Gene Transcription and Virulence Potential ofListeria monocytogenesStrains After Exposure to Acidic and NaCl Stress

Gene transcription and virulence potential of two strains of Listeria monocytogenes, EGD-e and 4140, were compared by quantitative real-time polymerase chain reaction and in a Caco-2 in vitro model after exposure to acidic (pH 5.5) and NaCl (4.5% w/v) stress. Strain-dependent differences in gene transcription were observed both after exposure to shock (six genes) and after long-term adaptation to stress (18 genes). In the shock experiments, a transient induction of clpC and clpE was seen for both strains, while transient induction of sigB, inlA, and inlB was observed for strain 4140 only; actA was only induced in EGD-e after NaCl shock. The long-term stress experiments were included to imitate the stress conditions encountered by L. monocytogenes when present in food products. Long-term adaptation of EGD-e to acidic stress induced transcription of iap and repressed flaA, while genes related to stress response and invasion (clpC, clpP, inlA, inlB, prfA, and sigB) were induced in 4140. Long-term adaptation of EGD-e to NaCl stress increased transcription of genes important for the intracellular life cycle (actA, hly, iap, inlA, inlB, plcA, plcB, and prfA), while few changes were observed for 4140. Experiments with Caco-2 confirmed that long-term adaptation of EGD-e and 4140 to acidic and NaCl stress is capable of increasing the virulence potential: an improved adhesion to Caco-2 was observed for both EGD-e and 4140 after acidic and NaCl stress, and increased invasion was seen for both strains after long-term NaCl stress. The fact that several virulence genes were up-regulated and that adhesion and invasion properties were increased demonstrate that certain environmental conditions in food products might influence the virulence potential of L. monocytogenes.