Identification and regulation of expression of a gene encoding a filamentous hemagglutinin-related protein in Bordetella holmesii

Background: Bordetella holmesii is a human pathogen closely related to B. pertussis, the etiological agent of whooping cough. It is able to cause disease in immunocompromised patients, but also whooping cough-like symptoms in otherwise healthy individuals. However, virtually nothing was known so far about the underlying virulence mechanisms and previous attempts to identify virulence factors related to those of B. pertussis were not successful. Results: By use of a PCR approach we were able to identify a B. holmesii gene encoding a protein with significant sequence similarities to the filamentous hemagglutinin (FHA) of B. avium and to a lesser extent to the FHA proteins of B. pertussis, B. parapertussis, and B. bronchiseptica. For these human and animal pathogens FHA is a crucial virulence factor required for successful colonization of the host. Interestingly, the B. holmesii protein shows a relatively high overall sequence similarity with the B. avium protein, while sequence conservation with the FHA proteins of the human and mammalian pathogens is quite limited and is most prominent in signal sequences required for their export to the cell surface. In the other Bordetellae expression of the fhaB gene encoding FHA was shown to be regulated by the master regulator of virulence, the BvgAS two-component system. Recently, we identified orthologs of BvgAS in B. holmesii, and here we show that this system also contributes to regulation of fhaB expression in B. holmesii. Accordingly, the purified BvgA response regulator of B. holmesii was shown to bind specifically in the upstream region of the fhaB promoter in vitro in a manner similar to that previously described for the BvgA protein of B. pertussis. Moreover, by deletion analysis of the fhaB promoter region we show that the BvgA binding sites are relevant for in vivo transcription from this promoter in B. holmesii. Conclusion: The data reported here show that B. holmesii is endowed with a factor highly related to filamentous hemagglutinin (FHA), a prominent virulence factor of the well characterized pathogenic Bordetellae. We show that like in the other Bordetellae the virulence regulatory BvgAS system is also involved in the regulation of fhaB expression in B. holmesii. Taken together these data indicate that in contrast to previous notions B. holmesii may in fact make use of virulence mechanisms related to those described for the other Bordetellae.