Behavior of Listeria monocytogenes when Incubated Together with Pseudomonas Species in Tryptose Broth at 7 and 13°C.

Tryptose broth (TB) was inoculated with Listeria monocytogenes (strain Scott A or California), Pseudomonas aeruginosa , Pseudomonas flourescens , or a combination of L. monocytogenes plus Pseudomonas species, and incubated at 7 or 13°C for 8 weeks. McBride Listeria Agar was used to determine numbers of L. monocytogenes and Pseudomonas Isolation Agar to enumerate Pseudomonas species at 0, 7, 14, 28, 42, or 56 d. At 13°C, presence of P. fluorescens had a slight negative effect on growth of L. monocytogenes strain Scott A, and was somewhat detrimental to its survival during the extended incubation. Growth of L. monocytogenes strain California was retarded by presence of P. fluorescens although the maximum population achieved by the pathogen was greater in the presence rather than absence of the pseudomonad; the pseudomonad did have a negative effect on survival of the pathogen. At the same temperature, P. aeruginosa had a negative effect on survival of L. monocytogenes strain California, but had essentially no effect on the other strain of the pathogen. Neither strain of L. monocytogenes affected growth of P. fluorescens nor P. aeruginosa . At 13°C the pH of TB generally decreased when L. monocytogenes grew by itself but increased when either pseudomonad grew by itself or together with the pathogen. At 7°C, growth of both pseudomonads was minimal. Presence of non-growing cells of P. fluorescens retarded somewhat growth of both L. monocytogenes strains early during the incubation. P. aeruginosa had no detectable effect on either strain of L. monocytogenes . The pH of TB decreased when L. monocytogenes grew by itself or together with either pseudomonad, and remained unchanged in TB inoculated with either pseudomonad.