Variation in isoniazid susceptibility of Mycobacterium avium during the cell cycle

A strain Mycobacterium avium that had limited susceptibility to isonicotinic acid hydrazide (INH) at 5 micrograms/ml of medium was tested for its susceptibility under cell cycle conditions. The INH-susceptible cells of M. avium were prevented from elongating if INH was present in the medium and, although there was an initial lag of 15 to 20 h, INH was bactericidal for the susceptible cells. Resurgence of viability in the INH-treated cultures was due to multiplication of INH-resistant cells. Ethidium bromide, if present in the INH-treated cultures, prevented multiplication of the INH-resistant cells.