Bacterial persisters are a subpopulation of cells that can tolerate lethal concentrations of antibiotics. However, the possibility of the emergence of genetically resistant mutants from antibiotic persister cell populations, upon continued exposure to lethal concentrations of antibiotics, remained unexplored. In the present study, we found that Mycobacterium tuberculosis cells exposed continuously to lethal concentrations of rifampin (RIF) or moxifloxacin (MXF) for prolonged durations showed killing, RIF/MXF persistence, and regrowth phases. RIF-resistant or MXFresistant mutants carrying clinically relevant mutations in the rpoB or gyrA gene, respectively, were found to emerge at high frequency from the RIF persistence phase population. A Luria-Delbruck fluctuation experiment using RIF-exposed M. tuberculosis cells showed that the rpoB mutants were not preexistent in the population but were formed de novo from the RIF persistence phase population. The RIF persistence phase M. tuberculosis cells carried elevated levels of hydroxyl radical that inflicted extensive genome-wide mutations, generating RIF-resistant mutants. Consistent with the elevated levels of hydroxyl radical-mediated genome-wide random mutagenesis, MXF-resistant M. tuberculosis gyrA de novo mutants could be selected from the RIF persistence phase cells. Thus, unlike previous studies, which showed emergence of genetically resistant mutants upon exposure of bacteria for short durations to sublethal concentrations of antibiotics, our study demonstrates that continuous prolonged exposure of M. tuberculosis cells to lethal concentrations of an antibiotic generates antibiotic persistence phase cells that form a reservoir for the generation of genetically resistant mutants to the same antibiotic or another antibiotic. These findings may have clinical significance in the emergence of drug-resistant tubercle bacilli.

The present study shows the existence of two specific sub-populations of Mycobacterium smegmatis and Mycobacterium tuberculosis cells differing in size and density, in the mid-log phase (MLP) cultures, with significant differential susceptibility to antibiotic, oxidative, and nitrite stress. One of these sub-populations (~10% of the total population), contained short-sized cells (SCs) generated through highly-deviated asymmetric cell division (ACD) of normal/long-sized mother cells and symmetric cell divisions (SCD) of short-sized mother cells. The other sub-population (~90% of the total population) contained normal/long-sized cells (NCs). The SCs were acid-fast stainable and heat-susceptible, and contained high density of membrane vesicles (MVs, known to be lipid-rich) on their surface, while the NCs possessed negligible density of MVs on the surface, as revealed by scanning and transmission electron microscopy. Percoll density gradient fractionation of MLP cultures showed the SCs-enriched fraction (SCF) at lower density (probably indicating lipid-richness) and the NCs-enriched fraction (NCF) at higher density of percoll fractions. While live cell imaging showed that the SCs and the NCs could grow and divide to form colony on agarose pads, the SCF, and NCF cells could independently regenerate MLP populations in liquid and solid media, indicating their full genomic content and population regeneration potential. CFU based assays showed the SCF cells to be significantly more susceptible than NCF cells to a range of concentrations of rifampicin and isoniazid (antibiotic stress), H2O2 (oxidative stress),and acidified NaNO2 (nitrite stress). Live cell imaging showed significantly higher susceptibility of the SCs of SC-NC sister daughter cell pairs, formed from highly-deviated ACD of normal/long-sized mother cells, to rifampicin and H2O2, as compared to the sister daughter NCs, irrespective of their comparable growth rates. The SC-SC sister daughter cell pairs, formed from the SCDs of short-sized mother cells and having comparable growth rates, always showed comparable stress-susceptibility. These observations and the presence of M. tuberculosis SCs and NCs in pulmonary tuberculosis patients' sputum earlier reported by us imply a physiological role for the SCs and the NCs under the stress conditions. The plausible reasons for the higher stress susceptibility of SCs and lower stress susceptibility of NCs are discussed.

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A classic example of antibiotic inactivating function in bacteria is the Mycobacterium smegmatis (Msm) encoded rifampicin-inactivating mono-ADP-ribosyl transferase (arr). Since its probable biological role has been proposed to be in DNA damage response, which is inflicted by reactive oxygen species (ROS), in the present study, we examined whether Msm Arr influences ROS levels. For this purpose, the levels of the ROS, hydroxyl radical and superoxide, were determined in the mid-log phase (MLP) cells of Msm arr knockout (arr-KO) strain, in comparison to those in the equivalently grown Msm arr+ wild-type (WT) strain. The MLP arr-KO cells generated significantly elevated levels of superoxide and hydroxyl radical, unlike the equivalently grown WT MLP cells. Complementation of arr-KO with arr, but not with empty vector, restored the ROS levels comparable to those in the WT strain. Elevated ROS levels in the arr-KO strain enabled selection of rifampicin-resistant mutants at 10-7 cfu/ml from the rifampicin-unexposed MLP cells of arr-KO, which is one-log10 higher than that for WT cells (10-8). Upon prolonged exposure to rifampicin, the susceptibility, persister formation, generation of elevated levels of hydroxyl radical by the persisters, rifampicin-resister generation frequency of the persisters and regrowth of the rifampicin-resistant mutants from the respective persisters were all comparable between the arr-KO and WT strains. These observations revealed that Arr influences ROS levels in the actively growing M. smegmatis cells but not in the rifampicin-exposed cells. We proposed the probable pathway through which Arr might be influencing ROS levels in the actively growing M. smegmatis cells.IMPORTANCEDiverse genera of bacteria consisting of pathogens, opportunistic pathogens and non-pathogens, possess Arr-type activities that confer equally efficient rifampicin resistance, thereby posing serious health hazard. Acquisition of this function by other bacteria through horizontal gene transfer enhances the hazard posed by the bacteria possessing it. M. smegmatis is an opportunistic human pathogen that causes infections of skin and soft tissues. Moreover, M. smegmatis is a genetically tractable model organism for M. tuberculosis with the potential to function even as tuberculosis vaccine. In view of these significant aspects of Arr and M. smegmatis, the study to find out the natural physiological role of Arr in M. smegmatis, gains importance for designing strategies to prevent antibiotic inactivation and to target the cellular function to contain the bacterium. Above all, the three-dimensional structure of M. smegmatis Arr reveals significant structural homology with eukaryotic ADP-ribosyltransferases and bacterial toxins, thereby giving the study broad significance.