GROWTH OF MIXED CULTURES OF LISTERIA-MONOCYTOGENES AND LISTERIA-INNOCUA IN BLUE-CRAB (CALLINECTES-SAPIDUS) MEAT

In most instances, when Listeria monocytogenes has been isolated in blue crab (Callinectes sapidus) processing plants, Listeria innocua has also been detected. This has led to the belief among blue crab processors that there is some association in the growth of these two Listeria spp., suggesting suppression of L. monocytogenes by L. innocua. In this study the growth of a L. monocytogenes strain (serotype 4b) and of a L. innocua strain, both isolated from crabmeat, was observed by inoculating pasteurized crab meat with different proportions of standardized inocula of the two microorganisms separately and together. Parallel samples inoculated with one of the microorganisms, or with a mixture of both in different proportions were incubated at two different temperatures (4 degrees and 36 degrees C) for up to 15 days. Mixture ratios of L. monocytogenes to L. innocua of 1:1, 9:1, and 1:9 were used to inoculate the crab meat. At each growth interval (0, 24, 36, 48, 72, and 96 h and 7, 10, and 15 days), 25-g samples were enumerated for each microorganism. Blood agar, with colistin sulfate (10 mg/l) and moxalactam (20 mg/l) added, was used for enumerating and differentiating between the two species. No significant differences (P > 0.05) in the growth of either microorganism separately or when in the presence of the other, at any inoculation level and temperature, were observed.