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Mechanistic stratification in electroactive biofilms of Geobacter sulfurreducens mediated by pilus nanowires.

ABSTRACT: Electricity generation by Geobacter sulfurreducens biofilms grown on electrodes involves matrix-associated electron carriers, such as c-type cytochromes. Yet, the contribution of the biofilm's conductive pili remains uncertain, largely because pili-defective mutants also have cytochrome defects. Here we report that a pili-deficient mutant carrying an inactivating mutation in the pilus assembly motor PilB has no measurable defects in cytochrome expression, yet forms anode biofilms with reduced electroactivity and is unable to grow beyond a threshold distance (?10??m) from the underlying electrode. The defects are similar to those of a Tyr3 mutant, which produces poorly conductive pili. The results support a model in which the conductive pili permeate the biofilms to wire the cells to the conductive biofilm matrix and the underlying electrode, operating coordinately with cytochromes until the biofilm reaches a threshold thickness that limits the efficiency of the cytochrome pathway but not the functioning of the conductive pili network.

SUBMITTER: Steidl RJ

PROVIDER: S-EPMC4974642 | biostudies-literature | 2016 Aug

REPOSITORIES: biostudies-literature

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Mechanistic stratification in electroactive biofilms of Geobacter sulfurreducens mediated by pilus nanowires.

Steidl Rebecca J RJ Lampa-Pastirk Sanela S Reguera Gemma G

Nature communications 20160802

Electricity generation by Geobacter sulfurreducens biofilms grown on electrodes involves matrix-associated electron carriers, such as c-type cytochromes. Yet, the contribution of the biofilm's conductive pili remains uncertain, largely because pili-defective mutants also have cytochrome defects. Here we report that a pili-deficient mutant carrying an inactivating mutation in the pilus assembly motor PilB has no measurable defects in cytochrome expression, yet forms anode biofilms with reduced el ...[more]

PMID: 27481214

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Project description:Geobacter sulfurreducens is commonly employed as a model for the study of extracellular electron transport mechanisms in the Geobacter species. Deletion of pilB, which is known to encode the pilus assembly motor protein for type IV pili in other bacteria, has been proposed as an effective strategy for evaluating the role of electrically conductive pili (e-pili) in G. sulfurreducens extracellular electron transfer. In those studies, the inhibition of e-pili expression associated with pilB deletion was not demonstrated directly but was inferred from the observation that pilB deletion mutants produced lower current densities than wild-type cells. Here, we report that deleting pilB did not diminish current production. Conducting probe atomic force microscopy revealed filaments with the same diameter and similar current-voltage response as e-pili harvested from wild-type G. sulfurreducens or when e-pili are expressed heterologously from the G. sulfurreducens pilin gene in Escherichia coli. Immunogold labeling demonstrated that a G. sulfurreducens strain expressing a pilin monomer with a His tag continued to express His tag-labeled filaments when pilB was deleted. These results suggest that a reinterpretation of the results of previous studies on G. sulfurreducens pilB deletion strains may be necessary. IMPORTANCE Geobacter sulfurreducens is a model microbe for the study of biogeochemically and technologically significant processes, such as the reduction of Fe(III) oxides in soils and sediments, bioelectrochemical applications that produce electric current from waste organic matter or drive useful processes with the consumption of renewable electricity, direct interspecies electron transfer in anaerobic digestors and methanogenic soils and sediments, and metal corrosion. Elucidating the phenotypes associated with gene deletions is an important strategy for determining the mechanisms for extracellular electron transfer in G. sulfurreducens. The results reported here demonstrate that we cannot replicate the key phenotype reported for a gene deletion that has been central to the development of models for long-range electron transport in G. sulfurreducens.

Project description:A family of five periplasmic triheme cytochromes (PpcA-E) was identified in Geobacter sulfurreducens, where they play a crucial role by driving electron transfer from the cytoplasm to the cell exterior and assisting the reduction of extracellular acceptors. The thermodynamic characterization of PpcA using NMR and visible spectroscopies was previously achieved under experimental conditions identical to those used for the triheme cytochrome c(7) from Desulfuromonas acetoxidans. Under such conditions, attempts to obtain NMR data were complicated by the relatively fast intermolecular electron exchange. This work reports the detailed thermodynamic characterization of PpcB, PpcD, and PpcE under optimal experimental conditions. The thermodynamic characterization of PpcA was redone under these new conditions to allow a proper comparison of the redox properties with those of other members of this family. The heme reduction potentials of the four proteins are negative, differ from each other, and cover different functional ranges. These reduction potentials are strongly modulated by heme-heme interactions and by interactions with protonated groups (the redox-Bohr effect) establishing different cooperative networks for each protein, which indicates that they are designed to perform different functions in the cell. PpcA and PpcD appear to be optimized to interact with specific redox partners involving e(-)/H(+) transfer via different mechanisms. Although no evidence of preferential electron transfer pathway or e(-)/H(+) coupling was found for PpcB and PpcE, the difference in their working potential ranges suggests that they may also have different physiological redox partners. This is the first study, to our knowledge, to characterize homologous cytochromes from the same microorganism and provide evidence of their different mechanistic and functional properties. These findings provide an explanation for the coexistence of five periplasmic triheme cytochromes in G. sulfurreducens.

Dataset Information

Mechanistic stratification in electroactive biofilms of Geobacter sulfurreducens mediated by pilus nanowires.

Publications

Mechanistic stratification in electroactive biofilms of Geobacter sulfurreducens mediated by pilus nanowires.

Similar Datasets

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