Characterization and taxonomic clarification of a non-emetic, radiation-resistant Bacillus cereus strain mrbd isolated from a 60Co irradiator pool

Abstract

This study isolated a radiation-resistant Bacillus species from the pool water of a ⁶⁰Co gamma irradiator. The bacterium was motile, a spore-former, rod-shaped, Gram-positive, and capable of withstanding 5 kGy of gamma radiation. Based on 16S rRNA gene sequencing, MLST, and gyrB gene phylogeny, the isolate was classified within the B. cereus group. Comparative whole-genome analysis demonstrated a high level nucleotide identity with B. cereus AH820 (99.49 %), B. anthracis (98.59-98.81 %), and B. cereus type strain ATCC 14579 (91.70 %). Despite the chromosomal similarity to B. cereus AH820, the isolate lacked the emetic toxin-producing plasmid (PER_AH820) and phenotypic traits of B. anthracis, such as enterotoxin production and penicillin resistance. In silico DNA-DNA hybridization (DDH) assays showed 88.4 % genome similarity to B. anthracis, while showing a lower similarity (44.7 %) to B. cereus. Detection of non-B. anthracis ancestral "G" allele in the plcR gene further excluded its identification as B. anthracis. Whole-genome phylogenetic analysis clustered the isolate with B. cereus strains previously identified from the International Space Station (ISS) and pathogenic variants of B. cereus. The isolate was designated as B. cereus strain mrbd.

Keywords: B. anthracis; B. cereus mrbd; Radiation-resistant; Taxonomy.