Functional Analysis of <i>Treponema denticola</i> Periplasmic Flagella: Roles of FlaB1, FlaB2, and FlaB3 in Morphology and Motility

Treponema denticola is a Gram-negative, anaerobic spirochete associated with periodontal disease. It navigates highly viscous environments using periplasmic flagella located between the inner and outer membranes. The flagellum is composed of three homologous flagellin proteins-FlaB1, FlaB2, and FlaB3-encoded by the flaB1, flaB2, and flaB3 genes, respectively, and is enwrapped by the sheath protein FlaA, encoded by the flaA gene. To investigate the roles of the three FlaB flagellins in cell morphology and motility, we constructed mutants lacking different combinations of the flaB genes, including double and triple deletions. The deletion of one or two flaB genes did not affect the transcription or protein expression of the remaining flaB gene(s). Normal flagellar filaments were observed when at least one FlaB was expressed but were absent in the complete flaB deletion mutant, which also exhibited an elongated cell shape. The flagella had no significant impact on bacterial growth. In liquid medium, most mutants exhibited rotational movement comparable to the parent strain, although some showed an increased population of cells with higher rotation rates. However, in a medium containing 0.5% agar, deletion of any two or three flaB genes significantly reduced motility. These findings suggest that the expression of at least two FlaB flagellins is required for the potent movement of T. denticola.