Diagnostic challenges within the Bacillus cereus-group: finding the beast without teeth

The Bacillus cereus group (B. cereus sensu lato) includes species such as Bacillus cereus sensu stricto, which are common, usually avirulent, contaminants of patient samples in routine microbiological diagnostics, as well as Bacillus anthracis which are highly virulent bacteria. Here we describe 16 clinical cases with isolates initially identified as Bacillus cereus-group using a MALDI-TOF MS standard database. Whole genome sequencing (WGS) data identified five isolates as B. anthracis species without carrying the typical virulence plasmids pXO1 and pXO2, four isolates B. paranthracis, three B. cereus sensu stricto, two B. thuringiensis, one B. mobilis, and one isolate represents a previously undefined species of Bacillus (B. basiliense sp. nov.). More detailed analysis using alternative MALDI-TOF MS databases, phenotype and diagnostic PCRs, gave further conflicting species results. These cases highlighted the difficulties in identifying avirulent B. anthracis within the B. cereus group using standard methods. WGS and alternative MALDI-TOF MS databases offers more accurate species identification, but is so far not routinely applied. We discuss the diagnostic resolution and discrepancies of various identification methods.