Mycobacterial Stationary Phase Induced by Low Oxygen Tension: Cell Wall Thickening and Localization of the 16-Kilodalton α-Crystallin Homolog

Most cases of tuberculosis are due to reactivation of endogenous infection which may have lain quiescent or dormant for decades. How Mycobacterium tuberculosis survives for this length of time is unknown, but it is hypothesized that reduced oxygen tension may trigger the tubercle bacillus to enter a state of dormancy. Mycobacterium bovis BCG and M. tuberculosis H37Rv were cultured under aerobic, microaerobic, and anaerobic conditions. Their ultrastructural morphology was analyzed by transmission electron microscopy (TEM), and protein expression profiles were compared by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). TEM revealed that the microaerobically and anaerobically cultured bacilli but not the aerobically cultured bacilli developed a strikingly thickened cell wall outer layer. The thickening was not observed in aerobically cultured stationary-phase bacilli or in anaerobically cultured Mycobacterium smegmatis. A highly expressed protein was detected by SDS-PAGE in microaerobic and anaerobic cultures and was identified as the 16-kDa small heat shock protein or α-crystallin homolog. Immunolocalization by colloidal gold immunoelectron microscopy identified three patterns of protein distribution in M. bovis BCG cultured under low oxygen tension. The 16-kDa protein was strongly associated with the cell envelope, fibrous peptidoglycan-like structures, and intracellular and peripheral clusters. These results suggest that tubercle bacilli may adapt to low-oxygen conditions by developing a thickened cell wall and that the 16-kDa protein may play a role in stabilizing cell structures during long-term survival, thus helping the bacilli survive the low oxygen tension in granulomas. As such, the cell wall thickening and the 16-kDa protein may be markers for the dormant state of M. tuberculosis.