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 (Prn), 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 naïve (control) mice to compare the proportion of colonisation with recent clinical Prn positive and Prn negative B. pertussis strains from Australia. The Prn negative strain colonised the respiratory tract more effectively than the Prn positive strain in immunised mice, out-competing the Prn positive strain by day 3 of infection. However, in control mice, the Prn positive strain out-competed the Prn negative strain. Our findings of greater ability of Prn negative strains to colonise ACV-immunised mice are consistent with reports of selective advantage for these strains in ACV-immunised humans.