHT73, the B. cytotoxicus plasmid pE81-53 (Figure 3), hosted by the same strain as the pXO2-like plasmid pE81-84 (Fayad et al, 2021b). Contrary to pE81-84, experimental evidence confirmed the conjugation potential of pE81-53 (Table 1).…”

“…pXO2, pAW63 and pBT9727 compose the core of the pXO2like plasmid family, but other conjugative -or putatively conjugative -plasmids also share some similarities. Three closely related plasmids from B. cytotoxicus were recently described as distantly related to the trio of pXO2-like plasmids (Fayad et al, 2021b). Two of them, pE81-84 and pE283-80, also harbor a region reminiscent of the tra region of pAW63 and kin, suggesting a conjugative potential for both plasmids.…”