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Journal of Clinical Microbiology, April 2000, p. 1375-1381, Vol. 38, No. 4
0095-1137/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.

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

Sigurdur E. Vilhelmsson,1,2 Alexander Tomasz,1 and Karl G. Kristinsson2,*

The Rockefeller University, New York, New York,1 and National University Hospital, Reykjavik, Iceland2

Received 29 October 1999/Returned for modification 24 December 1999/Accepted 22 January 2000

Since their first detection in 1988, penicillin-resistant Streptococcus pneumoniae 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 progeny of a single multidrug-resistant clone (Icelandic clone) with a homogeneous chromosomal macrorestriction profile and identical multilocus enzyme type expressing serotype 6B and resistance to penicillin, tetracycline, chloramphenicol, 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 and the availability of a complete collection of all non-penicillin-susceptible isolates of S. pneumoniae in Iceland prompted us to carry out a molecular epidemiological study to monitor the fate of the Icelandic clone between 1989 and 1996; in addition, we wished to extend the characterization to representative groups of all non-penicillin-susceptible serotype 6B pneumococci which showed variations in antibiotype and which were recovered in Iceland between late 1989 and the end of 1996. Also included in the study were non-penicillin-susceptible isolates of serogroup 23. Pulsed-field gel electrophoresis of SmaI-restricted chromosomal DNA and Southern hybridization with the lytA DNA probe and probes specific for antibiotic resistance genes were used to characterize pneumococcal isolates. The results show that (i) the 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 and/or resistant genes, singly or in combination, was documented during the surveillance period; and (iii) isolates belonging to the internationally spread multidrug-resistant serotype 23F clone were present in the Icelandic collection since late 1989 but did not increase in number during the subsequent years.

* Corresponding author. Mailing address: Department of Microbiology, National University Hospital, P. O. Box 1465, 121 Reykjavik, Iceland. Phone: (354) 560 1952. Fax: (354) 560 1957. E-mail: karl{at}rsp.is.

Journal of Clinical Microbiology, April 2000, p. 1375-1381, Vol. 38, No. 4
0095-1137/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.


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