Surface-associated filamentous hemagglutinin induces autoagglutination of Bordetella pertussis.

Filamentous hemagglutinin (FHA) is a major adhesin produced by Bordetella pertussis, the etiologic agent of whooping cough. FHA has been shown to be surface associated but is also secreted by virulent bacteria. Microscopic observations of lungs of mice infected with B. pertussis showed that the bacteria grow as clusters within the alveolar lumen. When B. pertussis was cultivated in vitro with chemically defined medium, bacteria grew as aggregates, mimicking growth observed in vivo. This aggregation was abolished by the addition of cyclodextrin (CDX) to the growth medium and depended on the production of FHA, because a mutant lacking the FHA structural gene failed to form aggregates in a CDX-free medium. Western blot (immunoblot) analyses revealed that, in the absence of CDX, FHA was attached to the bacterial surface and was not efficiently released into the growth medium. Hydrophobic chromatography of FHA showed that CDX drastically reduced the hydrophobicity of FHA, suggesting a direct binding of CDX to FHA, which was further supported by the partial protection of FHA from trypsin digestion in the presence of CDX. In addition, free FHA can interact in a CDX-inhibitable manner with solid phase-immobilized FHA. It can therefore be postulated that the B. pertussis aggregates are most likely due to direct FHA-FHA interaction.