Marked intra-strain variation in response of Listeria monocytogenes dairy isolates to acid or salt stress and the effect of acid or salt adaptation on adherence to abiotic surfaces

During food processing, and particularly in cheese manufacturing processes, Listeria monocytogenes may be exposed routinely to environments of low pH or high salt concentration. It has been suggested that these environmental conditions may contribute to bacterial adherence to abiotic surfaces and increased resistance to disinfection. In this study strains isolated from the environment of artisanal cheese-making dairies were used to investigate the behaviour of L. monocytogenes in response to acid and salt stress and clear differences between strains was observed. In planktonic culture, strains varied in resistance to low pH or high NaCl concentration and in the occurrence of an adaptive response to moderate acid or NaCl. There was dislocation in responses to salt and acid. Strains resistant, or adaptive, to acid were not resistant or adaptive to NaCl. The reverse also was observed. Exposure to moderate acid did not promote adherence to polystyrene but survival, at low pH or high NaCl concentration, of cells adherent to stainless steel was increased, even for strains that had no adaptive response planktonically, but the detail of these observations varied between strains. In contrast to acid adaptation, with some strains salt adaptation enhanced adherence of L. monocytogenes to polystyrene but this was not true for all strains. For some strains salt- or acid adaptation may enhance the survival of sessile cells exposed to hypochlorite disinfection.