RELIABILITY OF ESCHERICHIA-COLI COUNTS FOR VACUUM-PACKAGED GROUND-BEEF

Test strains of Escherichia coli were inoculated into fresh ground beef that had been irradiated or carefully excised and aseptically ground. Samples were vacuum-packaged and stored at 4-degrees-C. Plate counts on selective media incubated at 35 or 45-degrees-C were highly consistent during the 7- to 20-d storage periods. The standard most probable number (MPN) technique (lauryl tryptose broth at 35-degrees-C, followed by EC broth at 45-degrees-C) was also reliable. In contrast, direct inoculation into broths incubated at 45-degrees-C gave unreliable and highly variable results. The cause of the variability of the MPN counts at 45-degrees-C could not be determined. It was not due to lactic acid bacteria growing in the ground beef. E. coli in refrigerated, vacuum-packaged ground beef can be reliably detected by direct inoculation of selective plating media incubated at 45-degrees-C. Direct inoculation of selective broth media for the MPN technique at 45-degrees-C is not recommended.