Listeriolysin O production and pathogenicity of non-growing Listeria monocytogenes stored at refrigeration temperature

Three haemolytic, pathogenic strains of Listeria monocytogenes (a reference strain NCTC 7973, a food-derived strain L70 and a human strain L94) and a control strain of Listeria innocua L27 were held in phosphate-buffered saline (PBS) of pH 7.0 or 5.5 at 4 °C for 4 weeks. The number of viable cells did not change significantly during this storage (the cells were non-growing). Titers of Listeria listeriolysin O (LLO) activity against washed human erythrocytes and the pathogenicity of non-growing bacterial cells for 14-day-old chick embryos were determined before storage and after 1, 2, 3 and 4 weeks of storage. Prolonged storage at 4 °C affected both LLO production and pathogenicity of the non-growing cells, but effects were strain- and pH-dependent. At pH 7.0, all three L. monocytogenes strains had lost LLO activity after 2 weeks of storage. At pH 5.5, the reference and the food strains lost LLO activity 1 week later than when stored at neutral pH, and the human strain maintained LLO activity throughout the 4-week period. Pathogenicity of the reference strain stored at pH 7.0 and 5.5 and that of the food strain stored at pH 7.0 decreased during storage at 4 °C. However, the human strain stored at pH 7.0 and 5.5, and the food strain stored at pH 5.5, maintained their pathogenicity throughout the 4-week period. In all cases, non-growing L. monocytogenes cells that had ceased LLO production and/or had a reduced pathogenicity, recovered these characteristics after growth in media at 37 °C. This study indicates that prolonged storage of chilled foods in which L. monocytogenes is present, but not growing, may have the desirable result that the L. monocytogenes has a reduced ability to cause illness in humans. As well, pathogenicity testing involving growth of L. monocytogenes in laboratory media may not reflect the actual pathogenicity of the organism in the food as eaten.