Pertactin negative Bordetella pertussis demonstrates higher fitness under vaccine selection pressure in a mixed infection model

Whooping cough or pertussis is a highly infectious respiratory disease in humans caused by Bordetella pertussis. The use of acellular vaccines (ACV) has been associated with the recent resurgence of pertussis in developed countries including Australia despite high vaccination coverage where B. pertussis strains that do not express pertactin (Pm), a key antigenic component of the ACV, have emerged and become prevalent. In this study, we used an in vivo competition assay in mice immunised with ACV and in naive (control) mice to compare the proportion of colonisation with recent clinical Pm positive and Pm negative B. pertussis strains from Australia. The Pm negative strain colonised the respiratory tract more effectively than the Pm positive strain in immunised mice, out-competing the Pm positive strain by day 3 of infection. However, in control mice, the Pm positive strain out-competed the Pm negative strain. Our findings of greater ability of Pm negative strains to colonise ACV-immunised mice are consistent with reports of selective advantage for these strains in ACV-immunised humans. (C) 2015 Elsevier Ltd. All rights reserved.