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Antimicrobial Agents and Chemotherapy, September 1999, p. 2225-2230, Vol. 43, No. 9
0066-4804/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.

Novel Expansions of the Gene Encoding Dihydropteroate Synthase in Trimethoprim-Sulfamethoxazole-Resistant Streptococcus pneumoniae

Thanugarani Padayachee* and Keith P. Klugman

Pneumococcal Diseases Research Unit of the Medical Research Council, The South African Institute for Medical Research and the University of the Witwatersrand, Johannesburg, South Africa

Received 28 April 1999/Returned for modification 8 June 1999/Accepted 14 July 1999

A study of eight sulfonamide-resistant clinical isolates of Streptococcus pneumoniae revealed chromosomal mutations within the gene encoding dihydropteroate synthase that play a role in conferring resistance to sulfamethoxazole. The presence of the suld mutation, found previously only in a laboratory mutant, was shown to occur in three of the wild-type clinical isolates. The duplication of Ser61, the other previously defined mutation in the dihydropteroate synthase gene of S. pneumoniae, was observed in only one of the isolates characterized. We report two previously unidentified amino acid alterations, namely, a duplication of Arg58 and Pro59 and an insertion of an arginine residue between Gly60 and Ser61 in trimethoprim-sulfamethoxazole-resistant strains. The significance of these mutations was confirmed by site-directed mutagenesis and by the transformation of a susceptible strain of S. pneumoniae to sulfamethoxazole resistance. Two resistant isolates did not contain any mutations within the gene encoding dihydropteroate synthase. The results presented suggest the independent generation of resistant mutations among South African clinical isolates. It is also proposed that the mechanism of sulfonamide resistance in S. pneumoniae involves the expansion of a specific region within dihydropteroate synthase, which probably forms part of the sulfonamide binding site.

* Corresponding author. Present address: Department of Clinical Microbiology and Infectious Diseases, SAIMR, P.O. Box 1038, Johannesburg 2000, South Africa. Phone: 27-11-4899335. Fax: 27-11-4899332. E-mail: thanup{at}hotmail.com.

Antimicrobial Agents and Chemotherapy, September 1999, p. 2225-2230, Vol. 43, No. 9
0066-4804/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.


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