Influence of the adaptation of Listeria monocytogenes populations to structured or homogeneous habitats on subsequent growth on chilled processed meat

The influence of adaptation to habitat structure on the growth of Listeria monocytogenes on processed meats was investigated. Triplicate populations of each of two L. monocytogenes strains were adapted to growth in homogenous (Tryptic Soy Broth, TSB) or structured (Tryptic Soy Agar) environments by serial transfer in these media incubated at 25 °C for 48 h. Adapted (serial transfer for 56 days) and unadapted control (single 48 h growth period) populations were inoculated onto the surface of bologna or farmersʹ sausage disks, vacuum packaged and stored at 4 °C for 4 weeks. Initially and after 4 weeks, L. monocytogenes numbers were determined on Modified Oxford Agar incubated at 25 °C for 48 h. Control populations of both strains grown in the structured habitat reached significantly higher levels (1–1.5 log cfu g−1, P<0.05) on both products than their counterparts grown in homogeneous habitat. Populations of both strains adapted to homogeneous habitats grew to significantly higher levels (0.4–1 log cfu g−1, P<0.05), while those adapted to growth in structured habitats grew to significantly lower levels (0.3–1 log cfu g−1, P<0.05), on both products as compared to their unadapted control populations. Populations of both strains adapted to homogeneous habitats grew to significantly higher levels (0.2 to 0.5 log cfu g−1, P<0.05) on farmersʹ sausage, and significantly lower levels (0.2 to 0.4 log cfu g−1, P<0.05) on bologna, than those adapted to structured habitats. Habitat adaptation affected L. monocytogenes growth on processed meats, but the practical significance and reasons for these findings were not established.