The Salmonella typhi melittin resistance gene pqaB affects intracellular growth in PMA-differentiated U937 cells, polymyxin B resistance and lipopolysaccharide

Salmonella typhi is the causative agent of typhoid fever in humans. A cell-culture based assay involving the human monocyte macrophage cell line U937 has been developed to examine S typhi invasion and survival. An S typhi PhoP(-) (null) mutant was shown to be restricted in net growth in phorbol myristate acetate (PMA) differentiated U937 (PMA-U937) cells, and an S, typhi PhoP(c) (constitutive) mutant showed a defect in invasion. Neither of the phoP/Q mutants were growth impaired in HeLa cells, however the PhoP(c) mutant was impaired in invasion. As opposed to what was found for S, typhi, Salmonella typhimurium wild-type, PhoP(-) and PhoP(c) mutants grew equally well in PMA-U937 cells, indicating that the PhoP(-)-mediated net growth restriction in the PMA-U937 cells was S, typhi specific. An S, typhi mutation, pqaB::MudJ, recently shown to be a PhoP-activated locus, was shown to have a net growth defect in PMA-U937 cells. Sequencing of the S, typhi pqaB gene revealed it had 98% identity to the fifth gene in a S. typhimurium PmrA/B regulated operon necessary for 4-aminoarabinose lipid A modification and polymyxin B resistance. The pqaB locus was regulated by PmrA/B (whose activity is modulated by PhoP-PhoQ) and the pqaB transposon mutant was sensitive to polymyxin B, The lipopolysaccharides (LPS) of S, typhi and S, typhimurium wildtype, PhoP(-) and PhoP(c) mutants, were compared by SDS-PACE and silver staining. Differences in the LPS profile between the two Salmonella species were observed, and shown to be affected differently by the PhoP(c) mutation. Additionally, the pqaB::MudJ mutation affected S, typhi LPS, The effects on LPS may have ramifications for the difference between S typhi and S. typhimurium infection of hosts.