BipA Is Associated with Preventing Autoagglutination and Promoting Biofilm Formation in Bordetella holmesii

Bordetella holmesii causes both invasive and respiratory diseases in humans. Although the number of cases of pertussis-like respiratory illnesses due to B . holmesii infection has increased in the last decade worldwide, little is known about the virulence factors of the organism. Here, we analyzed a B . holmesii isolate that forms large aggregates and precipitates in suspension, and subsequently demonstrated that the autoagglutinating isolate is deficient in Bordetella intermediate protein A (BipA) and that this deletion is caused by a frame-shift mutation in the bipA gene. A BipA-deficient mutant generated by homologous recombination also exhibited the autoagglutination phenotype. Moreover, the BipA mutant adhered poorly to an abiotic surface and failed to form biofilms, as did two other B . holmesii autoagglutinating strains, ATCC 51541 and ATCC 700053, which exhibit transcriptional down-regulation of bipA gene expression, indicating that autoagglutination indirectly inhibits biofilm formation. In a mouse intranasal infection model, the BipA mutant showed significantly lower levels of initial lung colonization than did the parental strain ( P < 0.01), suggesting that BipA might be a critical virulence factor in B . holmesii respiratory infection. Together, our findings suggest that BipA production plays an essential role in preventing autoagglutination and indirectly promoting biofilm formation by B . holmesii .