Type variation of strains of Streptococcus pneumoniae in capsular serogroup 15.

The repeated finding of two capsular types of Streptococcus pneumoniae in serogroup 15 in infected exudate from the middle ear led to the demonstration of type variation in pneumococcal types 15B and 15C. Determination of the chemical composition of the capsular polysaccharides of the pneumococci in serogroup 15 showed that the observed variation was related to the presence of an O-acetyl group in the capsular polysaccharide of type 15B which was lacking from the otherwise identical polysaccharide of type 15C. The phenomenon appears similar to that reported in several other bacterial species in which it has been ascribed to labile inversion of a segment of DNA.