Dormant ovoid cells of Mycobacterium tuberculosis are formed in response to gradual external acidification.

It is believed that latent tuberculosis is associated with the persistence of Mycobacterium tuberculosis (MTB) in a dormant-like state. Dormant cells of MTB with coccoid morphology were produced in some in vivo studies, but similar forms were not produced in the known in vitro models in sufficient amounts to permit their characterization. This work demonstrates the efficient formation of phase-dark ovoid cells in MTB cultures within 150 days after the onset of stationary phase. During this time the medium underwent gradual acidification (pH 8.5 → 4.7) as a result of cellular metabolism. A rapid change in the external pH resulted in cell degradation and death. In common with the dormant forms found in other organisms, the ovoid cells had thickened cell walls, a low metabolic activity and elevated resistance to antibiotics and heating. The ovoid cells had lost the ability to form colonies on solid medium and were thus regarded as operationally «non-culturable». At an early stage in the acidification process (about 40 days post inoculation), the ovoid cells self-resuscitated when placed in fresh liquid medium. However, ovoid cells, stored for a prolonged time, required supernatant from active MTB cells, or externally added recombinant form of resuscitation promoting factor (Rpf) for successful resuscitation. It is suggested that the adaptation of cellular metabolism leading to gradual acidification of the external medium results in the formation of morphologically distinct dormant MTB cells in vitro. The model of MTB dormancy developed here could be a useful tool for the development of new drugs against latent TB.