Dynamics of penicillin-susceptible clones in invasive pneumococcal disease

In a 10-year period, 1987-1997, there was a >4-fold increase in the rate of pneumococcal bacteremia in Sweden. Invasive pneumococcal isolates (n = 113 6), which were obtained from 18 Swedish clinical microbiology laboratories from 1987 through 1997, and other national and international isolates were serotyped, and their clonal relationships were determined by molecular typi ng. The increase in invasive pneumococcal disease in Sweden during this per iod was associated particularly with an increase in isolates of serotypes 1 and 14. A 3-fold increase of type 14 was seen from 1987 through 1992, and a 10-fold increase of type 1 occurred from 1992 through 1997. One dominatin g penicillin-susceptible clone of type 14 was responsible for the increase of type 14 during the first 5 years. This clone also was found in Canada an d the United States and was shown by multilocus sequence typing to correspo nd to a previously identified hyper-virulent clone. A novel penicillin-susc eptible clone of type 1, which was not found among invasive isolates from 1 987 or 1992, was responsible for the increase of serotype I during the last 5 years. These results illustrate the ability of virulent penicillin-susce ptible pneumococcal clones to emerge and spread rapidly within a country.