Survival and proliferation of verotoxin-producing Escherichia coli in cattle.

摘要

The increase in number of outbreaks of foodborne illnesses has prompted extensive research to focus on prevalence, physiology, and toxicity of foodborne pathogens. Escherichia coli O157:H7 was first recognized as a human foodborne pathogen in 1982 following two major outbreaks of hemorrhagic colitis in Oregon and Michigan, U.S. Ruminants are considered reservoirs of the pathogen. In addition to E. coli O157:H7, there are approximately 60 other serotypes of verotoxin-producing E. coli (VTEC) that have been associated with human illnesses. To better understand the prevalence of VTEC in cattle, three one-year surveys were conducted on three cattle herds (two beef and one dairy) in Nevada. From dairy heifers grazing an irrigated pasture, O26 was isolated during winter. Several VTEC serotypes were isolated from beef heifers grazing an irrigated pasture. These were E. coli O6 (in the spring), O39 (in the fall), and both O157:H7 and O113 (in the winter). The third herd (beef heifers grazing native rangelands) revealed the presence of E. coli O118 and O138. A flow cytometric technique was developed to rapidly detect and quantify fluorescently-labeled E. coli O157:H7. This technique was used to assess ruminal survival and proliferation of E. coli O157:H7 (a VTEC model) as effected by pH. Eight pH treatments (5.5, 5.75, 6.0, 6.25, 6.5 6.75, 7.0, and 7.25) representing potential ruminal pH values due to feeding different diets (ranging from high concentrate to high forage) were examined in an artificial rumen (i.e., the dual-flow continuous culture fermenter system). E. coli O157:H7 was removed from the rumen environment in a quadratic fashion within 24 h after inoculation. It was concluded, therefore, that survival and proliferation of E. coli O157:H7 is modulated postruminally.