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Journal of Clinical Microbiology, March 2006, p. 872-875, Vol. 44, No. 3
0095-1137/06/$08.00+0     doi:10.1128/JCM.44.3.872-875.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

Sensitivity of Escherichia coli O157 Detection in Bovine Feces Assessed by Broth Enrichment followed by Immunomagnetic Separation and Direct Plating Methodologies

Food Animal Health Research Program, Ohio Agricultural Research and Development Center, The Ohio State University, 1680 Madison Ave., Wooster, Ohio 44691,¹ Department of Veterinary Microbiology and Pathology,² Department of Veterinary Clinical Sciences, College of Veterinary Medicine, Washington State University, Pullman, Washington 99164³

Received 18 January 2005/ Returned for modification 16 June 2005/ Accepted 23 December 2005

In order to more precisely predict food safety risks, the fecal presence of food-borne pathogens among animals at slaughter must be correctly determined. Quantification of Escherichia coli O157 is also desirable. In two separate experiments, detection and enumeration of a nalidixic acid-resistant strain of E. coli O157 in bovine feces was assessed by culture on MacConkey agar supplemented with nalidixic acid (MACnal) and compared to overnight broth enrichment followed by immunomagnetic separation (IMS) and to direct plating of dilutions of bovine feces onto sorbitol MacConkey agar containing cefixime and tellurite (SMACct). The sensitivity of detection of E. coli O157 by both direct plating and IMS was highly dependent upon the initial concentration of the target organism in the sample. Sensitivity of detection by IMS was poor below 100 CFU/g but was better, and not affected by initial E. coli O157 numbers, above this concentration. Sensitivity of detection of E. coli O157 in bovine feces at low initial concentrations is very poor for both direct plating and IMS. Direct plating of dilutions of bovine feces on SMACct can be used to determine the magnitude of fecal E. coli excretion among cattle excreting greater than 100 CFU/g. Among positive samples identified by direct plating on SMACct, the direct counts of E. coli O157:H7 were highly correlated with the estimates obtained with the MACnal plates (r = 0.88, P < 0.001). Because the majority of cattle excrete less than 10² CFU E. coli O157/g feces, most studies, including those using IMS methods, probably grossly underestimate the prevalence of E. coli O157 in cattle.

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