pertussis may be involved in facilitating transmission between hosts. Thus, a B. pertussis mutant carrying the bvgS-I1 mutation (GMT1i) persisted at wild-type levels only in the upper murine respiratory tract. Interestingly, a bipA deletion derivative of GMT1i displayed a reduced ability to colonize the nasal cavity of mice compared with GMT1i. However, in experimental mixed infections GMT1i expressing the Bvg i phase could establish an initial colonization in the nose and trachea of mice as efficiently as GMT1, but the wild-type strain outcompeted GMT1i at a later time point at all sites of the respiratory tract, suggesting that the Bvg i phase does not serve as a phenotypic phase specialized in colonization. Finally, even though B. pertussis expresses in vitro the Bvg i phase at the human nasal temperature, anti-BipA antibodies were undetectable in a large collection of sera from pertussis patients.