Morphological analysis of the sheathed flagellum of Brucella melitensis

BACKGROUND: It was recently shown that B. melitensis is flagellated. However, the flagellar structure remains poorly described. FINDINGS: We analyzed the structure of the polar sheathed flagellum of B. melitensis by TEM analysis and demonstrated that the Ryu staining is a good method to quickly visualize the flagellum by optical microscopy. The TEM analysis demonstrated that an extension of the outer membrane surrounds a filament ending by a club-like structure. The ΔftcR, ΔfliF, ΔflgE and ΔfliC flagellar mutants still produce an empty sheath. CONCLUSIONS: Our results demonstrate that the flagellum of B. melitensis has the characteristics of the sheathed flagella. Our results also suggest that the flagellar sheath production is not directly linked to the flagellar structure assembly and is not regulated by the FtcR master regulator.