CONSTITUTIVE EXPRESSION OF THE PHOP REGULON ATTENUATES SALMONELLA VIRULENCE AND SURVIVAL WITHIN MACROPHAGES

The phoP genetic locus is a two-component regulatory system (phoP-phoQ) that controls the expression of genes essential for Salmonella typhimurium virulence and survival within macrophages. Strains with a phoP constitutive mutation (phenotype PhoP(C)) showed up to 10-fold greater expression of phoP-activated genes (pag loci) than did strains with a wild-type phoP locus (phenotype PhoP+). While the phoP constitutive mutation resulted in increased expression of pag loci, it also dramatically reduced the expression of other protein species. Comparison of the protein content of PhoP+ and PhoP(C) strains by two-dimensional protein gel electrophoresis demonstrated that at least 40 separate protein species were changed in expression as a result of this mutation. The PhoP(C) S. typhimurium were found to be attenuated for virulence and survival within macrophages. This finding suggests that a balanced PhoP-PhoQ regulatory response, which allows expression of phoP-repressed as well as -activated genes, is required for full virulence of S. typhimurium. We have further shown that small numbers of PhoP(C) bacteria can be used as a live attenuated vaccine to protect against mouse typhoid. As few as 15 PhoP(C) bacteria protected mice against challenge with 105 50% lethal doses of wild-type organisms, suggesting that important protective antigens are regulated by the PhoP-PhoQ virulence regulon.