| dc.description.abstract | Mycobacterial populations have evolved various strategies to withstand diverse stress
conditions in the environment. One among these strategies is maintenance of phenotypic
heterogeneity, in terms of cell morphology, length, growth rate and division, metabolic status,
and gene expression among others, in a genetically identical population. Phenotypically
divergent but genetically identical subpopulations have been observed in mycobacterial
populations in Tuberculosis patients, in animal model studies as well as in in vitro cultures.
Phenotypic heterogeneity, in mycobacterial populations, based on cell length has been studied
but the molecular level differences and the benefit of such variations remain unknown.
In this regard, our laboratory had earlier reported the occurrence of highly deviated
asymmetric cell division in a subpopulation of septating mycobacterial cultures giving rise to
one short cell (SC) and one normal/long cell (NC). The proportion of cells undergoing highly
deviated asymmetric cell division in a mid-log phase population remains consistent. The
existence of short cells and normal cells was also observed in freshly diagnosed pulmonary TB
patient’s sputum samples, indicating the clinical relevance of cell-length based heterogeneity
in pathogenic Mycobacterium tuberculosis strains in TB patients. Our laboratory had also
reported the physiological relevance of cell-length based phenotypic heterogeneity of
mycobacterial SC and NC populations upon exposure to oxidative stress, nitrite stress and
antibiotics stress. The SC subpopulation was found to be significantly more susceptible to all
the stress conditions than the NC subpopulation. However, the molecular basis for the
differential susceptibility between the SC and NC subpopulations needed investigation.
Therefore, in the present study, we wanted to find out the molecular level differences between
the SC and NC subpopulations that are responsible for their differential tolerance/susceptibility
and the benefit such differences bestowed on the population upon antibiotic exposure.
The data chapter that presents the benefit of the differential
levels of hydroxyl radical generation in M. smegmatis SCs and NCs. Data showed that
differential levels of hydroxyl radical generation was the reason for differential susceptibility
of SCs and NCs, but with clear benefits to both the subpopulations that help their survival
against anti-tuberculosis antibiotics. Hydroxyl radical is known to create genome wide nonspecific
mutations which can give rise to resisters in a population. Differential levels of
hydroxyl radical generation in SCs and NCs inflicted mutations in their genome, resulting in
differential mutability or differential propensity for resister generation in SCs and NCs. This
was demonstrated by plating unexposed SCs and NCs on various anti-tuberculosis antibiotics,
selection for the resisters and confirming the presence of mutations on the hot spot regions of
the target genes. Data showed a differential propensity of resister generation between SCs and
NCs when selected on rifampicin, moxifloxacin and isoniazid. Mutation frequency of these
cells against different antibiotics was also calculated and compared. ROS scavengers or
inhibitors abolished the difference or reduced the mutation frequency in these cells. The overall
experiments and the results therefrom have been discussed in this Chapter, which is concluded
with a model depicting the overall data of the study | en_US |
