Dormant forms of Mycobacterium smegmatis with distinct morphology

1 Bach Institute of Biochemistry, Russian Academy of Sciences, Moscow, Russia 2 Winogradsky Institute of Microbiology, Russian Academy of Sciences, Moscow, Russia 3 Skryabin Institute of Biochemistry and Physiology of Microorganisms, Russian Academy of Sciences, Pushchino, Moscow oblast, Russia Correspondence Arseny Kaprelyants arseny{at}inbi.ras.ru Cultivation of Mycobacterium smegmatis cells in a nitrogen-limited minimal medium (SR-1) followed by prolonged storage at room temperature without shaking resulted in the gradual accumulation of morphologically distinct ovoid forms characterized by (i) low metabolic activity; (ii) elevated resistance to antibiotics and to heat treatment; and (iii) inability to produce colonies on standard agar plates (non-platable cells). Detailed microscopic examination confirmed that ovoid cells possessed an intact cell envelope, specific fine structure and large electron-transparent bodies in the cytoplasm. Cell staining with Nile red and analysis of the lipid content by TLC revealed the presence of significant amounts of apolar lipids in these bodies. The ovoid forms could be stored for significant periods (up to 5 months) and resuscitated afterwards in a modified Sauton's medium. Importantly, resuscitation of ovoid cells was accompanied by their transformation into the typical rod-shaped cells. We suggest that the observed ovoid cells represent dormant forms, resembling morphologically distinct cells of Mycobacterium tuberculosis previously isolated from tuberculosis patients and infected animals. Abbreviations: MTB, Mycobacterium tuberculosis ; PI, propidium iodide; TB, tuberculosis