A putative microcin amplifies Shiga toxin 2a production of Escherichia coli O157: H7

Escherichia coli O157:H7 is a foodborne pathogen, implicated in various multi-state outbreaks. It encodes Shiga toxin on a prophage, and Shiga toxin production is linked to phage induction. An E. coli strain, designated 0.1229, was identified that amplified Stx2a production when co-cultured with E. coli O157:H7 strain PA2. Growth of PA2 in 0.1229 cell-free supernatants had a similar effect, even when supernatants were heated to 100°C for 10 min, but not after treatment with Proteinase K. The secreted molecule was shown to use TolC for export and the TonB system for import. The genes sufficient for production of this molecule were localized to a 5.2 kb region of a 12.8 kb plasmid. This region was annotated, identifying hypothetical proteins, a predicted ABC transporter, and a cupin superfamily protein. These genes were identified and shown to be functional in two other E. coli strains, and bioinformatic analyses identified related gene clusters in similar and distinct bacterial species. These data collectively suggest E. coli 0.1229 and other E. coli produce a microcin that induces the SOS response in target bacteria. Besides adding to the limited number of microcins known to be produced by E. coli, this study provides an additional mechanism by which stx2a expression is increased in response to the gut microflora. Importance How the gut microflora influences the progression of bacterial infections is only beginning to be understood. Antibiotics are counter-indicated for E. coli O157:H7 infections, and therefore treatment options are limited. An increased understanding of how the gut microflora directs O157:H7 virulence gene expression may lead to additional treatment options. This work identified E. coli that enhance the production of Shiga toxin by O157:H7, through the secretion of a proposed microcin. This work demonstrates another mechanism by which non-O157 E. coli strains may increase Shiga toxin production, and adds to our understanding of microcins, a group of antimicrobials that are less well understood than colicins.