TUBERCULOSIS is responsible for one in four of all avoidable adult deaths in developing countries1. Increased frequency and accelerated fatality of the disease among individuals infected with human immunodeficiency virus has raised worldwide concern that control programmes may be inadequate2, and the emergence of multidrug-resistant strains of Mycobacterium tuberculosis has resulted in several recent fatal outbreaks in the United States3. Isonicotinic acid hydrazide (isoniazid, INH) forms the core of antituberculosis regimens; however, clinical isolates that are resistant to INH show reduced catalase activity and a relative lack of virulence in guinea-pigs4-7. Here we use mycobacterial genetics8,9 to study the molecular basis of INH resistance. A single M. tuberculosis gene, katG, encoding both catalase and peroxidase, restored sensitivity to INH in a resistant mutant of Mycobacterium smegmatis, and conferred INH susceptibility in some strains of Escherichia coli. Deletion of katG from the chromosome was associated with INH resistance in two patient isolates of M. tuberculosis.
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