Destruction of gram-negative food-borne pathogens by high pH involves disruption of the cytoplasmic membrane

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 O157:H7, Salmonella 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.