The cytochrome d oxidase complex regulated by fexA is an Achilles' heel in the <i>in vivo</i> survival of <i>vibrio vulnificus</i>

<i>Vibrio vulnificus</i> is a halophilic estuarine bacterium causing severe opportunistic infections. To successfully establish an infection, <i>V. vulnificus</i> must adapt to redox fluctuations in vivo. In the present study, we show that deletion of <i>V. vulnificus</i> fexA gene caused hypersensitivity to acid and reactive oxygen species. The <i>ΔfexA</i> mutant exhibited severe in vivo survival defects. For deeper understanding the role of <i>fexA</i> gene on the successful <i>V. vulnificus</i> infection, we analyzed differentially expressed genes in <i>ΔfexA</i> mutant in comparison with wild type under aerobic, anaerobic or in vivo culture conditions by genome-scale DNA microarray analyses. Twenty-two genes were downregulated in the <i>ΔfexA</i> mutant under all three culture conditions. Among them, <i>cydAB</i> appeared to dominantly contribute to the defective phenotypes of the <i>ΔfexA</i> mutant. The <i>fexA</i> deletion induced compensatory point mutations in the <i>cydAB</i> promoter region over subcultures, suggesting essentiality. Those point mutations (P_cydSMs) restored bacterial growth, motility, cytotoxicity ATP production and mouse lethality in the <i>ΔfexA</i> mutant. These results indicate that the <i>cydAB</i> operon, being regulated by <i>FexA</i>, plays a crucial role in <i>V. vulnificus</i> survival under redox-fluctuating in vivo conditions. The FexA-CydAB axis should serve an Achilles heel in the development of therapeutic regimens against <i>V. vulnificus</i> infection.