[Streptomycin-dependent mutants of a Pasteurella multocida strain isolated from poultry]

A culture of strain Pasteurella multocida 115, isolated from a hen affected with fowl cholera, was treated with 50 mcg/cm3 streptomycin and subjected to selection at an additional treatment with 200 mcg/cm3 of the same antibiotic. A total of 16 streptomycin-resistant mutant strains were obtained. All of them were shown to be avirulent in testing their pathogenicity for mice, however, two of the mutants were of lowered virulence in tests with birds. Fourteen days post vaccination birds that survived were challenged with 1000 LD of the initial wild strain 115. Two of the mutants only did not induce immunity. In cross immunologic testing of the birds with Pasteurella strains X73, 1059, 1662, and 3940 best immunologic properties showed mutants 115.2 and 115.3. In agglutination of sera from the vaccinated birds with HCl--and formalin-treated antigen of strain 115 the titers were low, ranging from 1:10 to 1:40.