Oxygen depletion induced dormancy in Mycobacterium smegmatis

We report here that the physiological behaviour of the fastgrowing saprophytic Mycobacterium smegmatis under in vitro oxygen-depletion and reactivation conditions is strikingly similar to the characteristics shown by the slowgrowing pathogenic M. tuberculosis. M. smegmatis died rapidly when shifted abruptly from aerobic to anaerobic conditions. In contrast to the lethal shock of abrupt oxygen depletion, the slow depletion through a selfgenerated oxygen gradient permitted an adaptation to a persistent state which showed increased resistance against the bactericidal effects of anaerobiosis. The anaerobic persistent culture did not synthesise DNA and showed synchronised division upon reactivation in oxygen rich medium, indicating that the persistent bacilli are uniformly arrested at a defined stage of the cell cycle. Upon reactivation the persistent culture started synthesising DNA only after the first cell division, suggesting that the persistent cells contain two chromosomes. Furthermore, the persistent culture developed sensitivity to metronidazole and resistance against ofloxacin. These results suggest that M. smegmatis might be useful as a fastgrowing non- pathogenic model for comparative molecular analyses of mycobacterial dormancy.