Escherichia coliO157; prevalence, survival, and stress responses during prolonged heat and cold shocks

Abstract

Escherichia coli O157 is a food borne pathogen of increasing public health concern worldwide. Cattle have been implicated as the primary reservoir of E. coli O157. The fact that the livestock industry has rapidly expanded in Saskatchewan makes it imperative to have a clear scientific understanding of the prevalence of E. coli O157 in this province as well as its survival in soil under ambient conditions.Longitudinal and point studies were employed to determine the prevalence of E. coli O157 among Saskatchewan’s cattle. During a 2-year period, 23 feedlot and cattle operations were examined and an overall prevalence of 15.6% was reported. The most important finding was that the prevalence rates were highly dependent on cattle density. All E. coli O157 isolates obtained from this study were characterized by using multiplex PCR, RAPD fingerprinting, a Vero cell cytotoxicity assay and antibiotic susceptibility tests. This characterization revealed a surprisingly highly virulent and heterogenous population of E. coli O157 isolates. Subsequently, the survival characteristics of E. coli O157:H7 ATCC 43894 in sterile soil and manure-amended soil microcosms, as well as in situ under ambient environmental conditions were examined. Findings from this work indicated that desiccation had the most lethal effect on cell viability, whereas nutritionally-rich soils significantly increased survival times of the pathogen population. A final study was designed to examine the survival strategy of hyper- and hypothermally adapted E. coli O157 cells exposed to high and low temperatures, with specific focus on the role of RpoS. Using wild type and its rpoS null allele E. coli O157 strains, in combination with 2D PAGE, It was found that both heat and cold post-acclimation stimulons consisted of two large sub-groups: (i) stress proteins, and (ii) housekeeping proteins. Comparative proteomic analyses revealed that the GroEL/S chaperonin complex and Pnp ribonuclease played a crucial role in growth resumption during high and low temperatures, respectively. Notably, RpoS had no control over key stress proteins in either stress stimulon. RpoS, however, showed a significantly more pronounced role during cold temperatures, where it was seen to regulate key proteins involved in homoeoviscous adaptation as well as various housekeeping proteins of both stress stimulons.