Transcriptional Downregulation of a Type III Secretion System under Reducing Conditions inBordetella pertussis

Abstract

ABSTRACTBordetella pertussisuses a type III secretion system (T3SS) to inject virulence proteins into host cells. Although theB. pertussisT3SS was presumed to be involved in host colonization, efficient secretion of type III secreted proteins fromB. pertussishas not been observed. To investigate the roles of type III secreted proteins during infection, we attempted to optimize culture conditions for the production and secretion of a type III secreted protein, BteA, inB. pertussis. We observed thatB. pertussisefficiently secretes BteA in ascorbic acid-depleted (AsA−) medium. When L2 cells, a rat lung epithelial cell line, were infected withB. pertussiscultured in the AsA−medium, BteA-dependent cytotoxicity was observed. We also performed an immunofluorescence assay of L2 cells infected withB. pertussis. Clear fluorescence signals of Bsp22, a needle structure of T3SS, were detected on the bacterial surface ofB. pertussiscultured in the AsA−medium. Since ascorbic acid is known as a reducing agent, we culturedB. pertussisin liquid medium containing other reducing agents such as 2-mercaptoethanol and dithioerythritol. Under these reducing conditions, the production of type III secreted proteins was repressed. These results suggest that inB. pertussis, the production and secretion of type III secreted proteins are downregulated under reducing conditions.IMPORTANCEThe type III secretion system (T3SS) ofBordetella pertussisforms a needlelike structure that protrudes from the bacterial cell surface.B. pertussisuses a T3SS to translocate virulence proteins called effectors into host cells. The culture conditions for effector production inB. pertussishave not been investigated. We attempted to optimize culture medium compositions for producing and secreting type III secreted proteins. We found thatB. pertussissecretes type III secreted proteins in reducing agent-deprived liquid medium and that BteA-secretingB. pertussisprovokes cytotoxicity against cultured mammalian cells. These results suggest that redox signaling is involved in the regulation ofB. pertussisT3SS.