Control of E. coli O157:H7 remains a significant challenge at both the pre- and post-harvest food safety levels due to the common presence of this pathogen in the gastrointestinal (GI) tract of cattle. While it is well known that cattle constitute the primary reservoir for E. coli O157:H7, the conditions that facilitate this host-pathogen interaction remain to be fully elucidated. In our previous study (Carlson et al., 2006), we showed that a small subpopulation of feedlot cattle persistently shed E. coli O157:H7 in their feces, as determined by six consecutive fecal samples (collected over 120 days) that tested positive for E. coli O157:H7. Molecular subtyping of E. coli O157:H7 isolates revealed that persistent and transient shedders were colonized by a predominant pulsed field gel electrophoresis (PFGE) type or molecular subtype, which represented > 50% of isolates that were characterized by PFGE typing. In addition, this specific PFGE type represented the most common molecular subtype isolated from the study population over all sample collections. Our preliminary data thus support the hypothesis that E. coli O157:H7 strains may differ in relative likelihood and ability to colonize healthy cattle and thus ultimately their transmissibility throughout the food chain and likelihood to cause human disease. Further work is clearly needed to gain additional insight into the underlying genetic mechanisms associated with differences in E. coli O157:H7 strain-specific virulence characteristics.
Shiga toxin-producing E. coli belonging to serotype O157:H7 was first recognized as an emerging human pathogen in the early 1980s and has since been linked to several multi-state outbreaks of human foodborne illness in the U.S. (Mead et al., 1998). E. coli O157:H7 infections have been associated with significant morbidity and mortality in the U.S. Specifically, E. coli O157:H7 was last estimated to cause > 70,000 cases of foodborne illness of which approximately 4% resulted in a serious disease known as hemolytic-uremic syndrome each year in the U.S. (Mead et al., 1999). Foodborne illnesses attributed to E. coli O157:H7 have been epidemiologically linked to consumption of meat, dairy products, produce, and water contaminated by animal feces along with person-to -person contact and direct contact with farm animals or the farm environment (Mead et al., 1998). Cattle are believed to be the primary reservoir of E. coli O157:H7 and are known to carry this pathogen in their GI tract without showing pathological symptoms (Blanco et al., 1996). Individual animals, however, may either shed the organism in their feces for a few days or become persistently colonized by this organism and as a result shed the organism in their feces for substantially longer periods (Grauke et al., 2002). The nature of E. coli O157:H7 fecal shedding in cattle and the conditions that mediate different host-pathogen interactions remain to be fully elucidated. Inside the host GI tract, intimin (encoded by eaeA) facilitates the ability of E. coli O157:H7 to intimately adhere to host epithelial cells and once bacteria are adherent they produce characteristic attaching and effacing lesions, which result in localized destruction of the brush border microvilli (Garmendia et al., 2005). Recent studies also suggest that the recto-anal junction is an important site for E. coli O157:H7 carriage in the GI tract of cattle and support that colonization of this site may be associated with shedding high levels (> 1x103 CFU/g) of this pathogen (Low et al., 2005; Naylor, 2003). While the mechanism and location of E. coli O157:H7 attachment have been widely studied, there is still a clear need to probe the underlying genetic markers associated with the nature of E. coli O157:H7 colonization of the bovine GI tract and to identify effective controls that interrupt this host-pathogen interaction. It is reasonable to believe that by effectively interrupting E. coli O157:H7 attachment in the bovine intestinal tract at the pre-harvest level, the load of this pathogen that enters the human food chain and ultimately the number of human foodborne illnesses attributed to this pathogen would in-turn be reduced.
The specific aims of this proposed project include (i) develop a greater understanding of the relationship between E. coli O157:H7 genomic diversity at the nucleotide level and distinct virulence phenotypes and (ii) evaluate mechanisms to interrupt E. coli O157:H7 attachment to the gastrointestinal tract, hide, carcass surface, and equipment surfaces.
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