Relationship between variations in pathogenicity and lag phase at 37°C of Listeria monocytogenes previously stored at 4°C

Abstract

Three haemolytic, pathogenic strains of Listeria monocytogenes (a reference strain, a food-derived strain and a human strain) were held at 4 degrees C for 4 weeks in phosphate-buffered saline pH 5.5 or 7.0, with and without 0.2% potassium sorbate or 0.3% sodium acetate. The number of viable cells did not change significantly during this storage. Pathogenicity of non-growing L. monocytogenes cells for 14-d-old chick embryos was determined before and after storage. Storage at 4 degrees C resulted in decreased pathogenicity, but effects were strain-, pH-and substrate-dependent. After 4 weeks storage at 4 degrees C non-growing bacterial cells were transferred to Brain Heart Infusion broth and growth characteristics were determined during incubation at 37 degrees C. Strains that showed decreased pathogenicity had significantly longer lag phases at 37 degrees C than strains that maintained pathogenicity. It is concluded that decreased pathogenicity of L. monocytogenes stored without growth at 4 degrees C for 4 weeks and subsequent long lag phase at 37 degrees C are correlated.