EFFECT OF GENETIC SWITCHING OF CAPSULAR TYPE ON VIRULENCE OF STREPTOCOCCUS-PNEUMONIAE

To assess the role of capsular serotypes in the virulence of Streptoco ccus pneumoniae, we have constructed isogenic derivatives differing on ly in the type of capsule expressed. Strains of types 2, 5, and 6B wer e converted to type 3 by transformation and selection for an erythromy cin resistance marker linked to the type 3 capsule locus. Characteriza tion studies revealed that these type 3 derivatives were indistinguish able from the type 2, type 5, and type 6B parental strains in terms of restriction enzyme fragment pattern and expression of pneumococcal su rface protein A (PspA). Expression of the type 3 capsule did not alter the mouse virulence of the similarly virulent type 2 strain. However, alteration of capsule type had a profound effect on the virulence of the type 5 and type 6B derivatives. The highly virulent type 5 strain was essentially avirulent when expressing the type 3 capsule. Converse ly, the 50% lethal dose of the relatively avirulent type 6B strain was reduced > 100-fold when the type 3 capsule was expressed. Thus, the s erotype of capsule expressed has a major effect on virulence, and this effect is dependent upon the genetic background of the recipient stra in.