SNPs in outbreak Escherichia coli O157:H7 isolates

An outbreak of E. coli O157:H7 infections occurred in the greater St. Louis area in late 2011 (Slayton, RB, et al, PLoS One. 2013;8(2):e55300. doi: 10.1371/journal.pone.0055300. Epub 2013 Feb 4). There was some discordance between reported exposure to the incriminated source (salad bars at a supermarket chain) and pulsed-field gel electrophoresis (PFGE) and multiple locus variable number of tandem repeat analysis (MLVA) patterns. We genome sequenced the infecting isolates from cases who claimed, denied, or were unsure about their exposure to the salad bars. Among several cases who denied exposure, we identified one or 2 single nucleotide polymorphisms (SNPs) that distinguished the case isolates from those of others in this outbreak. Also, we determined that a strain from a case who claimed exposure, but whose pulse field gel pattern was variant, did, in fact, belong to this cluster.