Mechanisms of Phenazine-Mediated Extracellular Electron Transfer by Pseudomonas aeruginosa

Citation

Saunders, Scott Harrison (2020) Mechanisms of Phenazine-Mediated Extracellular Electron Transfer by Pseudomonas aeruginosa. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/P4Z5-5445. https://resolver.caltech.edu/CaltechTHESIS:04022020-212557295

Abstract

Extracellular electron transfer (EET), the process whereby cells access electron acceptors or donors that reside many cell lengths away, enables metabolic activity by microorganisms, particularly under oxidant-limited conditions that occur in multicellular bacterial biofilms. Although different mechanisms underpin this process in individual organisms, a potentially widespread strategy involves extracellular electron shuttles, redox-active metabolites that are secreted and recycled by diverse bacteria. Here, I first review general aspects of the electron shuttling strategy, such as the chemical diversity and potential distribution of electron shuttle producers and users, and the costs associated with electron shuttle biosynthesis. Then I address the long-standing question: how do these electron shuttles catalyze electron transfer within biofilms without being lost to the environment? I show that phenazine electron shuttles mediate efficient EET through interactions with extracellular DNA (eDNA) in Pseudomonas aeruginosa biofilms, which are important in nature and disease. Retention of pyocyanin (PYO) and phenazine carboxamide in the biofilm matrix is facilitated by binding to eDNA. In vitro, different phenazines can exchange electrons in the presence or absence of DNA and phenazines can participate directly in redox reactions through DNA; the biofilm eDNA can also support rapid electron transfer between redox-active intercalators. Electrochemical measurements of biofilms indicate that retained PYO supports an efficient redox cycle with rapid EET and slow loss from the biofilm. Together, these results establish that eDNA facilitates phenazine metabolic processes in P. aeruginosa biofilms, suggesting a model for how extracellular electron shuttles achieve retention and efficient EET in biofilms.

Thesis Files