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).

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Bacterial nanowires offer a pathway for extracellular electron transfer (EET) by linking the respiratory chain of bacteria to external surfaces, including oxidized metals in the environment and engineered electrodes in renewable energy devices. Specifically, nanowires of the model metal-reducing bacterium Shewanella oneidensis MR-1 were previously shown to be conductive under non-physiological conditions. Despite the global, environmental, and technological consequences of bacterial nanowire-mediated EET, the composition, electron transport mechanism, and physiological relevance of these appendages remain unclear. The nanowires of S. oneidensis MR-1 were previously thought, but never shown, to be bacterial pili. In addition, the transport mechanism through bacterial nanowires has been the subject of intense debate, with ''metallic-like'' band transport and multistep redox hopping between multiheme cytochromes as the two proposed mechanisms. Here we report the first in vivo observations of the formation and respiratory impact of nanowires in S. oneidensis MR-1. Using live fluorescence measurements and quantitative gene expression analysis, we demonstrate that S. oneidensis MR-1 nanowires are extensions of the outer membrane and periplasm, rather than pilin-based structures. We show, through immunolabeling, that multiheme cytochromes localize to nanowires, in turn supporting the multistep redox hopping model as the transport mechanism. Furthermore, these bacterial nanowires are associated with outer membrane vesicles, structures ubiquitous in Gram-negative bacteria, and occasionally appear as membrane vesicle chains that transition to smoother filaments. Redox-functionalized membrane and vesicular extensions may represent a general microbial strategy for electron transport and energy distribution.

Over the past decade, regulatory proteolysis has emerged as a paradigm for transmembrane signal transduction in all organisms, from bacteria to humans. These conserved proteolytic pathways share a common design that involves the sequential proteolysis of a membrane-bound regulatory protein by two proteases. Proteolysis releases the regulator, which is inactive in its membrane-bound form, into the cytoplasm where it performs its cellular function. One of the best-characterized examples of signal transduction via regulatory proteolysis is the pathway governing the σ(E)-dependent cell envelope stress response in Escherichia coli. In unstressed cells, σ(E) is sequestered at the membrane by the transmembrane anti-sigma factor, RseA. Stresses that compromise the cell envelope and interfere with the proper folding of outer membrane proteins (OMPs) activate the proteolytic pathway. The C-terminal residues of unfolded OMPs bind to the inner membrane protease, DegS, to initiate the proteolytic cascade. DegS removes the periplasmic domain of RseA creating a substrate for the next protease in the pathway, RseP. RseP cleaves RseA in the periplasmic region in a process called regulated intramembrane proteolysis (RIP). The remaining fragment of RseA is released into the cytoplasm and fully degraded by the ATP-dependent protease, ClpXP, with the assistance of the adaptor protein, SspB, thereby freeing σ(E) to reprogram gene expression. A growing body of evidence indicates that the overall proteolytic framework that governs the σ(E) response is used to regulate similar anti-sigma factor/sigma factor pairs throughout the bacterial world and has been adapted to recognize a wide variety of signals and control systems as diverse as envelope stress responses, sporulation, virulence, and iron-siderophore uptake. In this chapter, we review the extensive physiological, biochemical, and structural studies on the σ(E) system that provide remarkable insights into the mechanistic underpinnings of this regulated proteolytic signal transduction pathway. These studies reveal design principles that are applicable to related proteases and regulatory proteolytic pathways in all domains of life.

In limiting oxygen as an electron acceptor, the dissimilatory metal-reducing bacterium Shewanella oneidensis MR-1 rapidly forms nanowires, extensions of its outer membrane containing the cytochromes MtrC and OmcA needed for extracellular electron transfer. RNA sequencing (RNA-Seq) analysis was employed to determine differential gene expression over time from triplicate chemostat cultures that were limited for oxygen. We identified 465 genes with decreased expression and 677 genes with increased expression. The coordinated increased expression of heme biosynthesis, cytochrome maturation, and transport pathways indicates that S. oneidensis MR-1 increases cytochrome production, including the transcription of genes encoding MtrA, MtrC, and OmcA, and transports these decaheme cytochromes across the cytoplasmic membrane during electron acceptor limitation and nanowire formation. In contrast, the expression of the mtrA and mtrC homologs mtrF and mtrD either remains unaffected or decreases under these conditions. The ompW gene, encoding a small outer membrane porin, has 40-fold higher expression during oxygen limitation, and it is proposed that OmpW plays a role in cation transport to maintain electrical neutrality during electron transfer. The genes encoding the anaerobic respiration regulator cyclic AMP receptor protein (CRP) and the extracytoplasmic function sigma factor RpoE are among the transcription factor genes with increased expression. RpoE might function by signaling the initial response to oxygen limitation. Our results show that RpoE activates transcription from promoters upstream of mtrC and omcA. The transcriptome and mutant analyses of S. oneidensis MR-1 nanowire production are consistent with independent regulatory mechanisms for extending the outer membrane into tubular structures and for ensuring the electron transfer function of the nanowires. IMPORTANCEShewanella oneidensis MR-1 has the capacity to transfer electrons to its external surface using extensions of the outer membrane called bacterial nanowires. These bacterial nanowires link the cell's respiratory chain to external surfaces, including oxidized metals important in bioremediation, and explain why S. oneidensis can be utilized as a component of microbial fuel cells, a form of renewable energy. In this work, we use differential gene expression analysis to focus on which genes function to produce the nanowires and promote extracellular electron transfer during oxygen limitation. Among the genes that are expressed at high levels are those encoding cytochrome proteins necessary for electron transfer. Shewanella coordinates the increased expression of regulators, metabolic pathways, and transport pathways to ensure that cytochromes efficiently transfer electrons along the nanowires.

BackgroundThe cell envelope of a bacterial pathogen can be damaged by harsh conditions in the environment outside a host and by immune factors during infection. Cell envelope stress responses preserve the integrity of this essential compartment and are often required for virulence. Bordetella species are important respiratory pathogens that possess a large number of putative transcription factors. However, no cell envelope stress responses have been described in these species. Among the putative Bordetella transcription factors are a number of genes belonging to the extracytoplasmic function (ECF) group of alternative sigma factors, some of which are known to mediate cell envelope stress responses in other bacteria. Here we investigate the role of one such gene, sigE, in stress survival and pathogenesis of Bordetella bronchiseptica.ResultsWe demonstrate that sigE encodes a functional sigma factor that mediates a cell envelope stress response. Mutants of B. bronchiseptica strain RB50 lacking sigE are more sensitive to high temperature, ethanol, and perturbation of the envelope by SDS-EDTA and certain β-lactam antibiotics. Using a series of immunocompromised mice deficient in different components of the innate and adaptive immune responses, we show that SigE plays an important role in evading the innate immune response during lethal infections of mice lacking B cells and T cells. SigE is not required, however, for colonization of the respiratory tract of immunocompetent mice. The sigE mutant is more efficiently phagocytosed and killed by peripheral blood polymorphonuclear leukocytes (PMNs) than RB50, and exhibits decreased cytotoxicity toward macrophages. These altered interactions with phagocytes could contribute to the defects observed during lethal infection.ConclusionsMuch of the work on transcriptional regulation during infection in B. bronchiseptica has focused on the BvgAS two-component system. This study reveals that the SigE regulon also mediates a discrete subset of functions associated with virulence. SigE is the first cell envelope stress-sensing system to be described in the bordetellae. In addition to its role during lethal infection of mice deficient in adaptive immunity, our results indicate that SigE is likely to be important for survival in the face of stresses encountered in the environment between hosts.

Dissimilatory metal-reducing bacteria can extract free energy from their environment by performing electron transfer to solid-phase minerals outside the cell. This extracellular electron transport (EET) has important implications in global elemental cycles as well as renewable energy technologies. Among

Bacterial nanowires made by Geobacter sulfurreducens were fırst described about a decade ago. These structures form only when cells are grown SUMMARY ➤ When Shewanella oneidensis bacteria are not near enough to the external electron acceptors that they require, one way the cells can close that gap is by producing structures called nanowires.➤ S. oneidensis cells facilitate extracellular electron transfer (EET) in at least three ways.➤ Unlike the electrically conductive pilus-like appendages studied in Geobacter sulfurreducens, S. oneidensis nanowires arise from chains of outer membrane vesicles that eventually convert to filaments.➤ Many other types of gram-negative bacteria, including some pathogens of humans, extrude similar vesicles but for other purposes.➤ How these outer membrane vesicles form and how this process is regulated in S. oneidensis is poorly understood.