Bacterial nanotubes and their role as bacterial nanowires in Pseudomonas aeruginosa biofilms

Microbial nanowires are nanofilaments that could offer an extracellular electron transfer (EET) pathway linking the bacterial respiratory chain to external surfaces, such as oxidized metals in the environment and engineered electrodes in renewable energy devices. Filaments proposed to function as nanowires have been reported in multiple bacteria, yet it remains largely unclear about the composition and electron transfer mechanism of bacterial nanowires. Pseudomonas aeruginosa is an environmental and electrochemically active bacterium. In this study, we found nanotube-like extracellular filaments in P. aeruginosa biofilms, which were bacterial membrane extensions similar to the nanowires reported in Shewanella oneidensis. Remarkably, conductive probe atomic force microscope showed measurable conductivity of these extracellular filaments, suggesting that they may function as nanowires in P. aeruginosa. Our results also indicated that the electron shuttle pyocyanin significantly affected the conductivity of P. aeruginosa nanowires, suggesting that the electron transfer mechanism of P. aeruginosa nanowires was different from S. oneidensis. Furthermore, factors that impact biofilm formation, such as flagella, type IV pili, and exopolysaccharides, were not essential for nanowires formation, while affect the formation and length of nanowires of P. aeruginosa. Taken together, this is the first report that investigated the role of electron shuttle on the conductivity of nanowires and factors that affected nanowires formation.