Generation of dormant forms by Mycobacterium smegmatis in the poststationary phase during gradual acidification of the medium

Persistent forms of the wild-type strain of Mycobacterium smegmatis and its mutants with inactivated devR and hlp genes were investigated. devR encodes the regulatory protein responsible for the formation of nonreplicating mycobacterial forms under hypoxia, and hlp codes for a histone-like protein. It has been found that a gradual decrease of pH in M. smegmatis wild-type poststationary cultures resulted in the formation of a special type of persisters. They significantly differed from vegetative cells in their properties and were represented by shortened ovoid forms with thickened cell walls. According to atomic force microscopy data, the size of the ovoid forms and vegetative cells was 1.2 × 0.9 μm and 3.7 × 0.8 μm, respectively. The metabolism level was markedly decreased in ovoid cells: the incorporation of [5,6- 3 H]uracil and thymidine was decreased 200- and 50-fold, respectively. The intracellular ATP content was lowered threefold. The ovoid forms that emerged in poststationary cultures in Sauton’s medium when the medium pH value was gradually decreased to 6.0 retained for a long time (9 months) the capacity to resume growth on rich solid and liquid medium. Compared to vegetative cells, the ovoid forms exhibited an elevated resistance to heating (60–80°C) and antibiotics (hygromycin, kanamycin, and tetracycline). The ovoid forms of the M. smegmatis wild-type strain were classified as dormant forms based on their survival capacity, resistance to deleterious factors, and structural peculiarities. The ovoid forms generated in poststationary cultures upon decreasing the pH value to 5.0 or below lost the colony-forming capacity. It was established that the capacity to form ovoid cells upon gradual decrease in the pH value to 6.0 was reduced in Δ devR and hlp-0 mutants compared to the wild-type strain (generation of 5–6 and 40% dormant forms, respectively) The amount of M. smegmatis dormant cells formed correlated with the acidification degree of the medium. The model developed can be used in tests of new antibacterial preparations that effectively inhibit resuscitating mycobacterial dormant forms that persist in the host organism.