P-004. Influence of Sodium Chloride and Temperature on Growth Rates of Strains Representing the Genetic Diversity of Listeria monocytogenes

T. Bergholz, M. Wiedmann;
Cornell Univ., Ithaca, NY.

Listeria monocytogenes causes a rare but severe foodborne disease, predominantly in neonates, the elderly, and in pregnant or immunocompromised individuals. As 99% of listeriosis cases result from consumption of contaminated food, prevention of listeriosis in humans depends on improving our understanding of how to control L. monocytogenes in foods. L. monocytogenes is capable of proliferating at low temperatures and under high osmotic stress, common conditions utilized for food preservation. L. monocytogenes isolates can be grouped into three genetic lineages based on molecular subtyping; isolates associated with sporadic and epidemic cases of human listeriosis belong to lineage I, isolates from foods and sporadic cases of human and animal listeriosis belong to lineage II, and isolates from animal listeriosis belong to lineage III. To test the hypothesis that L. monocytogenes lineages may exhibit different stress related phenotypes, isolates representing these three lineages were selected to assess if sodium chloride and temperature affected growth differently between the lineages. Strains were pre-adapted at 7 and 37ºC in brain heart infusion broth (BHI), and changes in cell density over time in BHI with 6% NaCl at 7 and 37ºC were quantified by plate counts and growth rates were determined using the Baranyi model. In BHI with 6% NaCl at 37ºC, the average specific growth rate for lineage III strains was 0.31 ± 0.04 log CFU/mL/h, which was significantly greater than the growth rate for lineage II strains, 0.23 ± 0.03 log CFU/mL/h, and lineage I strains, 0.27 ± 0.03 log CFU/mL/h. Under these conditions, lineage I strains had a significantly greater growth rate compared to lineage II isolates. Lag phase duration time upon transfer to BHI with 6% NaCl at 37ºC was significantly greater for the lineage III strains compared to the lineage I and II strains. At 7ºC in BHI + 6% NaCl, variation in growth rates within lineages was higher than at 37 ºC and all of the lineages had similar average growth rates (~ 0.02 log CFU/mL/h). This data, along with previously reported phenotypic stress resistance data suggest that the genetic lineages of L. monocytogenes may be adapted to thrive under different environmental conditions.