Mutation of rpoS gene decreased resistance to environmental stresses, synthesis of extracellular products and virulence of Vibrio anguillarum

Vibrio anguillarum is a gram-negative halophilic bacterium that causes vibriosis in marine fish, freshwater fish and other aquatic animals. Bacteria have developed strategies to survive in harsh environments. The alternative σ factor, RpoS (σ^S), plays a key role in surviving under stress conditions in some gram-negative bacteria. An rpoS mutant of pathogenic V. anguillarum W-1 was constructed by homologous recombination. The sensitivity of the rpoS mutant to osmotic stress 2.4 M NaCl in artificial seawater (ASW)] did not change obviously, but the sensitivity of the rpoS mutant to high temperature (45 °C in ASW), UV-irradiation and oxidative stress (5 mM H₂O₂ in ASW) increased 33-fold, sixfold and 10-fold, respectively. The production of extracellular phospholipase, diastase, lipase, caseinase, hemolysin, catalase and protease of the rpoS mutant decreased markedly compared with those of the wild-type strain. Virulence of the rpoS mutant strain was also decreased when it was inoculated intraperitoneally into zebra fish; the lethal dose 50% of the wild type and the mutant was 8.66 × 10⁴ and 2.55 × 10⁶ CFU per fish, respectively. These results indicated that the RpoS of V. anguillarum plays important roles in bacterial adaptation to environmental stresses and its pathogenicity.