Role of capsule in the pathogenesis of fowl cholera caused by Pasteurella multocida serogroup A

We have constructed a defined acapsular mutant in Pasteurella multocida X-7 3 (serogroup A:1) by disrupting the hexA gene through the insertion of a te tracycline resistance cassette, The genotype of the hexA::tet(M) strain was confirmed by PCR and Southern hybridization, and the acapsular phenotype o f this strain was confirmed by electron microscopy, The hexA::tet(M) strain was attenuated in both mice and chickens. Complementation of the mutant wi th an intact hexAB fragment restored lethality in mice but not in chickens. In contrast to the results described previously for P. multocida serogroup B (J. D. Boyce and B. Adler, Infect. Immun, 68:3463-3468, 2000), the hexA: :tet(M) strain was sensitive to the bactericidal action of chicken serum, w hereas the wildtype and complemented strains were both resistant. Following inoculation into chicken muscle, the bacterial count of the hexA:tet(M) st rain decreased significantly, while the wild-type and complemented strains both grew rapidly over 4 h, The capsule is thus an essential virulence dete rminant in the pathogenesis of fowl cholera.