Phenazine Homeostasis in Pseudomonas aeruginosa Biofilms

A bacterial biofilm is a community of sessile cells encased in a matrix composed of polysaccharides, proteins, and extracellular DNA that develops according to a reproducible morphogenic program. This morphogenic program is deeply influenced by prevailing redox conditions within the biofilm, which are established by a gradient of terminal electron acceptor through the depth of the biofilm. Terminal electron acceptor limitation leads to redox stress, measured as an elevated ratio of reduced to oxidized forms of the metabolic cofactor nicotinamide adenine dinucleotide, NAD(H). In biofilms of the gram-negative bacterium Pseudomonas aeruginosa, redox stress is relieved by the presence of diffusible redox-cycling molecules, phenazines, that are able to act as an electrical conduit between intracellular NADH and oxygen in the aerobic zone of the biofilm. This is most apparent in the dramatically hyperspread and hyperwrinkled morphologies observed in colony biofilms unable to produce phenazines. However, the ability of phenazines to act as a biologically relevant redox couple between the reducing equivalents of metabolism and atmospheric oxygen also renders them toxic to producing cells. In order to avoid phenazine toxicity, P. aeruginosa encodes self-resistance mechanisms under the control of the redox-sensitive transcription factor SoxR. Two components of the SoxR regulon, the efflux pump MexGHI-OpmD and the monooxygenase PumA, are known to be major contributors to survival in the presence of toxic concentrations of phenazines. This work further details the role of the small protein MexG (Chapter 3) and PumA in phenazine resistance (Chapter 4), and presents an electrochemical platform for studying the effects of a phenazine redox gradient in biofilm morphogenesis (Chapter 5).