Behaviour of Listeria monocytogenes on two processed meat products as influenced by temperature or attached growth during preincubation Original Research Article

The effect of preincubation at 4°C, 20°C and 37°C and the effect of preincubation as planktonic cells or cells attached to surfaces for 48 or 96 h at 37°C on the subsequent behaviour of Listeria monocytogenes during storage on chilled processed meats was determined. Two L. monocytogenes strains were grown in Tryptic Soy Broth under the above preincubation conditions, inoculated at ∼6 log cfu g−1 onto slices of both bologna and summer sausage, vacuum packaged and incubated at 4°C for 6 weeks. Initially and at weekly intervals L. monocytogenes numbers were determined by plating samples on Modifed Oxford Agar followed by incubation at 37°C for 48 h. Results indicated that under all preincubation conditions both strains could grow on the bologna and not on the summer sausage. Preincubation at 4°C resulted in a reduced lag phase of both strains of the pathogen on bologna as compared to the other temperatures. Preincubation at 37°C, on the other hand, resulted in reduced survival in one strain only on summer sausage as compared to the other temperatures. Preincubation under attached conditions for 48 h had no effect on the growth of either strain on the bologna, but enhanced survival of one of the strains on summer sausage as compared to 96 h attached and planktonic growth conditions. The importance of preincubation conditions to the growth and survival kinetics of L. monocytogenes on foods was confirmed by the present study and existing data was extended to include specific meat products. More significantly, however, a novel preincubation effect was described and the role of strain-dependent variations with respect to kinetic and predictive studies was highlighted.