Are type IV pili involved in Vibrio anguillarum virulence towards sea bass (Dicentrarchus labrax L.) larvae?

Vibrio anguillarum, an important bacterial fish pathogen, expresses a variety of virulence fac- tors contributing to its ability to cause vibriosis in fish. Many virulence factors of this pathogen remain however unknown. For example, a type IV pilus system was previously reported to be potentially involved in the virulence of this bac- terium but no experimental evidence was re- ported yet. In this study, complete genome se- quencing of V. anguillarum strain VIB15, shown to be highly virulent towards sea bass (Dicen- trarchus labrax L.) larvae, revealed the presence of a PilA pilin. A V. anguillarum VIB15 pilA mu- tant was constructed and the pathogenicity of this mutant was assessed in a gnotobiotic sea bass system developed for virulence screening. Our results suggest that the V. anguillarum pilA gene is not crucial for virulence towards sea bass larvae. Possibly, another type IV pilus system identified in V. anguillarum, showing homology to the mannose-sensitive hemagglu- tinin pilin of Vibrio cholerae, may complement the pilA mutation. Alternatively, the type IV pilus system has a role in infection of juvenile or adult fish, rather than in the larval phase. As such, further research is required to unravel the po- tential role of type IV pili in V. anguillarum viru- lence.status: publishe