Somatic and capsular factors of coliforms which affect resistance to bovine serum bactericidal activity

Killing of bacteria which cause acute coliform mastitis was studied in vitro, using serum without exogenous complement. Using 10% freshly collected whole serum, it was found that some strains were killed rapidly and could be classified as serum sensitive and others were killed less rapidly and could be classified as intermediate serum sensitive. With other strains, fewer were killed, and then the survivors multiplied; these could be classified as serum resistant. A 50-percentage kill of serum-resistant strains could not be achieved with serum (even in concentrations of greater than 90% serum), whereas 15 strains showed 50-percentage kill at appropriate serum concentrations. Resistance of Escherichia coli to serum bactericidal activity was greatest in strains with large amounts of heat-labile capsule present. The capsule could be demonstrated, using a test for somatic inagglutinability and a hemagglutination-inhibition test. Resistance of Klebsiella pneumoniae to serum bactericidal activity was not related to somatic inagglutinability. In K pneumoniae, resistance to serum bactericidal activity was related to presence of somatic antigens on the resistant strain.