High intracellular level of guanosine tetraphosphate in Mycobacterium smegmatis changes the morphology of the bacterium.

Almost one-third of the world population today harbors the tubercle bacillus asymptomatically. It is postulated that the morphology and staining pattern of the long-term persistors are different from those of actively growing culture. Interestingly, it has been found that the morphology and staining pattern of the starved in vitro population of mycobacteria is similar to the persistors obtained from the lung lesions. In order to delineate the biochemical characteristics of starved mycobacteria, Mycobacteria smegmatis was grown in 0.2% glucose as a sole carbon source along with an enriched culture in 2% glucose. Accumulation of the stringent factor guanosine tetraphosphate (ppGpp) with a concomitant change in morphology was observed for M. smegmatis under carbon-deprived conditions. In addition, M. smegmatis assumed a coccoid morphology when ppGpp was ectopically produced by overexpressing Escherichia coli relA, even in an enriched medium. The Mycobacterium tuberculosis relA and spoT homologue, when induced in M. smegmatis, also resulted in the overproduction of ppGpp with a change in the bacterium's growth characteristics.