Molecular characterization of isoniazid-resistant clinical isolates of Mycobacterium tuberculosis from the USA.

Drug-resistant tuberculosis poses a significant problem for treatment. The mechanisms of resistance to the front-line drug isoniazid (INH) are complex and can be mediated by katG, inhA and other unknown genes. To identify the percentage of INH-resistant strains with no katG or inhA mutation, this study characterized a panel of 28 clinical isolates of Mycobacterium tuberculosis and five mutants derived from H37Rv resistant to INH. Seventeen of 33 resistant strains (51 %) had katG mutations with 12 of the 17 strains having the most common KatG Ser315Thr mutation. Three of the 17 strains with the KatG 315 mutation had an additional mutation in the inhA promoter and were resistant to a high level of INH. Seventeen of the 33 INH-resistant strains (51 %) had inhA mutations. The most common inhA promoter mutation was -15C-->T and was present in 13 of the 17 inhA mutations. This promoter mutation occurred alone without katG mutations and was associated with a low level of INH and ethionamide resistance. However, other inhA mutations were associated with katG mutations. No mutations were found in the ndh gene. Three of 33 strains (9 %) had no mutations in katG, inhA or ndh, indicating that their resistance was due to a new mechanism of resistance. Detection of the KatG Ser315Thr mutation and the -15C-->T inhA mutation accounted for 76 % (25/33) of the INH-resistant strains and should be useful for rapid detection of INH-resistant strains by molecular tests.