RECOMBINATIONAL EXCHANGES AT THE CAPSULAR POLYSACCHARIDE BIOSYNTHETICLOCUS LEAD TO FREQUENT SEROTYPE CHANGES AMONG NATURAL ISOLATES OF STREPTOCOCCUS-PNEUMONIAE

Serotype 19F variants of the major Spanish multiresistant serotype 23F clone of Streptococcus pneumoniae have been proposed to have arisen b y recombinational exchanges at the capsular biosynthetic locus. Member s of the Spanish multiresistant serotype 23F clone and the serotype 19 F variants were confirmed to be essentially identical in overall genot ype, as they were indistinguishable by REP-PCR, and had identical sequ ences at three polymorphic housekeeping genes. Eight serotype 19F vari ants were studied and ail had large recombinational replacements at th e capsular biosynthetic locus. In all cases, one of the recombinationa l cross-over points appeared to be upstream of dexB, which flanks one end of the capsular locus, and in six of the variants the other cross- over point was downstream of aliA, which flanks the other end of the l ocus. In two strains a recombinational cross-over point between the in troduced serotype 19F capsular region and that of the Spanish serotype 23F clone could be clearly identified, within cpsN in one strain and within cpsM in the other. The differences in the recombinational junct ions and sequence polymorphisms within the introduced capsular genes, suggested that the eight serotype 19F variants emerged on at least fou r separate occasions. Changes in capsular type by recombination may th erefore be relatively frequent in pneumococci and this has implication s for the long-term efficacy of conjugate pneumococcal vaccines that w ill protect against only a limited number of serotypes.