Flagellin is the subunit protein which polymerizes to form the filaments of bacterial flagella. FlaA is essential for flagellar synthesis and full motility. Important for virulence at two different levels: is needed for crossing the fish integument and may play a role once the bacterium has entered the host. Heteromer of multiple flagellin subunits including FlaA, FlaB, FlaC, FlaD and possibly FlaE. Belongs to the bacterial flagellin family.