Optical O₂ sensors also respond to redox active molecules commonly secreted by bacteria

Abstract

From a metabolic perspective, molecular oxygen (O₂) is arguably the most significant constituent of Earth’s atmosphere. Nearly every facet of microbial physiology is sensitive to the presence and concentration of O₂, which is the most favorable terminal electron acceptor used by biology and also a dangerously reactive oxidant. As O₂ has such sweeping implications for physiology, researchers have developed diverse approaches to measure O₂ concentrations in natural and laboratory settings. Recent improvements to phosphorescent O₂ sensors piqued our interest due to the promise of optical measurement of spatiotemporal O₂ dynamics. However, we found that our preferred bacterial model, Pseudomonas aeruginosa PA14, secretes more than one molecule that quenches such sensors, complicating O₂ measurements in PA14 cultures and biofilms. Assaying supernatants from cultures of 9 bacterial species demonstrated that this phenotype is common: all supernatants quenched a soluble O₂ probe substantially. Phosphorescent O₂ probes are often embedded in solid support for protection, but an embedded probe called O₂ NS was quenched by most supernatants as well. Measurements using pure compounds indicated that quenching is due to interactions with redox-active small molecules including phenazines and flavins. Uncharged and weakly-polar molecules like pyocyanin were especially potent quenchers of O₂ NS. These findings underscore that optical O₂ measurements made in the presence of bacteria should be carefully controlled to ensure that O2, and not bacterial secretions, is measured, and motivate the design of custom O₂ probes for specific organisms to circumvent sensitivity to redox-active metabolites.