EFFECTS OF PH OR A(W) STRESS ON GROWTH OF LISTERIA-MONOCYTOGENES

The growth of three strains of Listeria monocytogenes at 20 degrees C in a meat broth of different pH or water activity was investigated. At inoculation or at the beginning of the exponential phase, cells were exposed to stress by the addition of NaOH or NH4+ acetic acid, NaCl or KCl, in order to reach a pH of either 9.0 or 5.6, or an a(w) of 0.950 or 0.965, respectively. The effects of the exposure to stress on the generation and lag times of each strain were analysed by turbidity mea surements for cultures in micro-titer plates. Results were confirmed b y conducting the same experiments in a fermenter, except for the maxim al population reached. The three strains showed similar behaviour. Cel ls were able to overcome the alkaline stress rapidly whereas acid and osmotic shocks induced important changes of the growth parameters. Cel ls exposed to acid or osmotic conditions from the time of inoculation were less affected than cells exposed at the beginning of the mid-expo nential phase. (C) 1998 Elsevier Science B.V.