Regulation of fatty acid metabolism by FadR is essential for Vibrio vulnificus to cause infection of mice

The opportunistic bacterial pathogen Vibrio vulnificus causes severe wound infection and fatal septicemia. We used alkaline phosphatase insertion mutagenesis in a clinical isolate of V. vulnificus to find genes necessary for virulence, and we identified fadR, which encodes a regulator of fatty acid metabolism. The fadR::mini-Tn5Km2phoA mutant was highly attenuated in a subcutaneously inoculated iron dextran-treated mouse model of V. vulnificus disease, was hypersensitive to the fatty acid synthase inhibitor cerulenin, showed aberrant expression of fatty acid biosynthetic (fab) genes and fatty acid oxidative (fad) genes, produced smaller colonies on agar media, and grew slower in rich broth than did the wild-type parent. Deletion of fadR essentially recapitulated the phenotypes of the insertion mutant, and the DeltafadR mutation was complemented in trans with the wild-type gene. Further characterization of the DeltafadR mutant showed that it was not generally hypersensitive to envelope stresses but had decreased motility and showed an altered membrane lipid profile compared to that of the wild type. Supplementation of broth with the unsaturated fatty acid oleate restored wild-type growth in vitro, and infection with oleate in the inoculum increased the ability of the DeltafadR mutant to infect mice. We conclude that fadR and regulation of fatty acid metabolism are essential for V. vulnificus to be able to cause disease in mammalian hosts.