Growth characteristics of Listeria monocytogenes, Listeria welshimeri and Listeria innocua strains in broth cultures and a sliced bologna-type product at 4 and 7 °C

The growth characteristics of meat processing plant-derived field strains of Listeria monocytogenes, L. welshimeri and L. innocua were analyzed. The strains were inoculated in BHI broth cultures and incubated at 4 and 7 °C. Growth curves were determined by colony counting for 28 days. Significant variations were detected in the growth properties of these field-derived strains. In particular some of the L. monocytogenes strains displayed better cold stress tolerance. These discrepancies in growth behavior were more apparent in the cultures at 4 °C compared to 7 °C. Similar growth characteristics were observed for selected L. monocytogenes strains also in food challenge tests based on a sliced bologna-type product. The results stress the need for more evaluation of field strain growth characteristics and incorporation of such information in relevant predictive microbial growth models for L. monocytogenes risk assessment in naturally contaminated food products.