Capsular transformation of a multidrug-resistant Streptococcus pneumoniae in vivo

Several multidrug-resistant (MDR) isolates of Streptococcus pneumoniae recovered from a variety of clinical sources and expressing serotypes 9N, 14, 19F, and 3 could not be distinguished from the capsular type 23 Spanish/USA epidemic clone in antibiotype, pulsed-field gel electrophoretic pattern, and restriction fragment length polymorphism types of their penicillin binding protein genes 1A, 2X, and 2B. When tested in a mouse model of virulence, isolates expressing capsular type 3 were lethal for 100% of infected animals within 2 days after intraperitoneal injection of 10(2) cfu/mouse. In contrast, several capsular type 23F isolates belonging to the MDR Spanish/USA clone and recovered from the same site were not lethal, even when injected at the dose of 10(7) cfu/mouse. These data suggest that the pneumococcal isolates expressing serotypes 9N, 14, 19F, and 3 represent products of in vivo capsular transformation events in which the MDR epidemic capsular type 23F Spanish/USA clone was the recipient.