Salt Stress Phenotypes in Listeria monocytogenes Vary by Genetic Lineage and Temperature

Listeria monocytogenes can survive and grow under wide-ranging environmental stress conditions encountered both in foods and in the host. The ability of certain L. monocytogenes subtypes to thrive under stress conditions present in specific niches was hypothesized to reflect genetic characteristics and phenotypic capabilities conserved among strains within a subtype. To quantify variations in salt stress phenotypes among 40 strains selected to represent the diversity of the three major L. monocytogenes genetic lineages and to determine if salt stress phenotypes were associated with genetic relatedness, we measured growth under salt stress at both 7 degrees C and 37 degrees C. At 7 degrees C, in brain-heart infusion with 6% NaCl, average growth rates among the lineages were similar. A comparison of doubling times after exposure to salt stress at 7 degrees C or 37 degrees C indicated that growth at 7 degrees C provided crossprotection to subsequent salt stress for strains in lineages I and II. At 37 degrees C, in brain-heart infusion with 6% NaCl, lineage I and III strains grew significantly faster (p < 0.0001) than lineage II strains. Under salt stress at 37 degrees C, differences in growth parameters were significantly (p < 0.005) associated with genetic relatedness of the strains. Compatible solute uptake is part of the L. monocytogenes salt stress response, but growth differences between the lineages were not related to differences in transcript levels of osmolyte transporter-encoding genes betL, gbuA, oppA, and opuCA. The combination of phylogenetic and phenotypic data suggests that L. monocytogenes lineage I and III strains, which are most commonly associated with human and animal disease, may be better adapted to osmotic stress at 37 degrees C, conditions that are present in the host gastrointestinal tract.