Sensitization of Listeria monocytogenes to low pH, organic acids, and osmotic stress by ethanol

The killing of Listeria monocytogenes following exposure to low pH, organic acids, and osmotic stress was enhanced by the addition of 5% (vol/vol) eth anol. At pH 3, for example, the presence of this agent stimulated killing b y more than 3 log units in 40 min of exposure, The rate of cell death at pH 3.0 was dependent on the concentration of ethanol, Thus, while the presenc e 10% (vol/vol) ethanol at pH 3.0 stimulated killing by more than 3 log uni ts in just 5 min, addition of 2.25% (vol/vol) ethanol resulted in less than 1 Log unit of killing in 10 min. The ability of 5% (vol/vol) ethanol to st imulate killing at low pH and at elevated osmolarity was also dependent on the amplitude of the imposed stress, and an increase in the pH from 3.0 to 4.0 or a decrease in the sodium chloride concentration from 25 to 2.5% led to a marked reduction in the effectiveness of 5% (vol/vol) ethanol as an au gmentative agent, Combinations of organic acids, low pH, and ethanol proved to be particularly effective bactericidal treatments; the most patent comb ination was pH 3.0, 50 mM formate, and 5 % (vol/vol) ethanol, which resulte d in 5 log units of killing in just 4 min. Ethanol-enhanced killing correla ted with damage to the bacterial cytoplasmic membrane.