Electron Flow in Biofilm Matrix: Impact of Chemical Composition and Context

Bacterial biofilms are a serious global health concern because they are notoriously difficult to eradicate. In biofilms, cells are subjected to resource limitations (e.g., O₂). To promote anaerobic survival, certain microbes, such as Pseudomonas aeruginosa (P.a.), can use diffusible extracellular electron shuttles to transport electrons to O₂ at a distance, a process known as extracellular electron transfer (EET). How do these small molecule electron shuttles catalyze EET in a complex and dynamically changing extracellular matrix without being lost to the environment? In other words, how do electrons move efficiently within the biofilm? Our recent work demonstrated that the extracellular P.a. biofilm matrix supports efficient electron transfer by retaining phenazines, a well-studied electron shuttle class, through binding extracellular DNA (eDNA). This discovery raises questions about the electron transfer mechanisms underpinning EET (e.g., molecular diffusion vs. electron hopping), and how they can be tuned as a function of matrix composition and environmental context. Here we show exopolysaccharides, another key component of the matrix, can compete with the binding between phenazines and eDNA by interacting with eDNA, and these interactions depend on the chemical environment within biofilms (e.g., redox and pH). To provide a mechanistic understanding of EET in biofilms, we fabricated well-defined synthetic hydrogels mimicking P.a. biofilm matrices, and characterized the mechanisms underpinning EET efficiency of both hydrogels and P.a. biofilms with distinct compositions under different conditions. This work begins to shed light on how microbes adapt to changing environmental conditions by producing matrices of different electronic properties, and raises the possibility that the extracellular matrix composition is a strategy whereby diverse microbes optimize EET.