Pasteurella multocida is the causative agent of fowl
cholera and other diseases of production animals. Isolates are classified into five groups based on capsular
antigens and into 16 serotypes based on LPS
antigens. Strains causing fowl
cholera are most frequently designated A:1, A:3 or A:4. Whole cell bacterins can provide some degree of protection, but only against the homologous LPS serotype. There is good evidence that cross-protective
antigens are expressed only under in vivo conditions. Empirically derived, live,
attenuated vaccines can protect against heterologous serotypes, but because the basis for attenuation is undefined, reversion to virulence is not uncommon. Work in our laboratory is aimed at using a variety of approaches to identify potential protective
antigens or virulence genes to be used as candidates for attenuating mutations or as the basis for
vaccine antigen delivery systems. The gene encoding an outer
membrane protein, Oma87, which is a homologue of the D15 protective
antigen of Haemophilus influenzae, was cloned and sequenced. Rabbit antiserum prepared against recombinant Oma87 could passively protect mice against
infection. Type 4 fimbriae form the basis of
vaccines against ovine footrot and bovine
keratoconjunctivitis. We have identified type 4 fimbriae on the surface of P. multocida, purified the fimbrial
subunit protein, PtfA, and determined its N-terminal amino acid sequence. Subsequent cloning of the ptfA gene and its inactivation will now be used to assess the importance of type 4 fimbriae in virulence. There has long been anecdotal evidence for the importance of
capsule in virulence, but unequivocal genetic evidence for such a role is lacking. We have cloned and characterised the
capsule biosynthetic locus in P. multocida A:1 and identified four bex genes involved in
capsule transport and genes encoding
enzymes involved in the biosynthesis and transfer of the
N-acetyl glucosamine and
glucuronic acid components of the
capsule. It has been suggested that the low concentration of available
iron in vivo acts as an environmental cue for expression of cross-protective
antigens. Accordingly, we have cloned and characterised the gene encoding
transferrin binding protein, Tbpl, so that its role in immunity and virulence can be investigated. Although P. multocida is not normally considered haemolytic, we have observed
haemolysis under anaerobic conditions. Standard library construction and screening resulted in the identification of the mesA gene which encodes an
esterase enzyme resulting in a haemolytic phenotype under anaerobic conditions. Virulence studies with mesA- mutants were performed to assess its role in pathogenesis. Using a promoterless phoA gene vector system, the cloning of
proteins homologous to known
surface proteins of other species as well as
proteins unique to P. multocida, allowing their potential as
vaccine components to be assessed.