The thermal resistance of Listeria monocytogenes associated with a milkborne outbreak of listeriosis was determined in buffer and whole milk. Thermal resistance was stable over a 2-year period and could not be altered by selecting heat-stressed survivors. The rate of inactivation was linear and did not differ significantly between pH 5.5 and 9.0. When portions of whole milk containing 1 × 105 cells of L. monocytogenes/ml were heated at seven temperatures from 52.2 to 74.4°C, the D-values ranged from 1683.7 to 0.7 s, respectively. The zD-value was 6.3°C. The D-value at 71.7°C was 0.9 s. L. monocytogenes would not survive the pasteurization process.

Morphological, cultural, biochemical, and serological characteristics of 79 strains of Vibrio parahemolyticus isolated from patients suffering from gastroenteric disease in Japan were compared with 17 suspected V. parahemolyticus cultures isolated from wound infections and 14 nonpathogenic vibrios isolated from an estuarine environment in the United States. These groups were differentiated on the basis of several key reactions which included: the range of growth temperature and salt tolerance; the production of catalase and acetoin; the hydrolysis of starch; the fermentation and utilization as single carbon source of sucrose, cellobiose, and arabinose; and the ability to swarm on 1 % agar. The separation of the groups on the basis of cultural and biochemical analyses was confirmed by means of slide agglutinations with specific anti-K antisera. The results of this study strongly suggest that the wound infection isolates are V. parahemolyticus species which are easily distinguished from the nonpathogenic estuarine vibrios. MATERIALS AND METHODS Cultures examined. A total of 110 cultures (Table 1) were used for the present studies. The first group of 79 V. parahemolyticus included 78 strains isolated in Japan from feces of patients suffering from gastroenteritis or from food implicated in food poisoning outbreaks. These strains represented all known K antigens with the exception of K-22 antigen. One strain, OY-G1-3, isolated from Puget Sound, was identified as V. parahemolyticus by specific bacteriophage susceptibility (1). The V. parahemolyticus group was obtained from the following donors:

The facultative halophile Vibrio parahaemolyticus, since its first isolation, has been shown to be toxic for mice and has produced experimental infection when fed to animals and humans. Pathogenicity has been shown to be associated with the presence of a heat-stable hemolysin-the Kanagawa-hemolysin. Previously, investigators reported that most strains of V. parahaemolyticus, regardless of virulence, could produce dilatation in the ligated rabbit gut. We have shown however, that ileal loop reactivity is strongly associated with Kanagawa-hemolysin and, thus, with virulence. Reactivity is obtained with live cell suspensions and the reaction time is dose dependent. Other investigators have reported that preparations of heated cells, cell lysates, culture filtrates, and purified Kanagawa-hemolysin will not produce dilatation in rabbit ileum nor edema in the mouse foot pad. In our studies, cell-free culture filtrates were without effect in the ligated ileal loop, suggesting absence of enterotoxin. Cell wall and heated cell preparations yielded varying ileal reactions. Present data are insufficient to describe the mechanism of virulence of V. parahaemolyticus.

Of 790 samples of oyster shellstock freshly harvested during a 12-month survey, 111 (most of which were harvested from June through August) contained Vibrio cholerae non-Ol (611 strains), and seven contained 01 Inaba (11 strains) organisms. None of the V. cholerae strains isolated were enterotoxigenic by immunological and biological tests.

The thermal resistance of Listeria monocytogenes strain Scott A that had been associated with a recent milkborne outbreak of listeriosis was determined in whole and skim milk, heavy cream, and ice cream mix. L. monocytogenes suspended at concentrations of approximately 1 × 105 cells/ml was heated at temperatures ranging from 52.2 to 79.4°C at various contact times. The D71.7°C values computed for milk samples ranged from 0.9 to 2.7 s. The D7.94°C value in ice cream mix was 0.5 s. The zD value for fluid products ranged from 5.8 to 7.1°C; the zF value for ice cream mix was 7.0°C. The L. monocytogenes suspensions would not survive a proper pasteurization process given to raw dairy products.