Effect of clonal and serotype-specific properties on the invasive capacity of Streptococcus pneumoniae

The present study compares the molecular epidemiology of Streptococcus pneumoniae causing invasive disease and carriage, respectively, in one geographic area (Stockholm, Sweden) during a specific point in time (the year 1997). A total of 273 invasive isolates (257 from adults and 16 from children) obtained from the 2 major hospitals in Stockholm, as well as 246 nasopharyngeal isolates recovered from children attending 16 day-care centers in the Stockholm area, were analyzed by serotyping, molecular typing (by pulsed-field gel electrophoresis and multilocus sequence typing), and antibiotic susceptibility testing. Of the 34 different serotypes plus nontypeable strains identified in the present study, 12 were never found among the 246 colonizing isolates, whereas only 3 were never found among the 273 invasive isolates. The isolates formed 2 major classes: 1 class that was found mainly among invasive isolates (type 1, 4, 7F, and 9V isolates) and was clonally highly related and 1 class that caused invasive disease but was also common in carriage (including type 6A, 6B, 14, and 19F isolates) and was genetically more diverse. Clones were found that belonged to the same serotype but had different abilities to cause invasive disease. Also, isolates belonging to the same clone were found, although they had different capsules because of serotype switch, and were found to have the same disease potential. Hence, properties associated with a particular clonal type, in addition to capsular serotype, are likely to be important for the potential of pneumococci to cause invasive disease.