Pasturella multocida is pathogenic for various animals and fowls.  Recently a Type A, serotype 111 strain has been isolated from a fatal case of septicemia in a harbor seal.  Past studies on vaccines for pasteurellosis have implicated both the capsular material and the 0-type antigen. The purpose of the present study was to isolate high molecular weight surface carbohydrates which were safe, immunogenic, and induced the appropriate immune response with a minimum number of injections.

The organisms were grown in 20 liters of Yeast-Extract - Proteose Peptone - Cystine broth (shaking culture, 24 brs) and then gently shaken for 24 hrs with glass beads.  The culture supernatant was treated with 40% ammonium sulfate, centrifuged, subjected to alkaline digestion (pH 9.0), neutralized, and separated on Sepharose 4B.  The high molecular weight fraction consisted of polysaccharide, 8% protein, no ketodeoxyoctonate, and trace nucleic acid. The polysaccharide was neutral as shown by affinity chromatography and the yield was 3.1 mg/liter.

LD₅₀ in mice injected intranasally (IN) was 3.5 x 10². Immunization of mice with 15 ug5 intraperitoneally protected 60% of the mice challenged IN with 3.5 x 10⁵ organisms.

California sea lions were injected intramuscularly with 500 ug polysaccharide and boosted with a similar dosage 3 months later.  Bactericidal activity of 8000 against the homologous organism and antibody titers of 1:400 as detected by the Elisa method were noted in the 7, 14, and 28 day post-booster sera.

The candidate vaccine was non-toxic and from results of the present study may prove to be an effective vaccine for long term protection against pasturellosis in marine mammals.