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The Bordetella bronchiseptica type III secretion system is required for persistence and disease severity but not transmission in swine

Tracy L. Nicholson, Susan L. Brockmeier, Crystal L. Loving, Karen B. Register, Marcus E. Kehrli Jr., Sarah M. Shore
Infection and Immunity 2014 v.82 no.3 pp. 1092-1103
Bordetella bronchiseptica, adaptive immunity, antibodies, blood serum, colonizing ability, disease severity, disease transmission, hosts, immune response, lesions (animal), models, mutants, nasal cavity, pathogenesis, pneumonia, rodents, swine, type III secretion system, virulence
Bordetella bronchiseptica is pervasive in swine populations and plays multiple roles in respiratory disease. Most studies addressing virulence factors of B. bronchiseptica utilize isolates derived from hosts other than pigs in conjunction with rodent infection models. Based on previous in vivo mouse studies, we hypothesized that the B. bronchiseptica type III secretion system (T3SS) would be required for maximal disease severity and persistence in the swine lower respiratory tract. To examine the contribution of the T3SS to the pathogenesis of B. bronchiseptica in swine, we compared the ability of a virulent swine isolate and an isogenic T3SS mutant to colonize, cause disease, and transmit host-to-host. We found the T3SS is required for maximal persistence throughout the swine respiratory tract, including the nasal cavity. Additionally, the T3SS contributes significantly to the development of nasal lesions and pneumonia. However, the T3SS mutant and the wild-type parent are equally capable of transmission among swine by both direct and indirect routes, demonstrating that transmission can occur even with attenuated disease. Our data further suggest the T3SS skews the adaptive immune response in swine by hindering the development of serum anti-Bordetella antibody levels and inducing an IL-10 cell-mediated response, likely contributing to the persistence of B. bronchiseptica in the respiratory tract. Overall, our results demonstrate that the Bordetella T3SS is required for maximal persistence and disease severity in pigs, but not for transmission.