Biochemical and immunological characteristics of Pasteurella multocida type A strains isolated from bovine pneumonia

Abstract

The aim of this study was to identify a vaccine strain that could give broad cross-protection and to determine a protective antigen that might lead to the improvement in the efficacy of vaccines against P. multocida type A strains.

Ninety three isolates of bacteria provisionally identified as P. multocida isolated from pneumonic calf lungs were subjected to biochemical, hyaluronidase decapsulation and serological tests in order to identify P. multocida of Carter's capsular serotype A. The type A strains were then used in immunological studies in mice. Their surface antigens (lipopolysaccharides, capsules and outer membranes) were also isolated and characterised.

Biochemical and serological tests indicated that only 22.5% of the isolates were typical P. multocida type A. Eighteen percent were identified as atypical P. multocida type A while 7.5% belonged to taxon 13. Variations were shown in the biochemical reactions of the type A strains but these variations did not correlate with the pathogenicity or immunogenicity of the strains. The antigenic analysis of the taxon 13 strains indicated that they were very similar to type A strains.

Adjuvant vaccines containing formalin-killed whole cells, heat-killed whole cells, lipopolysaccharides and capsules were used to immunise mice. Only formalin-killed and heat-killed vaccines gave good active protection. Diversity was also observed using heat-killed vaccine in which only one type A strain (W674) afforded significant cross-protection. None of the taxon 13 strains immunised mice against homologous challenge or gave significant heterologous protection.

The pathogenicity of the type A strains in mice as determined by LD₅₀ titratioris showed that the strains were not highly pathogenic. There was however, no correlation between their pathogenicity and their immunogenicity.

The type A and taxon 13 strains were classified into eleven groups based on the electrophoretic mobility of four of the major proteins in their outer membrane as analysed by SDS-polyacrylamide gel electrophoresis. The outer membrane protein patterns of the strains were found to be unrelated to their pathogenicity and immunogenicity.

The characterisation of OM and LPS using enzyme-linked immunoabsorbent assay (ELISA) and immunoblots and the characterisation of the capsular antigens by the use of fused rocket immunoelectrophoresis did not show that these extracts or their antigens re correlated with protection. The ELISA of the whole cell antigens with heterologous immune sera agreed wtih these results in that only shared group-specific antigens were detected but not protective antigens. This indicated that the whole cell vaccine induced immunity in mice against challenge infections, but the antigens responsible for protection were not identified by the techniques employed in this study.