Abstract
The Bacillus cereus group comprises multiple species capable of causing emetic or diarrheal foodborne illness. Despite being responsible for tens of thousands of illnesses each year in the U.S. alone, whole-genome sequencing (WGS) is not yet routinely employed to characterize B. cereus group isolates from foodborne outbreaks. Here, we describe the first WGS-based characterization of isolates linked to an outbreak caused by members of the B. cereus group. In conjunction with a 2016 outbreak traced to a supplier of refried beans served by a fast food restaurant chain in upstate New York, a total of 33 B. cereus group isolates were obtained from human cases (n = 7) and food samples (n = 26). Emetic (n = 30) and diarrheal (n = 3) isolates were most closely related to B. paranthracis (group III) and B. cereus sensu stricto (group IV), respectively. WGS indicated that the 30 emetic isolates (24 and 6 from food and humans, respectively) were closely related and formed a well-supported clade distinct from publicly available emetic group III genomes with an identical sequence type (ST 26). The 30 emetic group III isolates from this outbreak differed from each other by a mean of 8.3 to 11.9 core single nucleotide polymorphisms (SNPs), while differing from publicly available emetic group III ST 26 B. cereus group genomes by a mean of 301.7–528.0 core SNPs, depending on the SNP calling methodology used. Using a WST-1 cell proliferation assay, the strains isolated from this outbreak had only mild detrimental effects on HeLa cell metabolic activity compared to reference diarrheal strain B. cereus ATCC 14579. We hypothesize that the outbreak was a single source outbreak caused by emetic group III B. cereus belonging to the B. paranthracis species, although food samples were not tested for presence of the emetic toxin cereulide. In addition to showcasing how WGS can be used to characterize B. cereus group strains linked to a foodborne outbreak, we also discuss potential microbiological and epidemiological challenges presented by B. cereus group outbreaks, and we offer recommendations for analyzing WGS data from the isolates associated with them.
Highlights
The Bacillus cereus (B. cereus) group, known as B. cereus sensu lato (s.l.) is a complex of closely related species that vary in their ability to cause disease in humans
While B. cereus causes a considerable number of foodborne illness cases annually, outbreaks are rarely investigated with the methodological rigor that is increasingly used for surveillance and outbreak investigations targeting other foodborne pathogens
A specific challenge in the U.S is that, unlike for some other diseases, disease cases caused by B. cereus are typically not reportable, even though foodborne illnesses, regardless of etiology, are reportable in some states, including NY
Summary
The Bacillus cereus (B. cereus) group, known as B. cereus sensu lato (s.l.) is a complex of closely related species that vary in their ability to cause disease in humans. Foodborne illness caused by members of the group primarily manifests itself in one of two forms: (i) emetic intoxication that is caused by cereulide, a heat-stable toxin produced by B. cereus within a food matrix prior to consumption, or (ii) a diarrheal toxicoinfection, caused by enterotoxins produced by bacteria in the small intestine of the host (Ehling-Schulz et al, 2004; Schoeni and Wong, 2005; Stenfors Arnesen et al, 2008). B. cereus group isolates that have been linked to human clinical cases of foodborne disease rarely undergo whole-genome sequencing (WGS), as is becoming the norm for other foodborne pathogens (Joensen et al, 2014; Ashton et al, 2015; Moura et al, 2017)
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