Molecular and structural characterization of a Bacillus cereus strain producing an anthrax-like capsule

Bacillus cereus is a ubiquitous Gram-positive, spore-forming, rod-shaped saprophytic bacterium, occasionally reported to cause food-borne illnesses. However, instances of B. cereus strains harboring anthrax toxin and capsule genes have elevated certain strains as formidable pathogens and biothreats. This study focuses on the genomic analysis and the structural characterization of capsular material produced by the virulent B. cereus PATH2418 strain, isolated from the wound of a traumatic open fracture patient. The genome was sequenced using Nanopore MinION sequencing, revealing a chromosome of 5,270,283 bp and three plasmids. One plasmid, pATH1, was found to encode an operon for the biosynthesis of a bacterial capsule. This operon had sequence homology to the Bacillus anthracis capBCADE operon, which encodes the poly-γ-D-glutamate (PDGA) capsule. The capsule production in B. cereus PATH2418 was influenced by temperature and CO₂ levels. Structural analysis of the capsular material using a combined approach of nuclear magnetic resonance (NMR) and high-performance liquid chromatography (HPLC) techniques confirmed the presence of a high-molecular-weight poly-γ-glutamate capsule, with an enantiomeric composition of approximately 67% D-glutamic acid and 33% L-glutamic acid, matching that of B. anthracis.

This research provides genomic characterization and phylogenetic analysis of a Bacillus cereus strain producing a peptide capsule. Bacteriological techniques confirmed the conditions that induce the production of this capsule. Isolation of the capsule showed that it is high-molecular-weight and proteinase-resistant. The molecular structure of the capsule was characterized using spectroscopic and enantiomeric methods. The results revealed the absence of toxin genes; however, the presence of a poly-γ-glutamic acid (PGA) capsule with a high concentration of D-glutamic acid, akin to the poly-γ-D-glutamic acid (PDGA) capsule produced by B. anthracis, indicates that this strain can be a surrogate for studies of the anthrax PDGA capsule. The findings also contribute to the broader knowledge of B. cereus as a potential biothreat, demanding the development of effective diagnostic, prevention, and treatment measures.