R. M. Twedt

Thermal resistance of Listeria monocytogenes in milk

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.

Thermal Resistance of Listeria monocytogenes in Dairy Products

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.

Thermal Resistance of Disease-Associated Salmonella typhimurium in Milk

The thermal resistance of Salmonella typhimurium cultures that had been associated with a major milkborne oubreak of salmonellosis was determined in raw whole milk. Thirteen patient stool isolates and 24 implicated pasteurized milk isolates at concentrations of 1 × 105/ml were screened for heat resistance at 51.8°C. A representative milk strain was heated in replicate at four temperatures from 51.8 to 68.3°C. The zD value was calculated to be 5.3°C. Mean D-value estimates at 51.8°C were 24.0 and 22.8 min for patient and milk isolates, respectively. Extrapolated D71.7°C values were 0.24 and 0.22 s, and did not differ significantly (α = 0.05). These isolates would not survive proper pasteurization.

Thermal resistance of Listeria spp. in milk

The thermal resistance of one strain each of Listeria ivanovii , L. seeligeri , and L. welshimeri and three L. monocytogenes strains was determined in raw and sterile milk. Listeria spp. suspended in milk at concentrations of 1 × 105 cells/ml were heated at temperatures ranging from 52.2 to 71.1°C for various contact times. The heat resistance of L. monocytogenes appeared somewhat greater than that of the other Listeria spp. in both milks, but the difference was not statistically significant (α = 0.05). High-temperature, short-time processing is adequate for pasteurization of raw milk.

Quantitative comparison of two enrichment methods for isolating Listeria monocytogenes from seafoods

Two enrichment methods that had been used as standard procedures by the U.S. Food and Drug Administration (FDA) and the U.S. Department of Agriculture (USDA) were quantitatively compared for their ability to isolate Listeria monocytogenes from seafoods. Cultures of a clinical sample and a seafood isolate were inoculated into raw and cooked shrimp; cultures heated at 57.8°C for 5 min were added to surimi, cooked crabmeat, and cooked shrimp. With the FDA procedure, which used enrichment intervals of 24 h, 48 h, and 7 d, KOH culture treatment and enrichment for 24 h provided no advantage for Listeria recovery. The FDA procedure isolated heated L. monocytogenes from seafoods at a lower level than the USDA method; however, the two methods isolated unheated cells equally well. The greater selectivity of the USDA procedure may offer an advantage for isolating nonheat-stressed Listeria when the aerobic plate count of the product is high.

Effect of Temperature and Suspending Vehicle on Survival of Vibrio parahaemolyticus and Vibrio vulnificus

Four strains of Vibrio vulnificus and two strains of Vibrio parahaemolyticus from clinical and environmental sources were examined for their ability to survive storage at 4 and -20°C in shrimp homogenate and at -80°C in shrimp homogenate, fetal bovine serum and dimethyl sulfoxide. Cell counts declined with time at 4 and -20°C but they remained stable after freezing at -80°C. Dimethyl sulfoxide was the superior menstruum at -80°C because it protected against freezing lethality.

Potential Public Health Significance of Non-Escherichia coli Coliforms in Food

Several coliform species other than Escherichia coli are often associated with and possibly responsible for acute and chronic diarrheal disease. Recent evidence suggests that non- Escherichia coli coliforms may be capable of colonizing the human intestine and producing enterotoxin(s) in high-yield. Whether these organisms are newly capable of causing disease because of infestation with extrachromosomal factors mediating pathogenicity or simply because of inherent pathogenic capabilities that have gone unrecognized, they pose a potential health hazard. Food, medical, and public health microbiologists should be aware that the non- E. coli coliforms contaminating foods may be potential enteropathogens. This possibility may make determination of their pathogenic capabilities even more important than identification of their taxonomic characteristics.

Effects of purified altertoxins I, II, and III in the metabolic communication V79 system.

Purified Alternaria alternata altertoxins I, II, and III were evaluated for comparative cytotoxicity and ability to inhibit gap junction communication in the Chinese hamster lung metabolic cooperation assay. The noncytotoxic test range for each altertoxin was determined for the metabolic communication assays: altertoxin I, 1, 2, 3, 4, 5 micrograms/ml; altertoxin II, 0.02, 0.008, 0.006, 0.004, 0.002, 0.0008 micrograms/ml; and altertoxin III, 0.2, 0.1, 0.08, 0.06, 0.04 micrograms/ml. Altertoxin II was the most cytoxic in the V79 system, followed by altertoxins III and I. The last cytotoxic of the three, altertoxin I, weakly disrupted metabolic communication at two concentrations (4 and 5 micrograms/ml). Altertoxins III and II did not significantly inhibit gap junction communication more than the weak tumor promoter 4-O-methyl ether tetradecanoylphorbol 13-acetate.

Evidence That Clostridium perfringens Produces Only One Enterotoxin

Thirteen Clostridium perfringens isolates classified as nonenterotoxigenic by radioimmunoassay (RIA) were tested for biological activity in rabbit ileal loops to determine whether these organisms produced enterotoxins serologically unrelated to the classical C. perfringens enterotoxin. None of these strains was active in the ileal loop assays. The large number of RIA-negative isolates obtained from food-poisoning outbreaks is more likely due to the failure to isolate causative strains rather than to the existence of novel enterotoxins.

Evaluation of the Enteropathogenicity of Klebsiella pneumoniae Isolates from Summer-Harvested Louisiana Oysters

Klebsiella pneumoniae isolates were recovered in increased numbers from oysters harvested during the warm months of the year from approved and classified waters. A total of 76 oyster isolates was examined using in vivo and in vitro assays. The nonpathogenic responses of the strains studied suggest that environmental strains are not a public health risk.