Fig 8. Acid sensing via the stressosome triggers upregulation of the inlAB invasion operon and influences Caco-2 cell invasion without affecting PrfA activity.

(A) inlA, (B) inlB, (C) prfA and (D) actA expression was obtained from mid-log phase cultures of strains used in this study. Cultures were untreated (-) and treated (+) in pH 5.0 for 15 min at 37°C. (E) Caco-2 invasion experiments performed on untreated (-) and treated (+) cultures in pH 5.0 for 15 min. L. monocytogenes wild type and mutant strains were in contact with the Caco-2 epithelial cells for 1 h and extracellular bacteria were eliminated with extensive washing and the supplementation of 50 μg.mL^-1 of gentamicin to fresh tissue culture medium. Quantification of internalized L. monocytogenes was obtained from lysed Caco-2 cells 1 h after the gentamicin treatment. The untreated wild type strain was arbitrary converted to 0% and all samples were normalized to percentage of CFU.mL^-1 relative to the untreated wild type. Statistical analysis was performed using a paired Student’s t-test relative to the wild type untreated after 15 min (*, P value of <0.05; **, P value of <0.01; ***, P value of <0.001).