Island which had been designated as approved or prohibited by the National Shellfish Sanitation Program. Fecal coliforms counts, aerobic plate counts, and Vibrio parahaemolyticus densities were determined for the samples. Mean V. parahaemolyticus density was more than 100 times greater in oysters than in water, whereas density of fecal coliforms was approximately 10 times higher in oysters. Seasonal and geographical distributions of V. parahaemolyticus were related to water temperature, with highest densities in samples collected in the spring and the summer along the Gulf coast. The synthetic DNA probe for thermostable direct hemolysin hybridized with 2 of 50 isolates, 1 of which was positive by the Kanagawa test. Vibrio parahaemolyticus is an enteric pathogen transmitted to humans primarily through consumption of raw or mishandled seafood (2, 23). Like other members of the genus Vibrio, it is a gram-negative, halophilic bacterium that occurs naturally in estuarine environments (15). Its distribution is worldwide, but reported densities of V. parahaemolyticus in the environment and in seafood vary greatly according to season, location, sample type, fecal pollution, and analytical methodology (3, 5, 8, 16, 27). The public health significance of V. parahaemolyticus contamination in seafoods is debatable, since few environmental or seafood isolates produce the thermostable-direct hemolysin (TDH) or Kanagawa hemolysin associated with most (96.5%) pathogenic strains found among clinical isolates (12, 20, 21). However, a TDH-related hemolysin has been isolated from Kanagawa phenomenon-negative V. parahaemolyticus strains associated with gastroenteritis in Southeast Asia (13). In an evaluation (5) of four methods for the enumeration of V. parahaemolyticus in seawater and oysters, we found that the hydrophobic grid membrane filter method described by Entis and Boleszczuk (9) gave the highest density estimates. This method had been used to evaluate the effects of thermal stress on artificially contaminated seafoods but had not been used in environmental surveys. Past V. parahaemolyticus surveys were conducted regionally but did not compare distribution of the organism among the Pacific, Gulf, and Atlantic coasts. The present study reports the incidence of V. parahaemolyticus in U.S. oysters and seawater as determined by the hydrophobic grid membrane filter method. The National Shellfish Sanitation Program classification of shellfish-growing areas is based on fecal coliform densities in the overlying waters (14); however, a statistical evaluation is needed to correlate V. parahaemolyticus densities in shellfish with either fecal coliform or V. parahaemolyticus counts for the overlying waters. In shellfish, V. parahaemolyticus

An improved selective medium, Tryptose-sulfite-cycloserine (TSC) agar, for the enumeration of Clostridium perfringens is described. It consists of the same basal medium as Shahidi-Ferguson-perfringens (SFP) agar, but with 400 p,g of D-cyclOserine per ml substituted for the kanamycin and polymyxin. Tolerance of C. perfringens for D-cycloserine, its production of lecithinase, and its ability to reduce sulfite were used as the basis for development of this medium. Comparisons were made between TSC and SFP agars for the recovery of vegetative cells of C. perfringens by using statistical methods. The results showed that TSC allowed virtually complete recovery of most of the C. perfringens strains while inhibiting practically all facultative anaerobes tested. SFP agar allowed a slightly higher rate of recovery of C. perfringens but was found to be much less selective.

The effect of prior heat shock on thermotolerance of Listeria monocytogenes and Salmonella typhimurium in broth culture was determined. Bacteria were grown at the permissive temperature of 35°C, sublethally heated at 35 (control), 42, 48, and 52°C (nonpermissive control) for various times, and inactivated at either 57.8 or 52°C. The induction of increased thermotolerance by heat shock, although consistent within each experiment, was generally not significant for L. monocytogenes; the increase was significant for S. typhimurium. Temperature shift experiments with L. monocytogenes suggested that induced thermotolerance was not long lived unless the shock temperature was maintained. Exposure to temperatures above the range for normal cell growth (the normal of Arrhenius range) (13) leads to progressive loss of bacterial viability. When bacteria are shifted for a short period from lower to higher temperatures within or slightly above their normal growth range, a degree of protection against the lethal effects of a subsequent shift to a higher temperature or an acquired thermotolerance is achieved (17,20,26).Several investigators (20, 29) concluded that acquired thermotolerance develops during the brief incubation period at elevated but nonlethal temperature as a result of heat shock response, i.e., induction of a specific set of proteins known as heat shock proteins that mediate recovery to stress-induced damage (16,20,26). A direct cause-and-effect relationship between heat shock protein synthesis and acquired thermotolerance, however, is controversial (23, 26).The effect of heat shock response on food safety may be important because certain foods are thermally processed to ensure safety (11). Mackey and Derrick (17, 18) pioneered such studies with Salmonella spp. in broth, liquid whole egg, and reconstituted dried milk and concluded that marginal heat treatments may not be adequate in certain instances. Recent foodborne outbreaks of listeriosis were epidemiologically linked to the consumption of dairy products and meats; hence, the thermal processing parameters of these foods are being reexamined (5,8). This study was undertaken to determine the degree of acquired thermotolerance conferred on Listeria monocytogenes and Salmonella typhimurium by heat shock response and the kinetics and duration of its induction.(Results of this study were presented at the 12th Food Microbiology Research Conference, Chicago, Ill., 8 to 12 November 1989.) L. monocytogenes F5069 was obtained from Robert Weaver, Centers for Disease Control, Atlanta, Ga. This strain belongs to serotype 4b and was isolated from raw bovine milk (3, 4). S. typhimurium 42ScBs was obtained from J. G. Bradshaw, Food and Drug Administration, Cincinnati, Ohio. The strain was isolated from 2% lowfat milk, epidemiologically linked to the 1983 Chicago milk outbreak, and carries the outbreak strain plasmid profile (22).

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.

were conducted on 10 types of fresh produce from two Minneapolis area supermarkets to detect Listeria spp. The produce included broccoli, cabbage, carrots, cauliflower, cucumbers, lettuce, mushrooms, potatoes, radishes, and tomatoes. The vegetables were tested by the Food and Drug Administration method for isolation of Listeria spp., with the addition of LiClphenylethanol-moxalactam agar in the last 280 tests; 8.6 and 11.4% of these tests were positive by modified McBride and LiCl-phenylethanol-moxalactam agars, respectively. Listeria monocytogenes was isolated from cabbage, cucumbers, potatoes, and radishes; L. innocua was isolated from cucumbers, lettuce, mushrooms, potatoes, and radishes; L. seeligeri was isolated from cabbage and radishes; and L. welshimeri was isolated from cucumbers, potatoes, and radishes. The isolates were of various serotypes; however, the L. monocytogenes isolates were predominantly serotype 1 (82%). Only potatoes (25.8% positive) and radishes (30.3% positive) showed significant amounts of L. monocytogenes contamination.