Summary: 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 invasion and survival. An 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 (constitutive) mutant showed as defect in invasion. Neither of the phoP/O, mutants were growth impaired in HeLa cells, however the PhoP mutant was impaired in invasion. As opposed to what was found for wild-type, PhoP and PhoP mutants grew equally well in PMA-U937 cells, indicating that the PhoP-mediated net growth restriction in the PMA-U937 cells was specific. An mutation, ::Mud recently shown to be a PhoP-activated locus, was shown to have a net growth defect in PMA-U937 cells. Sequencing of the gene revealed it had 98% identity to the fifth gene in a PmrA/B regulated operon necessary for 4-aminoarabinose lipid A modification and polymyxin B resistance. The locus was regulated by PmrA/B (whose activity is modulated by PhoP-PhoQ) and the transposon mutant was sensitive to polymyxin B. The lipopolysaccharides (LPS) of and wi/d type, PhoP and Phop mutants, were compared by SDS-PAGE and silver staining. Differences in the LPS profile between the two species were observed, and shown to be affected differently by the PhoP mutation. Additionally, the ::Mud mutation affected LPS. The effects on LPS may have ramifications for the difference between and infection of hosts.

Keyword(s): PhoP , PmrA/B , polymyxin B , pqa/pqr , Q regulon and virulence

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