DESTRUCTION OF GRAM-NEGATIVE FOOD-BORNE PATHOGENS BY HIGH PH INVOLVESDISRUPTION OF THE CYTOPLASMIC MEMBRANE

High pH has been shown to rapidly destroy gram-negative food-borne pat hogens; however, the mechanism of destruction has not yet been elucida ted. Escherichia coli O157:H7, Salmonella enteritidis ATCC 13706, and Listeria monocytogenes F5069 were suspended in NaHCO3-NaOH buffer solu tions at pH 9, 10, 11, or 12 to give a final cell concentration of app roximately 5.2 x 10(8) CFU/ml and then held at 37 or 45 degrees C. At 0, 5, 10, and 15 min the suspensions were sterilely filtered and each filtrate was analyzed for material with A(260). 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. A(260) increased dramatically wi th pH and temperature for both E.increased dramatically with pH and te mperature for both E. coli and S. enteritidis; however, with L. monocy togenes material with A(260) 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 A(260) and death rate of the gram-negative pathogens (r = 0.997). At pH 12, gram-negative test cells appeared co llapsed and showed evidence of lysis while gram-positive L. monocytoge nes did not, when observed by scanning and transmission electron micro scopy. It was concluded that destruction of gram-negative food-borne p athogens by high pH involves disruption of the cytoplasmic membrane.