Differences in pathogenicity for chick embryos and growth kinetics at 37 °C between clinical and meat isolates of Listeria monocytogenes previously stored at 4 °C

Fifteen clinical strains of Listeria monocytogenes (eight strains of serogroup 4 and seven strains of serogroup 1) and 15 meat isolates (all serogroup 1) were stored with no growth in phosphate-buffered saline (pH 7.0) at 4 °C for 4 weeks. Pathogenicity for 14 day old chick embryos and growth kinetics in brain heart infusion (BHI) broth at 37 °C of the strains were determined before and after storage. Although no differences in pathogenicity between clinical and meat strains were found when tested as fresh cultures, significant differences became apparent after cold storage. Firstly, the pathogenicity of clinical strains was not affected by storage, whereas the average mortality of embryos inoculated with meat strains decreased from 98.7 to 68.0%. Secondly, clinical strains subcultured at 37 °C had a significantly shorter average lag phase than meat strains after cold storage. The results of this study indicate that strains that caused human listeriosis have a higher resistance to the effects of unfavourable storage conditions than meat strains, with respect to pathogenicity and lag phase duration at body temperature.