In-vitro activity of rifabutin against rifampicin-resistant Mycobacterium tuberculosis isolates with known rpoB mutations

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The relationship between resistance to rifampicin and rifabutin and genetic alterations in the rpoB gene of 41 rifampicin-resistant isolates of Mycobacterium tuberculosis was evaluated. Although 35 isolates with rifampicin MICs ≥ 32 mg/L were also rifabutin-resistant, six isolates with rifampicin MICs of 2–16 mg/L were susceptible to rifabutin (MIC ≤ 0.5 mg/L). Mutations Asp516Val, Asp516Tyr, Leu533Pro and the double mutation Met515Ile and Leu533Pro influenced susceptibility to rifampicin, but not to rifabutin. All mutations at codons 531 and 526, except one isolate with a His526Cys mutation, correlated with resistance to both compounds.

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