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).

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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 intracellular and freely suspended Listeria monocytogenes that was associated with a milkborne outbreak of listeriosis was studied by using the sealed tube and slug flow heat exchanger methods. Test temperatures for the former method were 57.

Land disposal of sewage sludge and effluent is becoming a common practice in the United States. The fertilizer content and humus value of such wastes are useful for agricultural purposes, and the recycling of sewage onto the land eliminates many of our stream pollution problems. The potential exists for crops grown in such irrigated soil to be contaminated by viruses that may be present in the sewage. Studies were initiated to determine viral persistence in soil and on crops grown under natural conditions in field plots that had been flooded to a depth of 1 inch (2.54 cm) with poliovirus 1-inoculated sewage wastes. Lettuce and radishes were planted in sludgeor effluent-flooded soil. In one study, the vegetables were planted 1 day before flooding, and in another they were planted 3 days after the plots were flooded. Survival of poliovirus 1 in soil irrigated with inoculated sewage sludge and effluent was determined during two summer growing seasons and one winter period. The longest period of survival was during the winter, when virus was detected after 96 days. During the summer, the longest survival period was 11 days. Poliovirus 1 was recovered from the mature vegetables 23 days after flooding of the plots had ceased. Lettuce and radishes are usually harvested 3 to 4 weeks after planting.