Isoniazid is active against susceptible bacteria only when they are undergoing cell division. Susceptible bacteria may continue to undergo one or two divisions before multiplication is arrested. Isoniazid can inhibit the synthesis of mycolic acids, which are essential components of mycobacterial cell walls. The mycobacterial enzyme cata-lase-peroxidase KatG activates the administered isoni-azid to its biologically active form. The target sites for the activated isoniazid action are acyl carrier protein AcpM and Kas A, a (3-ketoaceyl carrier protein synthetase that blocks mycolic acid synthesis. Isoniazid exerts its lethal effects at the target sites by forming covalent complexes.
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