Genetic relatedness within serotypes of penicillin-susceptible Streptococcus pneumoniae isolates

The molecular epidemiological characteristics of all Streptacoccus pneumoni ae strains isolated in a nationwide manner from patients with meningitis in The Netherlands in 1994 were investigated. Restriction fragment end labeli ng analysis demonstrated 52% genetic clustering among these penicillin-susc eptible strains, a value substantially Ion-er than the percentage of cluste ring among Dutch penicillin-nonsusceptible strains. Different serotypes Her e found within 8 of the 28 genetic clusters, suggesting that horizontal tra nsfer of capsular genes is common among penicillin-susceptible strains. The degree of genetic clustering Has much higher among serotype 3, 7F, 9V; and 14 isolates than among isolates of other serotypes, i.e., 6A, 6B, 18C, 19F , and 23F. We further studied the molecular epidemiological characteristics of pneumococci of serotype 3, which is considered the most virulent seroty pe and which is commonly associated with invasive disease in adults. Fifty epidemiologically unrelated penicillin-susceptible serotype 3 invasive isol ates originating from the United States (n = 27), Thailand (n = 9), The Net herlands (n = 8), and Denmark (n = 6) were analyzed. The vast majority of t he serotype 3 isolates (74%) belonged to two genetically distinct clades th at Here observed in the United States, Denmark, and The Netherlands. These data indicate that two serotype 3 clones have been independently disseminat ed in an international manner. Seven serotype 3 isolates Here less than 85% genetically related to the other serotype 3 isolates. Our observations sug gest that the latter isolates originated from horizontal transfer of the ca psular type 3 gene locus to other pneumococcal genotypes. In conclusion, ep idemiologically unrelated serotype 3 isolates Here genetically more related than those of other serotypes, This observation suggests that serotype 3 h as evolved only recently or has remained unchanged over long periods.