Can food-related environmental factors induce different behaviour in two key serovars, 4b and 1/2a, of Listeria monocytogenes?

Listeria monocytogenes isolates (81 in total; 42 isolated from cases of human listeriosis; 39 isolated from food), belonging to serovars 1/2a or 4b, were studied for any group differences between serovars to selected factors associated with foods (two bacteriocins and mild heat treatment), growth kinetics at 37°C and pathogenicity for chick embryos. The isolates were tested for sensitivity to two bacteriocins at 4°C and 37°C, and were tested for the remaining parameters both before and after exposure to cold storage (4°C) with starvation. In addition, the isolates were typed using pulsed field gel electrophoresis (PFGE), multilocus enzyme electrophoresis (MEE) and phage typing to find any correlation between the types and group differences in the chosen parameters. Considerable strain diversity within each L. monocytogenes serovar with respect to the chosen parameters was observed, especially after exposure to cold storage. Nevertheless, the serovar 1/2a isolates, as a group, tended to be more resistant to the two antilisterial bacteriocins at 4°C than the group of serovar 4b isolates. In contrast, after cold storage at 4°C, L. monocytogenes serovar 4b isolates, as a group, tended to be more resistant to heat treatment at 60°C than the group of 1/2a isolates. In addition, the serovar 4b group tended to have shorter lag phases and higher pathogenicity, when transferred from cold storage to body temperature (37°C), than the group of serovar 1/2a isolates. No correlation between PFGE-, MEE- and phage-types and the tested parameters was found. Although the above serovar-related differences were observed only when mean values of the groups were compared (not all isolates within each group followed the group pattern), the results indicate interesting directions for further research.