The Enduring Hypoxic Response of Mycobacterium tuberculosis

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Abstract

Background

A significant body of evidence accumulated over the last century suggests a link between hypoxic microenvironments within the infected host and the latent phase of tuberculosis. Studies to test this correlation have identified the M. tuberculosis initial hypoxic response, controlled by the two-component response regulator DosR. The initial hypoxic response is completely blocked in a dosR deletion mutant.

Methodology/Principal Findings

We show here that a dosR deletion mutant enters bacteriostasis in response to in vitro hypoxia with only a relatively mild decrease in viability. In the murine infection model, the phenotype of the mutant was indistinguishable from that of the parent strain. These results suggested that additional genes may be essential for entry into and maintenance of bacteriostasis. Detailed microarray analysis of oxygen starved cultures revealed that DosR regulon induction is transient, with induction of nearly half the genes returning to baseline within 24 hours. In addition, a larger, sustained wave of gene expression follows the DosR-mediated initial hypoxic response. This Enduring Hypoxic Response (EHR) consists of 230 genes significantly induced at four and seven days of hypoxia but not at initial time points. These genes include a surprising number of transcriptional regulators that could control the program of bacteriostasis. We found that the EHR is independent of the DosR-mediated initial hypoxic response, as EHR expression is virtually unaltered in the dosR mutant.

Conclusions/Significance

Our results suggest a reassessment of the role of DosR and the initial hypoxic response in MTB physiology. Instead of a primary role in survival of hypoxia induced bacteriostasis, DosR may regulate a response that is largely optional in vitro and in mouse infections. Analysis of the EHR should help elucidate the key regulatory factors and enzymatic machinery exploited by M. tuberculosis for long-term bacteriostasis in the face of oxygen deprivation.

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Most cited references 43

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Evaluation of a nutrient starvation model of Mycobacterium tuberculosis persistence by gene and protein expression profiling.

Joanna Betts, Pauline Lukey, Linda Robb … (2002)

The search for new TB drugs that rapidly and effectively sterilize the tissues and are thus able to shorten the duration of chemotherapy from the current 6 months has been hampered by a lack of understanding of the metabolism of the bacterium when in a 'persistent' or latent form. Little is known about the condition in which the bacilli survive, although laboratory models have shown that Mycobacterium tuberculosis can exist in a non-growing, drug-resistant state that may mimic persistence in vivo. Using nutrient starvation, we have established a model in which M. tuberculosis arrests growth, decreases its respiration rate and is resistant to isoniazid, rifampicin and metronidazole. We have used microarray and proteome analysis to investigate the response of M. tuberculosis to nutrient starvation. Proteome analysis of 6-week-starved cultures revealed the induction of several proteins. Microarray analysis enabled us to monitor gene expression during adaptation to nutrient starvation and confirmed the changes seen at the protein level. This has provided evidence for slowdown of the transcription apparatus, energy metabolism, lipid biosynthesis and cell division in addition to induction of the stringent response and several other genes that may play a role in maintaining long-term survival within the host. Thus, we have generated a model with which we can search for agents active against persistent M. tuberculosis and revealed a number of potential targets expressed under these conditions.

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Persistence of Mycobacterium tuberculosis in macrophages and mice requires the glyoxylate shunt enzyme isocitrate lyase.

John McKinney, Kerstin zu Bentrup, Ernesto Muñoz-Elías … (2000)

Mycobacterium tuberculosis claims more human lives each year than any other bacterial pathogen. Infection is maintained in spite of acquired immunity and resists eradication by antimicrobials. Despite an urgent need for new therapies targeting persistent bacteria, our knowledge of bacterial metabolism throughout the course of infection remains rudimentary. Here we report that persistence of M. tuberculosis in mice is facilitated by isocitrate lyase (ICL), an enzyme essential for the metabolism of fatty acids. Disruption of the icl gene attenuated bacterial persistence and virulence in immune-competent mice without affecting bacterial growth during the acute phase of infection. A link between the requirement for ICL and the immune status of the host was established by the restored virulence of delta icl bacteria in interferon-gamma knockout mice. This link was apparent at the level of the infected macrophage: Activation of infected macrophages increased expression of ICL, and the delta icl mutant was markedly attenuated for survival in activated but not resting macrophages. These data suggest that the metabolism of M. tuberculosis in vivo is profoundly influenced by the host response to infection, an observation with important implications for the treatment of chronic tuberculosis.

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An in vitro model for sequential study of shiftdown of Mycobacterium tuberculosis through two stages of nonreplicating persistence.

L. Wayne, L G Hayes (1996)

It was demonstrated previously that abrupt transfer of vigorously aerated cultures of Mycobacterium tuberculosis to anaerobic conditions resulted in their rapid death, but gradual depletion of available O2 permitted expression of increased tolerance to anaerobiosis. Those studies used a model based on adaptation of unagitated bacilli as they settled through a self-generated O2 gradient, but the model did not permit examination of homogeneous populations of bacilli during discrete stages in that adaptation. The present report describes a model based on culture of tubercle bacilli in deep liquid medium with very gentle stirring that keeps them in uniform dispersion while controlling the rate at which O2 is depleted. In this model, at least two stages of nonreplicating persistence were seen. The shift into first stage, designated NRP stage 1, occurred abruptly at a point when the declining dissolved O2 level approached 1% saturation. This microaerophilic stage was characterized by a slow rate of increase in turbidity without a corresponding increase in numbers of CFU or synthesis of DNA. However, a high rate of production of glycine dehydrogenase was initiated and sustained while the bacilli were in this state, and a steady ATP concentration was maintained. When the dissolved O2 content of the culture dropped below about 0.06% saturation, the bacilli shifted down abruptly to an anaerobic stage, designated NRP stage 2, in which no further increase in turbidity was seen and the concentration of glycine dehydrogenase declined markedly. The ability of bacilli in NRP stage 2 to survive anaerobically was dependent in part on having spent sufficient transit time in NRP stage 1. The effects of four antimicrobial agents on the bacilli depended on which of the different physiologic stages the bacilli occupied at a given time and reflected the recognized modes of action of these agents. It is suggested that the ability to shift down into one or both of the two nonreplicating stages, corresponding to microaerophilic and anaerobic persistence, is responsible for the ability of tubercle bacilli to lie dormant in the host for long periods of time, with the capacity to revive and activate disease at a later time. The model described here holds promise as a tool to help clarify events at the molecular level that permit the bacilli to persist under adverse conditions and to resume growth when conditions become favorable. The culture model presented here is also useful for screening drugs for the ability to kill tubercle bacilli in their different stages of nonreplicating persistence.

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Author and article information

Contributors

: Role: Academic Editor

Journal

Journal ID (nlm-ta): PLoS ONE

Journal ID (publisher-id): plos

Journal ID (pmc): plosone

Title: PLoS ONE

Publisher: Public Library of Science (San Francisco, USA )

ISSN (Electronic): 1932-6203

Publication date Collection: 2008

Publication date (Electronic): 30 January 2008

Volume: 3

Issue: 1

Electronic Location Identifier: e1502

Affiliations

[1 ]Seattle Biomedical Research Institute, Seattle, Washington, United States of America

[2 ]Department of Pathobiology, University of Washington, Seattle, Washington, United States of America

[3 ]Fred Hutchinson Cancer Research Center, Seattle, Washington, United States of America

Wellcome Trust Sanger Institute, United Kingdom

Author notes

* To whom correspondence should be addressed. E-mail: david.sherman@ 123456sbri.org

Conceived and designed the experiments: DS TR MH. Performed the experiments: TR RL MH. Analyzed the data: DS TR MH. Wrote the paper: DS TR.

Article

Publisher ID: 07-PONE-RA-02393R1

DOI: 10.1371/journal.pone.0001502

PMC ID: 2198943

PubMed ID: 18231589

SO-VID: 09b1ece0-aa73-487c-a1e7-b60cbbd79bf2

Copyright © Rustad et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

History

Date received : 3 October 2007

Date accepted : 28 December 2007

Page count

Pages: 8

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The Enduring Hypoxic Response of Mycobacterium tuberculosis

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Methodology/Principal Findings

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Point-Of-Care Testing for Tuberculosis

Most cited references 43

Evaluation of a nutrient starvation model of Mycobacterium tuberculosis persistence by gene and protein expression profiling.

Persistence of Mycobacterium tuberculosis in macrophages and mice requires the glyoxylate shunt enzyme isocitrate lyase.

An in vitro model for sequential study of shiftdown of Mycobacterium tuberculosis through two stages of nonreplicating persistence.

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