MOLECULAR MECHANISMS OF ISONIAZID RESISTANCE IN MYCOBACTERIUM-TUBERCULOSIS AND MYCOBACTERIUM-BOVIS

Genetic and biochemical studies have suggested a link between reduced catalase activity and resistance to isoniazid in Mycobacterium tubercu losis. In this study, we examined the molecular mechanisms of resistan ce to isoniazid with six in vitro mutants of the M. tuberculosis compl ex (Mycobacterium bovis and M. tuberculosis), Five of six mutants resi stant to isoniazid were negative by catalase assays. Immunoblot analys es using a polyclonal antibody against the katG gene product (catalase -peroxidase) demonstrated that the enzyme is not produced in four of t hese isoniazid-resistant strains. A complete deletion of the katG gene was detected in only one of these isoniazid-resistant M. tuberculosis complex strains by Southern blot analyses, In two other resistant str ains, partial deletions of the katC gene were identified, A point muta tion which resulted in the insertion of a termination codon in the kat G coding sequence caused a catalase-negative phenotype in a fourth str ain, Of the two resistant strains which produce the enzyme, one was sh own to be negative by a catalase assay. Single-stranded conformational polymorphism and DNA sequence analyses identified a mutation in the k atC gene of this strain which may contribute to reduced enzymatic acti vity and subsequent isoniazid resistance. These data demonstrate that genetic alterations to the katG gene other than complete deletions are prevalent and may contribute significantly to the number of cases of isoniazid-resistant tuberculosis.