MntR Mediates LiaSR-Regulated gadT2/gadD2 Expression and Acid Resistance in Listeria monocytogenes 10403S

Listeria monocytogenes is a foodborne pathogen capable of persisting under acid, osmotic, oxidative, thermal, and other environmental stresses. The glutamate decarboxylase (GAD) system is a major determinant of survival under acidic conditions. Previous work showed that the two-component system LiaSR negatively regulates the gadT2/gadD2 locus in L. monocytogenes 10403S, but the intervening regulator was unknown. Here, we combined transcription-factor mutant screening, promoter-reporter assays, RT-qPCR, Western blotting, acid-survival assays, and electrophoretic mobility shift assays to define this regulatory relationship. Deletion of mntR reduced P_gadT2-gfp activity, gadD2 transcript abundance, and GadD2 protein levels at pH 4.5, 7, and 9, whereas complementation restored expression toward the wild-type level. The ΔmntR strain also showed reduced survival under inorganic and organic acid stresses. LiaSR deletion increased P_mntR-gfp activity, and LiaR bound directly to P_mntR. Conversely, mntR deletion increased LiaR abundance, although MntR did not bind to P_liaSR, indicating indirect feedback regulation. The liaSR/mntR double mutant showed reduced gadT2/gadD2 expression under acidic conditions and impaired acid survival. Together, these findings support a LiaSR-MntR-gadT2/gadD2 pathway in which MntR promotes GAD-mediated acid resistance.