Improvements, challenges, and validation of enumeration of Escherichia coli O157 and sampling methods for Escherichia coli O157 and Salmonella in feedlot cattle

Escherichia coli O157 and Salmonella have been targeted as food-borne pathogens of concern to the beef industry. Enumeration of and proper detection protocols for E. coli O157 and Salmonella has become an important part of pre-harvest surveillance and testing of intervention strategies. The objectives of this research were: (1) to improve and validate a Most Probable Number (MPN) / Immunomagnetic Separation (IMS) method to enumerate E. coli O157 in fecal samples from feedlot cattle; and (2) to determine the distribution of E. coli O157 and Salmonella on cattle hides (back, flank, hock, neck, perineum, and ventrum), the oral cavity, rectal-anal junction, and feces of feedlot cattle. Known quantities of E. coli O157:H7 were inoculated into feces and populations were determined by direct plating and compared to a MPN/IMS method. The MPN/IMS methodology compared to direct plating was tested in five experiments with varying E. coli O157 concentrations and it was found that there were differences (P < 0.01) between the two methodologies with negligible biological importance due to a low difference between least squares means (LS-means) (0.29 ± 0.01). The overall correlation coefficient was high (r = 0.93) which indicated that the MPN/IMS method was precise when compared to the direct plating method. Nine samples were obtained from feedlot cattle from the above mentioned locations (objective 2) and subjected to E. coli O157 and Salmonella detection protocols. Several measurements were taken: (1) prevalence of both pathogens at each location; (2) odds ratios comparing the fecal grab samples to all other locations; and (3) odds ratios comparing the perineum hide swab samples to all other hide swab samples. These data suggest that using multiple sample locations are useful when isolating these pathogens from feedlot cattle.