Survival and Prevalence of Non-O157 Shiga Toxin Producing Escherichia coli (STEC) in Beef Cattle Preharvest and in the Environment

Abstract

Cattle are primary reservoirs of shiga toxin producing Escherichia coli (STEC) and are considered “super-shedders” if they shed more than 104 CFU/g of E. coli O157:H7. In addition to E. coli O157:H7, six non-O157 STEC serogroups “big six” were added to the zero-tolerance adulterant list in June 2012. Limited information is available on the prevalence of non-O157 serogroups in beef cattle, especially during the pre-harvest stages. Alabama has a large number of cow/calf operations. Most calves are birthed, weaned, and shipped from Alabama to commercial feedlots in the Midwest. Thus, the first purpose of this study was to determine the prevalence of non-O157 STEC among the beef cattle in Alabama. This was accomplished by acquiring fecal samples from cattle at 6 different Alabama Agricultural Experiment Station research centers and testing them for STEC. The second purpose was to identify shedding level changes in calves before and after weaning. For this study, 110 fecal samples were taken from calves located at E.V. Smith research center, both before and after weaning. These samples were tested for the presence of STEC. The third purpose was to isolate and serotype the STEC strains from “super-shedder” calves. The same E. V. Smith calf fecal samples as were used for purpose 2 were also used for purpose 3. Those fecal samples exhibiting a “super-shedder” load of STEC had their dominant STEC isolated and sent for serotyping. Based on findings, O26 proved to be the dominant “big six” member in both the research center and the calf study. STEC O26 isolates also displayed different virulence profiles in a multiplex PCR comparison, targeting three major virulence genes. For these reasons, the fourth purpose was to investigate how O26’s possession of different virulence profiles affected its survival in ground beef and different environmental samples. Overnight cultures of O26 strains containing two different virulence gene profiles were inoculated at various concentrations into separate samples of ground beef, cattle feces, bedding material, and trough water. STEC levels were observed over a 10 day period for the ground beef samples and a 30 day period for the environmental materials. Results from these studies show that there were a greater number of positive STEC shedders in research centers sampled during the summer months and in herds containing calves. “Big six” STEC were isolated from the high shedders at 3 out of 6 research centers. STEC O103 was isolated from one research center and O26 was isolated from two others. The calf study shows that calves begin shedding STEC early in their lives and that weaning does generate an impact on STEC shedding levels. Nine out of 110 calves were found to be “super-shedders” both before and after weaning. “Big six” member O26 was also isolated from these “super-shedder” samples before and after weaning. Results show that virulence genes do not have an impact on STEC’s ability to survive in ground beef and in the environment (P > 0.05), while the inoculation levels and sample types do (P < 0.05). Further research should be conducted on control methods pertaining to the pre-harvest stage of cattle production. From this study’s results, the gut microflora appear to be easily influenced since the dominant STEC before weaning was not the same as after. Also, more emphasis should be placed on studying and controlling STEC O26, as it was the dominant “big six” member in these studies.