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Antimicrobial Agents and Chemotherapy, December 2003, p. 3799-3805, Vol. 47, No. 12
0066-4804/03/$08.00+0     DOI: 10.1128/AAC.47.12.3799-3805.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.

ethA, inhA, and katG Loci of Ethionamide-Resistant Clinical Mycobacterium tuberculosis Isolates

Glenn P. Morlock,* Beverly Metchock, David Sikes, Jack T. Crawford, and Robert C. Cooksey

Division of AIDS, STD, and TB Laboratory Research, National Center for HIV, STD and TB Prevention, Centers for Disease Control and Prevention, Atlanta, Georgia 30333

Received 16 June 2003/ Returned for modification 24 July 2003/ Accepted 19 September 2003

Ethionamide (ETH) is a structural analog of the antituberculosis drug isoniazid (INH). Both of these drugs target InhA, an enzyme involved in mycolic acid biosynthesis. INH requires catalase-peroxidase (KatG) activation, and mutations in katG are a major INH resistance mechanism. Recently an enzyme (EthA) capable of activating ETH has been identified. We sequenced the entire ethA structural gene of 41 ETH-resistant Mycobacterium tuberculosis isolates. We also sequenced two regions of inhA and all or part of katG. The MICs of ETH and INH were determined in order to associate the mutations identified with a resistance phenotype. Fifteen isolates were found to possess ethA mutations, for all of which the ETH MICs were >=50 µg/ml. The ethA mutations were all different, previously unreported, and distributed throughout the gene. In eight of the isolates, a missense mutation in the inhA structural gene occurred. The ETH MICs for seven of the InhA mutants were >=100 µg/ml, and these isolates were also resistant to >=8 µg of INH per ml. Only a single point mutation in the inhA promoter was identified in 14 isolates. A katG mutation occurred in 15 isolates, for which the INH MICs for all but 1 were >=32 µg/ml. As expected, we found no association between katG mutation and the level of ETH resistance. Mutations within the ethA and inhA structural genes were associated with relatively high levels of ETH resistance. Approximately 76% of isolates resistant to >=50 µg of ETH per ml had such mutations.

* Corresponding author. Mailing address: Tuberculosis/Mycobacteriology Branch, Centers for Disease Control and Prevention, Mail stop F-08, Atlanta, GA 30333. Phone: (404) 639-0147. Fax: (404) 639-5491. E-mail: gmorlock{at}cdc.gov.

Antimicrobial Agents and Chemotherapy, December 2003, p. 3799-3805, Vol. 47, No. 12
0066-4804/03/$08.00+0     DOI: 10.1128/AAC.47.12.3799-3805.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.



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