Virulence and recovery of stressed Escherichia coli O157:H7

Escherichia coli O157:H7, a causative agent of worldwide foodborne and waterborne outbreaks, results in potentially life threatening illnesses. An incomplete understanding of the relationship between virulence and survival mechanisms of E. coli O157:H7 hampers the design of effective control systems in food processing and patient therapy. Experiments using starvation conditions and exposure to hypochlorite stresses in conjunction with in vitro and in vivo assays were conducted to understand the effects of stress on recovery and virulence of E. coli O157:H7. Recovery of nonculturable E. coli O157:H7 was tested using microbiological methods targeted at enrichment and recovery of stressed cells; however, starvation and hypochlorite induced nonculturable cells were not detectable using these methods. Nonculturable E. coli O157:H7 were also intrcxluced into an in vivo system to determine if in vivo factors contribute E. coli O157:H7 recovery; moreover, virulence due to Shiga toxin production was also examined. Results suggest that starvation and hypochlorite-induced noncultutable E. coli O157:H7 are not able to recover in vivo and do not retain virulence. Studies were designed to address effects of sublethal stresses on E. coli O157:H7 virulence using attachment and Vero-cytotoxicity assays; in addition, the relationship between recovery and quorum sensing (using conditioned media) was also examined. Results show that conditioned media promoted recovery and growth of stressed cells; moreover, stressed E. coli O157:H7 undergoing recovery and growth in the presence of conditioned media augmented Vero-cytotoxicity and attachment compared recovery and growth in nonconditioned media. Conditioned media from recovering cell suspensions was tested for the presence of the interspecies communicating molecule, autoinducer-2. Indeed, AI-2 was detected in conditioned media from recovering bacteria; however, results do not conclusively demonstrate the involvement of AI-2 in recovery of stressed E. coli O157:H7. Results of the studies presented here in conjunction with the occurrence of E. coli O157:H7 waterborne outbreaks suggest that stressed cells recover in vivo due to chemical signals secreted by normal flora; moreover, exposure to secreted signals from conditioned media enhances Shiga toxin production and attachment of bacteria to intestinal cells.