Rapid enrichment strategy for isolation of Listeria from bovine hide, carcass, and meat samples

Since the outbreak of foodborne illness linked to Escherichia coli O157:H7 bacteria in ground beef in the early 1980s, the beef processing industry has focused on increasing the safety of beef products by implementing procedures for surveying live cattle, carcasses, and beef products for bacterial pathogens. Effective methods are in place for screening cattle and beef products for the presence of E. coli O157:H7 contamination, and recent work has established the acceptability of these methods for surveillance of Salmonella. In keeping with the need to continually improve the food safety of beef products, new work investigating pathogen prevalence now includes surveillance for Listeria monocytogenes. Tryptic soy broth (TSB) has been documented as a robust nonselective medium for the enrichment of both E. coli and Salmonella from bovine hide, carcass, and meat samples. The University of Vermont modification medium is most often used as the primary enrichment medium for surveillance of Listeria spp. In this study, samples from bovine hides (n=50), preevisceration carcasses (n=50), and beef trim (n=193) were used to evaluate TSB as a primary enrichment medium for the isolation of Listeria spp., including L. monocytogenes. No significant difference (P > 0.05) between TSB and the University of Vermont modification medium was observed when all three sample types underwent primary enrichment for the isolation of Listeria spp. Furthermore, the standard secondary enrichment ratio for Fraser broth used for Listeria recovery can be modified to accommodate a high-throughput method for processing multiple samples.