The lipoprotein NlpD in responds to acid stress and regulates macrophage resistance and virulence by maintaining membrane integrity.

, an emerging opportunistic pathogen, is implicated in severe foodborne outbreak infections in premature and full-term infants. Generally, acid tolerance is vital for the pathogenesis of foodborne pathogens; however, its role in virulence remains largely unknown. To screen out acid-tolerance determinants from transposon mutants, anovel counterselection method using gentamicin and acid was developed. Using the counterselection method and growth assay, we screened several acid-sensitive mutants and found that  encodes an acid-resistance factor in .  Compared to the wild-type strain, the mutant exhibited attenuated virulence in a rat model. Using macrophage THP-1 cells and a pH probe, we verified that enables bacteria to resist macrophages by resisting acidification. Finally, we confirmed that maintains membrane integrity in acid using propidium iodide permeabilization assays via flow cytometry. Our results confirm that is a novel virulence factor that permits to survive under acid stress conditions. Considering that NlpD is a conserved lipoprotein located in the bacterial outer membrane, NlpD could be used as a target for drug development.