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

Abstract

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 PhoPc (constitutive) mutant showed a defect in invasion. Neither of the phoP/Q mutants were growth impaired in HeLa cells, however the PhoPc mutant was impaired in invasion. As opposed to what was found for S. typhi, Salmonella typhimurium wild-type, PhoP- and PhoPc 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. typhipqaB 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 wild-type, PhoP- and PhoPc mutants, were compared by SDS-PAGE and silver staining. Differences in the LPS profile between the two Salmonella species were observed, and shown to be affected differently by the PhoPc 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.