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The Nuanced Effects of Redox-Active Metabolites on Bacterial Physiology and Antibiotic Susceptibility


Andrade Meirelles, Lucas (2022) The Nuanced Effects of Redox-Active Metabolites on Bacterial Physiology and Antibiotic Susceptibility. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/67p2-q992.


The production of secondary metabolites is widespread throughout the tree of life. Bacteria, including many relevant opportunistic pathogens, can make redox-active secondary metabolites, both in the environment and while causing infections. Yet, their physiological consequences for the microbial communities exposed to them are much less understood. This thesis investigates the multifaceted and nuanced effects that such metabolites can have on their producers and other bacteria found in the producer's vicinity, focusing on the role these molecules play as modulators of antibiotic susceptibility. I start by presenting a literature review addressing the link between secondary metabolite production and resilience to clinical antibiotics in diverse opportunistic and enteric bacterial pathogens.

Next, using Pseudomonas aeruginosa (a widespread opportunistic pathogen) and its endogenously produced metabolite called pyocyanin, I explore the nuanced effects of the metabolite's production throughout the producer's lifecycle. Pyocyanin is part of a class of redox-active molecules made by P. aeruginosa called phenazines. I show that the production of pyocyanin, due to its self-poisoning effects, is a "double-edged sword," where the ultimate consequences for the producer are directly dependent on the physiological and environmental conditions. Carbon source limitation plays a major role in the self-poisoning effect of pyocyanin, a process responsible for killing a subpopulation of cells that, through extracellular DNA release, seems critical for proper biofilm development.

Despite pyocyanin's toxicity, P. aeruginosa is remarkably tolerant to its harmful effects. For this reason, I then explore how P. aeruginosa handles the stress caused by the metabolite. I present results using a functional genomics approach (transposon-sequencing) to screen for genes involved in P. aeruginosa tolerance to pyocyanin. Defenses involved in pyocyanin tolerance are similar to ones involved in tolerance to clinical antibiotics. These shared mechanisms lead to testing the hypothesis that defenses induced by the production of or exposure to "natural antibiotics" (such as pyocyanin) may affect the efficacy of treatments with clinical antibiotics. Supporting this hypothesis, exposure to pyocyanin significantly induces tolerance and resistance to certain clinical drugs, both in P. aeruginosa and other opportunistic pathogens within the Burkholderia cepacia complex (Bcc). Pyocyanin and the drugs affected, such as fluoroquinolones, share molecular structure similarities, which is likely responsible for the shared protection.

Finally, based on these results, I explore the broader role of redox-active metabolites as modulators of antibiotic resilience in opportunistic pathogens. I show that pyocyanin, another phenazine called phenazine-1-carboxylic acid, and a non-phenazine redox-active molecule called toxoflavin can all modulate antibiotic susceptibility in Bcc species. Depending on the antibiotic's class, the metabolites' presence can either antagonize or potentiate the drug's efficacy. All the studied metabolites are produced by clinical isolates that infect cystic fibrosis and other immunocompromised patients. I demonstrate that the modulator effect of redox-active molecules in the pathogens is dependent on the transcription factor SoxR, which senses the presence of the metabolites and induces specific redox-regulated efflux systems that are effective in transporting both the metabolites and the structurally related drugs. To end, I provide a proof-of-principle that including such metabolites during clinical drug susceptibility tests may lead to a more accurate assessment of pathogens' resistance profile.

Taken together, the findings presented in this thesis demonstrate that redox-active secondary metabolites have profound effects on the physiology and antibiotic sensitivity levels of opportunistic pathogens. Their modulator effect on antibiotic susceptibility is likely a widespread phenomenon in polymicrobial communities that has been overlooked and may have direct consequences for the evolution of antibiotic resistance. Understanding the physiological roles of these metabolites at the molecular level is essential for accurate predictions of the drugs and pathogens affected, which may lead to more effective treatment strategies.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Secondary metabolites; bacteria; antibiotics; antibiotic tolerance; antibiotic resistance; redox-active metabolites; opportunistic pathogens; bacterial physiology; Pseudomonas aeruginosa; Burkholderia cepacia complex; efflux pumps; oxidative stress; phenazines; pyocyanin; toxoflavin
Degree Grantor:California Institute of Technology
Division:Biology and Biological Engineering
Major Option:Biology
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Newman, Dianne K.
Thesis Committee:
  • Elowitz, Michael B. (chair)
  • Bronner, Marianne E.
  • Leadbetter, Jared R.
  • Newman, Dianne K.
Defense Date:17 December 2021
Record Number:CaltechTHESIS:02152022-081613451
Persistent URL:
Related URLs:
URLURL TypeDescription adapted for Chapter 2 adapted for Chapter 3 adapted for Chapter 4 adapted for Chapter 5
Andrade Meirelles, Lucas0000-0003-3194-7136
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:14497
Deposited By: Lucas Andrade Meirelles
Deposited On:07 Mar 2022 17:16
Last Modified:08 Nov 2023 00:46

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