First documentation of isoniazid reversion in Mycobacterium tuberculosis.
BACKGROUND
Drug-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.


OBJECTIVE
To document the occurrence of isoniazid (INH) reversion in a patient with multidrug-resistant tuberculosis (TB) and investigate its association with fitness cost.


METHODS
Genotypic 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.


RESULTS
Genotyping 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.


CONCLUSIONS
This 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.
Department of Pathology, Stanford University School of Medicine, Stanford, California, USA.
E T
ET
Richardson
S-Y G
SY
Lin
B A
BA
Pinsky
E
E
Desmond
N
N
Banaei
D000995:Antitubercular Agents / Q000627:therapeutic use*; D015373:Bacterial Typing Techniques; D002374:Catalase / Q000235:genetics; D004269:DNA, Bacterial / Q000302:isolation & purification; D024901:Drug Resistance, Multiple, Bacterial / Q000235:genetics; D004359:Drug Therapy, Combination; D029968:Escherichia coli Proteins / Q000235:genetics; D005260:Female; D056084:Genetic Fitness; D005838:Genotype; D006801:Humans; D007538:Isoniazid / Q000627:therapeutic use*; D008826:Microbial Sensitivity Tests; D008875:Middle Aged; D009154:Mutation; D009169:Mycobacterium tuberculosis / Q000187:drug effects* / Q000235:genetics / Q000472:pathogenicity; D018384:Oxidative Stress; D010641:Phenotype; D012641:Selection, Genetic; D013183:Sputum / Q000382:microbiology; D016896:Treatment Outcome; D018088:Tuberculosis, Multidrug-Resistant / Q000188:drug therapy* / Q000382:microbiology
D000995:Antitubercular Agents; D004269:DNA, Bacterial; D029968:Escherichia coli Proteins; D002374:Catalase; C051253:katG protein, E coli; D007538:Isoniazid