Elucidating virulence effectors important for M. tuberculosis survival in the host macrophage

AbstractElucidating virulence effectors important for M. tuberculosis survival in the host macrophageBy Katie A. LienDoctor of Philosophy in Molecular and Cell BiologyUniversity of California, BerkeleyProfessor Sarah Stanley, ChairThe host-pathogen interaction during M. tuberculosis infection is dynamic and complicated. Many mechanisms of cell intrinsic control are up-regulated in response to M. tuberculosis infection. However, M. tuberculosis regulates its immediate environment and forms a replicative niche inside phagosomes, the same vesicles in which most pathogens die. In the introductory chapter of this dissertation, I highlight the major ways in which macrophages, the cell type M. tuberculosis predominantly infects, are activated during infection to become inhospitable to invading pathogens. The most important are mechanisms mediated by IFN-γ, a cytokine that regulates an intricate network of pathways. I then address a few ways in which M. tuberculosis is able to inhibit or counteract host antimicrobial responses and persist in the phagosome. Specifically, I address the bacteria’s ability to survive and mitigate the effects of acid and oxidative stress in the phagolysosome. In the second chapter, I profile differences between the mouse and human macrophage environment. IFN-γ stimulated human macrophages infected with M. tuberculosis up-regulate mechanisms of cell intrinsic control, but are unable to restrict bacterial growth. This is in stark contrast to mouse macrophages that display significant restriction of M. tuberculosis growth when activated with IFN-γ. The differences in macrophage environment have physiological consequences for the bacteria. During M. tuberculosis infection of human macrophages, bacteria undergo nitrogen metabolism and produce robust amounts of nitrite. During mouse macrophage infection, nitrite detected is produced by the host as a byproduct of IFN-γ activation and production of NO. To indiscriminately identify genes that are essential for survival in the different physiological environments of human and mouse macrophages, I perform a transposon-sequencing screen. Preliminary analysis of this data set reveals identification of new genes that will be used to spur future research and elucidate mechanisms of survival.In chapter three, I characterize a bacterial organelle that is essential for M. tuberculosis survival under conditions of oxidative and pH stress. The nanocompartment is a proteinaceous organelle that packages a dye-decoloring peroxidase (DypB) inside. Unlike most peroxidases, DypB is optimally active at acidic pH levels and has an essential function in conditions where other enzymes are inactivated. This is in part due to its optimal activity at low pH, but also because the peroxidase is packaged in the protective environment of the nanocompartment shell. Packaging of the dye-decoloring peroxidase inside of the nanocompartment is necessary for maximal survival of M. tuberculosis when bacteria are exposed to oxidative and acid stress. Additionally, nanocompartment mutants are attenuated for growth during macrophage infection, proving nanocompartments mediate bacterial survival within the phagolysosome.I close with a final chapter in which I speculate on the future directions of each project. In particular, I discuss the newer technology of random barcoded transposon sequencing (Bar-seq). Bar-seq will have a tremendous impact on improving the essentiality predictions of M. tuberculosis genes as it is better suited for analyzing slow growing bacteria that have smaller magnitudes of separation between attenuated and fully-virulent mutants. Additionally, the simple and cost-effective design of Bar-seq will allow for high-throughput screening of M. tuberculosis mutants through hundreds of chemical conditions to improve gene annotation. As for the role of bacterial nanocompartments, one of the looming questions is whether the organelle is stable enough to function extracellularly in a phagolysosome and protect bacteria. Future directions will further characterize the organelle versus organelle battle between host phagolysosomes and bacterial nanocompartments