A major focus of the laboratory has been defining the mechanisms by which mycobacterial populations diversify and the functional consequences of this heterogeneity. We have demonstrated that Mtb is relatively genetically stable yet we, and others, have shown that there is dramatic phenotypic heterogeneity at a single cell level. We have found that this phenotypic variation arises through many mechanisms, including asymmetric growth and division and DNA methylation-based regulation of gene expression and can generate privileged bacterial subpopulations that are capable of surviving antibiotic stress.
We have also discovered that the variability in infection outcomes that we observe in TB lesions in humans and nonhuman primates is recapitulated at the single cell level. When an immunophenotypically homogenous population of human macrophages is infected with Mtb, bulk analysis suggests a slight expansion of the bacterial population. However, at a single cell level we find tremendous variability in outcomes. Even in the absence of exogenous activator, human macrophages kill many bacteria but a subpopulation of Mtb survives and replicates robustly. The variable outcomes of Mtb infection of human macrophages are likely to reflect heterogeneity in the bacterial and macrophage populations, providing a tractable system in which to identify determinants of natural heterogeneity in infection outcome. We hypothesize that there is heterogeneity in the bacterial population that contributes to differences in the outcome of infection at a single cell level and that these differences set differences in lesion trajectory in vivo. We take multidisciplinary approaches to test this hypothesis.
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