Mycobacterial populations are known for the heterogeneity in terms of cell size, morphology, and metabolic status, which are believed to help the population survive under stress conditions. Such population heterogeneity had been observed in TB patients, in animal models, and in in vitro cultures. Also, the physiological relevance of population heterogeneity under nutrient starvation has been studied. However, the physiological significance of population heterogeneity in oxidative and nitrite stress has not been addressed yet. Our laboratory had earlier shown that a subpopulation of mycobacterial mid-log phase cultures divide by highly deviated asymmetric division, generating short cells and normal-sized/long cells. This proportion has been found to be consistent and reproducible, and has been found in the freshly diagnosed pulmonary tuberculosis patients’ sputum, which is known to have high levels of oxidative stress. The highly deviated asymmetric cell division has been found to be one of the mechanisms that mycobacteria use to generate cell size heterogeneity in the population. However, the physiological significance of the population heterogeneity generated by the highly deviated asymmetric division remained to be addressed. Therefore, in the present study, we addressed the physiological significance of the generation of population heterogeneity in terms of cell size in Mycobacterium smegmatis and Mycobacterium tuberculosis. In this regard, we explored whether the minor subpopulation of short cells generated in the population has any relevance in the response of mycobacteria to oxidative and nitrite stress for survival. The Chapter 1, which forms the Introduction to the thesis, gives an extensive literature survey on the phenotypic heterogeneity in diverse bacterial systems and the physiological significance of such heterogeneity. Subsequently, an account of the phenotypic heterogeneity reported in mycobacteria is given, with examples of its significance implicated for survival under nutrient stress. Then an account of various studies on the oxidative and nitrite stress response of mycobacteria and on the genes involved in those processes are given. Further, the present study is justified by stating that so far there has not been any study to find out the physiological relevance of phenotypic heterogeneity on oxidative and nitrite stress response in mycobacteria. Finally, the Introduction is concluded by stating that the present study investigates and reports for the first time the physiological significance of the minor subpopulation of short cells for survival under oxidative and nitrite stress conditions. The Chapter 2 forms the Materials and Methods used in the present study. Here a detailed description of the methods used for the separation of the short cells, their characterisation, stress response, and so on are given in great detail. The Chapter 3 forms the first data chapter that presents results on the nature of response of Mycobacterium smegmatis and Mycobacterium tuberculosis against oxidative and nitrite stress. Here the cell size natural distribution, in terms of short cells and normal-sized/long cells in the mid-log phase population, the fractionation and enrichment of these subpopulations, differential susceptibility of the cells in the fractions to the stress conditions, the enhanced survival of the population upon mixing of these cell populations at the natural proportion, and the decreased survival upon mixing them at unnatural proportion are presented. The differential survival of the short cells and normal-sized/long cells was studied at a variety of stress concentrations for oxidative (H2O2) and nitrite (acidified sodium nitrite, pH 5), cell densities and exposure time to show the robustness of the phenomenon. Enhanced survival upon extended exposure to stress also has been documented. Essentially the data in this chapter shows that although the different sized populations show differential stress susceptibility to the stress conditions, their combined presence at the proportion that naturally exists in the mid-log phase population enhances the survival of the population, at the cost of the highly susceptible short cells for the enhanced survival of the less susceptible normal-sized/long cells, kin selection and altruism. The Chapter concludes with a discussion on the results. The Chapter 4 delineates the mechanism of the altruistic phenomenon that results in the enhanced survival of the population at the sacrifice of the minor subpopulation of short cells. Here we present evidence that hydroxyl radical generated through Fenton reaction is responsible for the enhanced survival through the induction of the synthesis of catalase-peroxidase (KatG) for the degradation of H2O2. The free iron deficient short cells acquire more iron, which in turn becomes stoichiometrically detrimental to them due to the high levels of hydroxyl generation in the presence of H2O2. On the contrary, the free iron containing normal-sized/long cells do not acquire iron and hence the hydroxyl radical produced in the population becomes stoichiometrically beneficial to them. Thus, the deficiency of free iron which consequentially necessitates the short cells to acquire more iron becomes a maladaptive trait in the presence of H2O2 but gets co-opted in kin selection, for the survival of the normal-sized/long cells that form major proportion of the population – a phenomenon reminiscent of altruism. The Chapter concludes with a model depicting the entire phenomenon and a discussion on the results and their implications.