Abstract
A structured collection of P. multocida strains, containing isolates from different host species, geographic regions and disease categories was assembled from lyophilised preserved isolates. Each strain characterized for capsule serotype and for the known or putative virulence determinants. In 94% of strains, an indirect haemagglutination test allowed unequivocal allocation to a serotype. Capsular antibody raised in rabbits with a course of intravenous injection of formalized suspension furnished high potency and specificity, though differences were found in their antibody content. (serotypes A and D appeared to be less antigenic than B, E and F serotypes). The high success-rate in serotyping was attributed to the quality of the antisera prepared. The virulence determinants investigated were not uniformly present in all serotypes. Capsule and neuraminidase were present in all serotypes but the size of the capsule zone and the amount of neuraminidase varied between serotypes and within each serotype. Protein toxin was detected in 8 strains (5 from serotype A and 3 from D). Four of these strains were of ovine origin. Hyaluronidase (which acted also as chondroitin sulphatase) was produced only by serotype B. Capsular hyaluronic acid was present in only serotype A; a similar, capsular substance in serotype D resisted hyaluronidase and was thought to be an acidic polysaccharide differing from hyaluronic acid of serotype A. NaCl gradient ion exchange paper chromatography proved an effective method for the study of the capsular polysaccharides. A single mouse model for virulence of P. multocida strains was derived from previous work and tested. Statistical analysis revealed significant correlation between the high specific neuraminidase activity of serotypes A, D and F and their high mouse virulence. The hyaluronidase of serotype B appeared also to have some association with high mouse virulence. Mouse avirulent strains of serotype A had higher mean capsule zone size and contained more hyaluronic acid than the virulent strains. The development of severe hypothermia followed by death of mouse at the latter stage of virulent P. multocida infection suggested a possible role of endotoxin in disease: the mode of action of the endotoxin in mice may, at least in part, be through hypothermia.