Isolation or group B streptococci defective for the type antigen Ia by transposon-mediated mutagenesis

Transposon-mediated mutability has been used to isolate the isogenic strains of the group B serotype Ia streptococci with the mutations in the genes coding for the production of capsular antigen. The transconjugants have lost the ability to bind type Ia antiserum as demonstrated by immunoblotting technique. The loss of type-specific antigen by the strains has resulted in a dramatic decrease in virulence for mice while the absorbtion indexes of transconjugants increased 2-3 fold. The mutant clones deficient in capsule expression had the increased buoyant density in the percoll gradient as compared with the parent strains of group B. The stable mutants impaired in ability to produce the polysaccharide capsule on the cell surface were obtained as a result of site-specific insertion of transposon Tn916 into the genome of group B serotype I streptococcus.