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Title:Oxidative stress in anoxic habitats
Author(s):Khademian, Maryam
Director of Research:Imlay, James
Doctoral Committee Chair(s):Imlay, James
Doctoral Committee Member(s):Metcalf, William; Cronan, John; Gennis, Robert
Department / Program:Microbiology
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Oxidative stress
obligate anaerobiosis
reactive oxygen species
hydrogen peroxide
cytochrome c peroxidase, pyruvate:ferredoxin oxidoreductase
pyruvate formate lyase
Abstract:Oxygen and its reactive species —hydrogen peroxide (H2O2), superoxide (O2-) and hydroxyl radicals (HO•)— can cause a plethora of damages in the cell, from DNA damage to inactivation of iron containing enzymes. Virtually all life forms experience oxidative stress to some degree, and they are all equipped with mechanisms to protect themselves and to repair the damaged moieties. While oxidative stress is an obvious caveat of oxic life, it can also affect the organisms in anoxic environments and at oxic- anoxic interfaces. The present body of work looks into the latter condition by investigation the effects of oxidative stress endured by a facultative anaerobe (Escherichia coli) and an obligate anaerobe (Bacteroides thetaiotaomicron). E. coli lives adjacent to the epithelia in mammalian intestines. The encounter of oxygen penetrating from the epithelial layer and the reduced molecules like H2S from the center of the lumen creates an oxic-anoxic interface with different reactive oxygen species that can affect E. coli. B. thetaiotaomicron is another gut-dwelling bacterium living in the intestinal lumen, where it occasionally experiences oxidative stress. Bacteroides species in general are staple members of the gut microbiome because of their role in breaking down complex carbohydrates and thereby providing carbon sources for the rest of the community. Both E. coli and B. thetaiotaomicron should deal with high levels of oxidative stress during pathogenesis and the transition periods between hosts. The first chapter is an introduction to oxidative stress, its targets and the defense mechanisms against it in both of the bacteria discussed above. The second chapter focuses on finding a new periplasmic H2O2 degrading enzyme in E. coli that is only induced in the absence of oxygen and in presence of H2O2. The regulation mechanism of this enzyme, along with the detailed calculations of H2O2 fluxes through cellular membranes, revealed that it cannot protect the cytoplasm against exogenous H2O2. Instead, it enables E. coli to use H2O2 as a terminal electron acceptor for anaerobic respiration. The third chapter of this thesis looks into the molecular basis of obligate anaerobiosis. Previous reports have identified three points of oxidative damage in the central metabolism of B. thetaiotaomicron. Two of these enzymes, fumarase and pyruvate formate lyse (PFL), were reported to be damaged by O2- and molecular O2 respectively. In this study, we show that the third enzyme, pyruvate ferredoxin oxidoreductase (PFOR), is also damaged by molecular O2 itself. Pyruvate dissimilation in this bacterium depends upon PFL and PFOR; therefore, even in the absence of other reactive species O2 can directly inhibit this key metabolic node and block growth. The rate of fumarase damage by O2- approaches the PFL and PFOR inactivation by O2. We infer that the titer of scavenging enzymes in B. thetaiotaomicron is set so targets of H2O2 and O2- are neither more nor less sensitive to aeration than is pyruvate dissimilation. The role of oxygen in the phenomenon of obligate anaerobiosis is therefore multifaceted. In the fourth chapter, I provide a detailed summary of the works done in chapters two and three. Some important questions raised by these studies are also discussed in this chapter. Overall, this thesis attempts to explore different aspects of oxidative stress in anoxic habitats.
Issue Date:2020-06-25
Rights Information:Copyright 2020 Maryam Khademian
Date Available in IDEALS:2020-10-07
Date Deposited:2020-08

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