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

Bordetella pertussis uses a type III secretion system (T3SS) to inject virulence proteins into host cells. Although the B. pertussis T3SS was presumed to be involved in host colonization, the efficient secretion of type III secreted proteins from B. pertussis has 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, in B. pertussis. We observed that B. pertussis efficiently secretes BteA in ascorbic acid-depleted (AsA-) medium. When L2 cells, a rat lung epithelial cell line, were infected with B. pertussis cultured in the AsA- medium, BteA-dependent cytotoxicity was observed. We also performed an immunofluorescence assay of L2 cells infected with B. pertussis. The clear fluorescence signals of Bsp22, a needle structure of T3SS, were detected on the bacterial surface of B. pertussis cultured in the AsA- medium. Since ascorbic acid is known as a reducing agent, we cultured B. pertussis in 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 in B. pertussis, the production and secretion of type III secreted proteins are downregulated under reducing conditions. IMPORTANCEThe type III secretion system (T3SS) of Bordetella pertussis forms a needle-like structure that protrudes from the bacterial cell surface. B. pertussis uses T3SS to translocate virulence proteins called effectors into host cells. The culture conditions for effector production in B. pertussis have not been investigated. We attempted to optimize culture medium compositions for producing and secreting type III secreted proteins. We found that B. pertussis secretes type III secreted proteins in reducing agent-deprived liquid medium, and that BteA-secreting B. pertussis provokes cytotoxicity against cultured mammalian cells. These results suggest that redox signaling is involved in the regulation of B. pertussis T3SS.