Transmission of multidrug-resistant serotype 23F Streptococcus pneumoniae in group day care: evidence suggesting capsular transformation of the resistant strain in vivo.

Surveillance for nasopharyngeal colonization with Streptococcus pneumoniae was maintained in a research day care center between 1985 and 1992. An outbreak of nasal carriage of a multi-drug-resistant (MDR) serotype 23F organism occurred between May 1990 and December 1991 involving 14 of 52 children. Electrophoresis of penicillin-binding proteins (PBP) and pulsed-field gel electrophoresis (PFGE) of chromosomal DNA indicated that the MDR serotype 23F organism was closely related to a serotype 23F MDR clone that has been prevalent in Spain since the early 1980s. In June 1991, an MDR serotype 14 organism was isolated from a child who had previously carried the MDR serotype 23F strain. PFGE and PBP typing revealed that the MDR serotype 14 organism was very similar to the circulating MDR serotype 23F strain, suggesting serotype transformation. Dissemination of MDR pneumococcal strains and possibly spread of the MDR phenotype to additional serotypes may be facilitated in group day care.