Contribution of six flagellin genes to the flagellum biogenesis of Vibrio vulnificus and in vivo invasion.

Vibrio vulnificus is a halophilic pathogenic bacterium that is motile due to the presence of a single polar flagellum. V. vulnificus possesses a total of six flagellin genes organized into two loci (flaFBA and flaCDE). We proved that all six of the flagellin genes were transcribed, whereas only five (FlaA, -B, -C, -D, and -F) of the six flagellin proteins were detected. To understand roles of the six V. vulnificus flagellins in motility and virulence, mutants with single and multiple flagellin deletions were constructed. Mutations in flaB or flaC or the flaCDE locus resulted in a significant decrease in motility, adhesion, and cytotoxicity, whereas single mutations in the other flagellin genes or the flaFBA locus showed little or no effect. The motility was completely abolished only in the mutant lacking all six flagellin genes (flaFBA flaCDE). Surprisingly, a double mutation of flaB and flaD, a gene sharing 99% identity with the flaB at the amino acid level, resulted in the largest decrease in motility, adhesion, and cytotoxicity except for the mutant in which all six genes were deleted (the hexa mutant). Additionally, the 50% lethal doses (LD50s) of the flaB flaD and the flaFBA flaCDE mutants increased 23- and 91-fold in a mouse model, respectively, and the in vitro and in vivo invasiveness of the mutants was significantly decreased compared to that of the wild type. Taken together, the multiple flagellin subunits differentially contribute to the flagellum biogenesis and the pathogenesis of V. vulnificus, and among the six flagellin genes, flaB, flaD, and flaC were the most influential components.