• E T Richardson, S-Y G Lin, B A Pinsky, E Desmond, and N Banaei.
• Department of Pathology, Stanford University School of Medicine, Stanford, California, USA.
• Int. J. Tuberc. Lung Dis. 2009 Nov 1;13(11):1347-54.

BackgroundDrug-resistant strains of Mycobacterium tuberculosis are increasing worldwide and pose a major threat to global health. However, it remains unsettled whether drug-resistant mutants are fixed in the bacterial population or if they would revert in the absence of drug pressure.ObjectiveTo document the occurrence of isoniazid (INH) reversion in a patient with multidrug-resistant tuberculosis (TB) and investigate its association with fitness cost.DesignGenotypic and phenotypic assays were used to characterize the reversion of INH resistance in isolates from a patient with pulmonary TB. The pre-reversion katG mutation was reconstructed in a pan-susceptible laboratory strain (H37Rv DeltakatG::katG W300G) and tested for susceptibility to INH and oxidative stress.ResultsGenotyping and drug susceptibility testing showed that an isogenic strain of M. tuberculosis reverted from an INH-resistant to a susceptible phenotype in the absence of INH therapy. The genotypic basis of this reversion was mapped to the katG codon 300 which reverted from GGG (glycine, G) to a wild-type codon, TGG (tryptophan, W). The H37Rv DeltakatG::katG W300G mutant was resistant to INH, but also showed a deficiency in coping with oxidative stress.ConclusionThis study confirms that, in the absence of INH pressure, some INH-resistant mutants will revert to a drug-susceptible phenotype. This finding may have broader implications for INH-resistant strains and for the clinically useful lifespan of INH.

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