Shewanella oneidensis MR-1 nanowires are outer membrane and periplasmic extensions of the extracellular electron transport components

Abstract

Bacterial nanowires offer an extracellular electron transport (EET) pathway for linking the respiratory chain of bacteria to external surfaces, including oxidized metals in the environment and engineered electrodes in renewable energy devices. Despite the global, environmental, and technological consequences of this biotic-abiotic interaction, the composition, physiological relevance, and electron transport mechanisms of bacterial nanowires remain unclear. We report, to our knowledge, the first in vivo observations of the formation and respiratory impact of nanowires in the model metal-reducing microbe Shewanella oneidensis MR-1. Live fluorescence measurements, immunolabeling, and quantitative gene expression analysis point to S. oneidensis MR-1 nanowires as extensions of the outer membrane and periplasm that include the multiheme cytochromes responsible for EET, rather than pilin-based structures as previously thought. These membrane extensions are associated with outer membrane vesicles, structures ubiquitous in Gram-negative bacteria, and are consistent with bacterial nanowires that mediate long-range EET by the previously proposed multistep redox hopping mechanism. Redox-functionalized membrane and vesicular extensions may represent a general microbial strategy for electron transport and energy distribution. R eduction-oxidation (redox) reactions and electron transport are essential to the energy conversion pathways of living cells (1). Respiratory organisms generate ATP molecules-life's universal energy currency-by harnessing the free energy of electron transport from electron donors (fuels) to electron acceptors (oxidants) through biological redox chains. In contrast to most eukaryotes, which are limited to relatively few carbon compounds as electron donors and oxygen as the predominant electron acceptor, prokaryotes have evolved into versatile energy scavengers. Microbes can wield an astounding number of metabolic pathways to extract energy from diverse organic and inorganic electron donors and acceptors, which has significant consequences for global biogeochemical cycles (2-4).

Microbial nanowires: an electrifying tale

“…A recent study has reported that MNWs in S. oneidensis are composed of extended periplasmic and outer membranes embedded with cytochromes ( Fig. 3c) which further supports the electron hopping model (Pirbadian et al, 2014). However, in S. oneidensis MNWs, it is yet to be proved conclusively that cytochromes are closely spaced enough (1-2 nm) to carry out charge transport over micrometre distances.…”

“…However, MNWs in S. oneidensis are made up of outer membrane vesicle chains which subsequently elongate and become MNWs ( Fig. 3c) (Pirbadian et al, 2014). Unlike pili and flagella, which are mostly homopolymers of a single subunit type, MNWs in S. oneidensis are a concoction of different cytochromes and periplasmic as well as outer membrane components.…”

“…However, so far, it was not ruled out that other extracellular structures (pili and flagella) in S. oneidensis cannot conduct electrons. Also in their study, electrical conductivity measurements of extended membrane extensions were not done (Pirbadian et al, 2014). All extracellular structures produced by S. oneidensis should be isolated and studied independently for their conductive behaviour.…”

Nutritional stress induces exchange of cell material and energetic coupling between bacterial species

“…This hypothesis is compatible with the fact that D. vulgaris does not grow when physical interactions are blocked. Several authors have demonstrated that direct electron transfer might also take place through nanowires [44][45][46][47] and it seems to be the primary way of electron transfer in some methanogenic environments. However, proteins known to be involved in this process are absent from D. vulgaris and C. acetobutylicum.…”

Harnessing the power of microbial nanowires

“…The model representative Shewanella oneidensis MR‐1 was also assumed to produce pilus ‘nanowires’ (Gorby et al ., 2006; El‐Naggar et al ., 2010), although MR‐1 T4P had previously been shown to be nonconductive (Reguera et al ., 2005). These ‘nanowires’ were later found to be dehydrated forms of outer membrane extensions, which this bacterium forms by fusing outer membrane vesicles (Pirbadian et al ., 2014). Theoretical studies suggest that the clusters of outer membrane c‐ type cytochromes (c‐Cyts) that decorate the MR‐1 extensions could diffuse and collide to enable charge transport in vivo (Subramanian et al ., 2018).…”