Stepping stones towards a consensus Bacillus cereus group taxonomy in the age of whole genome sequencing

Delimiting microbial species boundaries remains an important challenge and becomes urgent when unresolved taxonomy threatens public health and food security. An example of this is the taxonomic situation for the Bacillus cereus group. These closely related Gram-positive spore-forming bacteria include the opportunistic pathogen B. cereus (sensu stricto), frequently implicated in food poisoning and food spoilage worldwide, Bacillus anthracis, the cause of the lethal disease anthrax, and Bacillus thuringiensis, an insect pathogen which is the world’s most widely used biological pesticide. These three species are largely defined by their toxicity phenotypes. Over the last decade, however, 20 additional species, and even up to 57 genomospecies, have been proposed in the group based on numerous genetic analyses, leaving the B. cereus group taxonomy in collapse. B. cereus group bacteria are thus at the centre of a taxonomic conflict, as the different “species” are hard to identify based on genetic criteria and, at the same time, the main distinguishing phenotypic (toxicity) features are conferred by mobile genetic elements (MGEs). We have started a collaborative effort to constitute a research network that addresses the issues of B. cereus group taxonomy, accommodating the phylogenetic approach and reconciling the contribution of MGEs, while still retaining some of the clinical, environmental, and socioeconomic aspects associated with the “classical” species. Therefore, this round table aims at addressing the inherent taxonomical challenges of this bacterial group in the whole-genome sequencing era. This collaborative effort by the “B. cereus group community” will not only provide guidance to have, in the near future, a taxonomic work frame for the classification of B. cereus group strains, but also will have a clear impact in clinical microbiology, the food and biopesticides industries, and in the legal framework used by different stakeholders and organizations, by assuring correct identification and safe utilization of these species.