Molecular evolution in a multidrug-resistant lineage of Streptococcus pneumoniae: Emergence of strains belonging to the serotype 6B Icelandic clone that lost antibiotic resistance traits

Since their first detection in 1988, penicillin-resistant Streptococcus pne umoniae isolates have rapidly spread in Iceland to account for close to 20% of all pneumococcal disease in that country by 1993. The major component ( 70%) of the resistant pneumococci identified from 1989 to 1992 was the prog eny of a single multidrug-resistant clone (Icelandic clone) with a homogene ous chromosomal macrorestriction profile and identical multilocus enzyme ty pe expressing serotype 6B and resistance to penicillin, tetracycline, chlor amphenicol erythromycin, and trimethoprim-sulfamethoxazole. The rest of the non-penicillin-susceptible isolates included bacteria with serotype 6A and serogroups 19 and 23, The unique geographic and epidemiological setting an d the availability of a complete collection of all non-penicillin-susceptib le isolates of S. pneumoniae in Iceland prompted us to carry out a molecula r epidemiological study to monitor the fate of the Icelandic clone between 1989 and 1996; in addition, we wished to extend the characterization to rep resentative groups of all non-penicillin-susceptible serotype 611 pneumococ ci which showed variations in antibiotype and which were recovered in Icela nd between late 1989 and the end of 1996. Also included in the study mere n on-penicillin-susceptible isolates of serogroup 23. Pulsed-field gel electr ophoresis of SmaI-restricted chromosomal DNA and Southern hybridization wit h the lytA DNA probe and probes specific for antibiotic resistance genes we re used to characterize pneumococcal isolates. The results show that (i) th e Icelandic clone remained the predominant type among penicillin-resistant S. pneumoniae through 1996; (ii) the emergence of variants of the Icelandic clone which had lost one or more of the antibiotic resistance phenotypes a nd/or resistant genes, singly or in combination, was documented during the surveillance period; and (iii) isolates belonging to the internationally sp read multidrug-resistant serotype 23F clone were present in the Icelandic c ollection since late 1989 but did not increase in number during the subsequ ent gears.