Listeria monocytogenes F5069 was suspended in either Trypticase soy broth-.6% yeast extract (TSBYE) or sterile, whole milk and heated at 62.8°C in sealed thermal death time tubes. Severely heat-injured cells were recovered in TSBYE within sealed thermal death time tubes because of the formation of reduced conditions in the depths of the TSBYE. Also, the use of strictly anaerobic Hungate techniques significantly increased recovery in TSBYE containing 1.5% agar compared with aerobically incubated controls. The exogenous addition of catalase, but not superoxide dismutase, slightly increased the recovery of heat-injured cells in TSBYE containing 1.5% agar incubated aerobically. Growth of cells at 43°C caused a greater increase in heat resistance as compared with cells heat shocked at 43°C or cells grown at lower temperatures. Growth of L. monocytogenes at 43°C and enumeration by the use of strictly anaerobic Hungate techniques resulted in D62.8°C values that were at least sixfold greater than those previously obtained by using cells grown at 37°C and aerobic plating. Results indicate that, under the conditions of the present study, high levels of L. monocytogenes would survive the minimum low-temperature, long-time treatment required by the U.S. Food and Drug Administration for pasteurizing milk. The possible survival of low levels of L. monocytogenes during high-temperature, short-time pasteurization and enumeration of injured cells by recovery on selective media under strictly anaerobic conditions are discussed.

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High pH has been shown to rapidly destroy gram-negative food-borne pathogens; however, the mechanism of destruction has not yet been elucidated. Escherichia coli 0157:H7, Salnonella enteritidis ATCC 13706, and Listeria monocytogenes F5069 were suspended in NaHCO3-NaOH buffer solutions at pH 9, 10, 11, or 12 to give a final cell concentration of approximately 5.2 x 108 CFU/ml and then held at 37 or 45°C. At 0, 5, 10, and 15 min the suspensions were sterilely filtered and each filtrate was analyzed for material with A260, Viability of the cell suspensions was evaluated by enumeration on nonselective and selective agars. Cell morphology was evaluated by scanning electron microscopy and transmission electron microscopy. A260 increased dramatically with pH and temperature for both E. coli and S. enteritidis; however, with L. monocytogenes material with A260 was not detected at any of the pHs tested. At pH 12, numbers of E. coli and S. enteritidis decreased at least 8 logs within 15 s, whereas L. monocytogenes decreased by only 1 log in 10 min. There was a very strong correlation between the initial rate of release of material with A260 and death rate of the gram-negative pathogens (r = 0.997). At pH 12, gram-negative test cells appeared collapsed and showed evidence of lysis while gram-positive L. monocytogenes did not, when observed by scanning and transmission electron microscopy. It was concluded that destruction of gram-negative food-borne pathogens by high pH involves disruption of the cytoplasmic membrane.

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