Antimicrobial resistance profiling and phenotypic characterization of archived clinical Bacillus paranthracis strains

Bacillus paranthracis was formally defined as a species in 2017, after decades of carrying the name "emetic Bacillus cereus" based on cereulide production and clustering within the B. cereus sensu lato phylogenetic group III. Commonly associated with foodborne intoxication, reports rarely link B. paranthracis to non-foodborne clinical illness. As such, the new taxonomy and close resemblance of the name to the biothreat pathogen Bacillus anthracis cause confusion in diagnostic and public health settings. To address this issue, B. paranthracis clinical strains (n = 20) from the CDC collection were tested with microbiological methods used for the identification of B. anthracis and antimicrobial susceptibility testing. Some B. paranthracis phenotypes were similar to B. anthracis; however, others were inconsistent across strains. Like B. anthracis, three strains tested capsule positive, five were non-hemolytic on blood agar, and nine were non-motile. All B. paranthracis strains were resistant to gamma phage lysis, which differentiated them from B. anthracis. Treatment regimens for B. paranthracis infections are not well established, as antimicrobial therapy is not indicated for emetic intoxication caused by B. paranthracis. Notably, six B. paranthracis strains had elevated minimal inhibitory concentrations to anthrax-recommended antibiotics: one for ciprofloxacin, three for doxycycline and tetracycline, and two for clindamycin. Rapid MinION sequencing was assessed for antimicrobial resistance detection and prediction but had limited value when using PiMA v.1. These microbiological observations and susceptibility profiles of B. paranthracis expand our understanding of this pathogen, strengthening our ability to differentiate this bacterium from B. anthracis to improve diagnosis and patient outcomes.IMPORTANCEThis study describes the in vitro characterization of 20 archived clinical strains of Bacillus paranthracis, an opportunistic pathogen identified more frequently in recent reports. Our findings highlight phenotypic differences and similarities between B. paranthracis and Bacillus anthracis using standard microbiological methods and drug susceptibility profiling. We also assess a rapid B. anthracis-specific MinION long-read genome sequencing workflow with B. paranthracis. This report highlights the overlapping morphological features shared by B. paranthracis and B. anthracis to improve future laboratory diagnosis and strengthen anthrax preparedness. This article will effectively reach an audience of public health professionals and microbiologists, strengthening anthrax preparedness.