Influence of the natural microbial flora on the acid tolerance response ofListeria monocytogenes in a model system of fresh meat decontamination fluids

Depending on its composition and metabolic activity, the natural flora that may be established in a meat plant environment can affect the survival, gr owth, and acid tolerance response (ATR) of bacterial pathogens present in t he same niche, To investigate this hypothesis, changes in populations and A TR of inoculated (10(5) CFU/ml) Listeria monocytogenes were evaluated at 35 degreesC in water (10 or 85 degreesC) or acidic (2% lactic or acetic acid) washings of beef with or without prior filter sterilization, The model exp eriments were performed at 35 degreesC rather than lower (less than or equa l to 15 degreesC) temperatures to maximize the response of inoculated L, mo nocytogenes in the washings with or without competitive Bora. Acid solution washings were free (<1.0 log CFU/ml) of natural flora before inoculation ( day 0), and no microbial growth occurred during storage (35 degreesC, 8 day s), Inoculated L, monocytogenes died off (negative enrichment) in acid wash ings within 24 h, In nonacid (water) washings, the pathogen increased (appr oximately 1.0 to 2.0 log CFU/ml), irrespective of natural flora, which, whe n present, predominated (>8.0 log CFU/ml) by day I The pH of inoculated wat er washings decreased or increased depending on absence or presence of natu ral flora, respectively, These microbial and pH changes modulated the ATR o ft. monocytogenes at 35 degreesC. In filter-sterilized water washings, inoc ulated L. monocytogenes increased its ATR by at least 1.0 log CFU/ml from d ays I to 8, while in unfiltered water washings the pathogen was acid tolera nt at day 1 (0.3 to 1.4 log CFU/ml reduction) and became acid sensitive (3. 0 to >5.0 log CFU/ml reduction) at day 8, These results suggest that the pr edominant gram-negative flora of an aerobic fresh meat plant environment ma y sensitize bacterial pathogens to acid.