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°C. Plate counts on selective media incubated at 35 or 45°C were highly consistent during the 7- to 20-d storage periods. The standard most probable number (MPN) technique (lauryl tryptose broth at 35°C, followed by EC broth at 45°C) was also reliable. In contrast, direct inoculation into broths incubated at 45°C gave unreliable and highly variable results. The cause of the variability of the MPN counts at 45°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 °C. Direct inoculation of selective broth media for the MPN technique at 45°C is not recommended.