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Antimicrobial Agents and Chemotherapy, February 2000, p. 355-361, Vol. 44, No. 2
0066-4804/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.

New Gene Cassettes for Trimethoprim Resistance, dfr13, and Streptomycin-Spectinomycin Resistance, aadA4, Inserted on a Class 1 Integron

Peter V. Adrian,1,* Christopher J. Thomson,2 Keith P. Klugman,1 and Sebastian G. B. Amyes2

Department of Medical Microbiology, University of the Witwatersrand, and the South African Institute for Medical Research, Johannesburg, South Africa,1 and Molecular Chemotherapy, Department of Medical Microbiology, University of Edinburgh, Edinburgh, United Kingdom2

Received 11 June 1999/Returned for modification 23 August 1999/Accepted 3 November 1999

In a previous survey of 357 trimethoprim-resistant isolates of aerobic gram-negative bacteria from commensal fecal flora, hybridization experiments showed that 25% (90 of 357) of the isolates failed to hybridize to specific oligonucleotide probes for dihydrofolate reductase types 1, 2b, 3, 5, 6, 7, 8, 9, 10, and 12. Subsequent cloning and sequencing of a plasmid-borne trimethoprim resistance gene from one of these isolates revealed a new dihydrofolate reductase gene, dfr13, which occurred as a cassette integrated in a site-specific manner in a class 1 integron. The gene product shared 84% amino acid identity with dfr12 and exhibited a trimethoprim inhibition profile similar to that of dfr12. Gene probing experiments with an oligonucleotide probe specific for this gene showed that 12.3% (44 of 357) of the isolates which did not hybridize to probes for other dihydrofolate reductases hybridized to this probe. Immediately downstream of dfr13, a new cassette, an aminoglycoside resistance gene of the class AADA [ANT(3")(9)-I], which encodes streptomycin-spectinomycin resistance, was identified. This gene shares 57% identity with the consensus aadA1 (ant(3")-Ia) and has been called aadA4 (ant(3")-Id). The 3' end of the aadA4 cassette was truncated by IS26, which was contiguous with a truncated form of Tn3. On the same plasmid, pUK2381, a second copy of IS26 was associated with sul2, which suggests that both integrase and transposase activities have played major roles in the arrangement and dissemination of antibiotic resistance genes dfr13, aadA4, blaTEM-1, and sul2.

* Corresponding author. Mailing address: Department of Medical Microbiology, SAIMR, P.O. Box 1038, Johannesburg 2000, South Africa. Phone: 27 11 489 9335. Fax: 27 11 489 9332. E-mail: petera{at}mail.saimr.wits.ac.za.

Antimicrobial Agents and Chemotherapy, February 2000, p. 355-361, Vol. 44, No. 2
0066-4804/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.


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