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).

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.

Nuts have been identified as a vector for salmonellosis. The objective of this project was to estimate the prevalence and contamination level of Salmonella in raw tree nuts (cashews, pecans, hazelnuts, macadamia nuts, pine nuts, and walnuts) at retail markets in the United States. A total of 3,656 samples of six types of tree nuts were collected from different types of retail stores and markets nationwide between October 2014 and October 2015. These samples were analyzed using a modified version of the Salmonella culture method from the U.S. Food and Drug Administration's Bacteriological Analytical Manual. Of the 3,656 samples collected and tested, 32 were culturally confirmed as containing Salmonella. These isolates represented 25 serotypes. Salmonella was not detected in pecans and in-shell hazelnuts. Salmonella prevalence estimates (and 95% confidence intervals) in cashews, shelled hazelnuts, pine nuts, walnuts, and macadamia nuts were 0.55% [0.15, 1.40], 0.35% [0.04, 1.20], 0.48% [0.10, 1.40], 1.20% [0.53, 2.40], and 4.20% [2.40, 6.90], respectively. The rates of Salmonella isolation from major or big chain supermarkets, small chain supermarkets, discount, variety, or drug stores, and online were 0.64% [0.38, 1.00], 1.60% [0.80, 2.90], 0.00% [0.00, 2.40], and 13.64% [2.90, 35.00], respectively (Cochran-Mantel-Haenszel test: P = 0.02). The rates of Salmonella isolation for conventional and organic nuts were not significantly different. Of the samples containing Salmonella, 60.7% had levels less than 0.003 most probable number (MPN)/g. The highest contamination level observed was 0.092 MPN/g. The prevalence and levels of Salmonella in these tree nut samples were comparable to those previously reported for similar foods.

Flow cytometry is a potentially powerful tool for analyzing the interactions of facultative intracellular bacteria and macrophages on a cellular level, particularly when fluorochromes are used to label the bacteria. We labeled Listeria monocytogenes and Salmonella typhimurium with a lipophilic dye, PKH-2, and used flow cytometry to investigate phagocytosis by J774A.1 cells and short-term bacterial survival. Labeled and unlabeled bacteria were identical in terms of viability, growth kinetics, and survival within macrophages, although recovery per macrophage was much greater for L. monocytogenes than for S. typhimurium. Using L. monocytogenes as a prototypical facultative intracellular bacterium, we estimated bacterial survival during phagocytosis on the basis of linear fluorescence measurements of infected J774A.1 cells and recovery of L. monocytogenes from sorted cells. The lower percentage of surviving L. monocytogenes in macrophages containing higher bacterial loads indicated the accumulation of nonviable bacteria within phagocytes. Removal of the external source of viable bacteria by washes and gentamicin treatment reduced the percentage of surviving intracellular L. monocytogenes to a baseline level, and all baseline levels were similar, regardless of bacterial load. Listeria enrichment recoveries, derived from individually sorted J774A.1 cells, demonstrated the heterogeneity of macrophages in intracellular bacterial survival, especially within heavily infected cells. These results indicated that survival of L. monocytogenes was dependent on the adaptations of a small fraction of bacteria within a population of macrophages which permit intracellular growth.