Role of Capsule in the Pathogenesis of Fowl Cholera Caused byPasteurella multocidaSerogroup A

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Abstract
We have constructed a defined acapsular mutant inPasteurella multocidaX-73 (serogroup A:1) by disrupting thehexAgene through the insertion of a tetracycline resistance cassette. The genotype of thehexA::tet(M) strain was confirmed by PCR and Southern hybridization, and the acapsular phenotype of this strain was confirmed by electron microscopy. ThehexA::tet(M) strain was attenuated in both mice and chickens. Complementation of the mutant with an intacthexABfragment restored lethality in mice but not in chickens. In contrast to the results described previously forP. multocidaserogroup B (J. D. Boyce and B. Adler, Infect. Immun. 68:3463–3468, 2000), thehexA::tet(M) strain was sensitive to the bactericidal action of chicken serum, whereas the wild-type and complemented strains were both resistant. Following inoculation into chicken muscle, the bacterial count of thehexA::tet(M) strain decreased significantly, while the wild-type and complemented strains both grew rapidly over 4 h. The capsule is thus an essential virulence determinant in the pathogenesis of fowl cholera.