Bidirectional extracellular electron transfer pathways of Geobacter sulfurreducens biofilms: Molecular insights into extracellular polymeric substances.

The basis for bioelectrochemical technology is the capability of electroactive bacteria (EAB) to perform bidirectional extracellular electron transfer (EET) with electrodes, i.e. outward- and inward-EET. Extracellular polymeric substances (EPS) surrounding EAB are the necessary media for EET, but the biochemical and molecular analysis of EPS of Geobacter biofilms on electrode surface is largely lacked. This study constructed Geobacter sulfurreducens-biofilms performing bidirectional EET to explore the bidirectional EET mechanisms through EPS characterization using electrochemical, spectroscopic fingerprinting and proteomic techniques. Results showed that the inward-EET required extracellular redox proteins with lower formal potentials relative to outward-EET. Comparing to the EPS extracted from anodic biofilm (A-EPS), the EPS extracted from cathodic biofilm (C-EPS) exhibited a lower redox activity, mainly due to a decrease of protein/polysaccharide ratio and α-helix content of proteins. Furthermore, less cytochromes and more tyrosine- and tryptophan-protein like substances were detected in C-EPS than in A-EPS, indicating a diminished role of cytochromes and a possible role of other redox proteins in inward-EET. Proteomic analysis identified a variety of redox proteins including cytochrome, iron-sulfur clusters-containing protein, flavoprotein and hydrogenase in EPS, which might serve as an extracellular redox network for bidirectional EET. Those redox proteins that were significantly stimulated in A-EPS and C-EPS might be essential for outward- and inward-EET and warranted further research. This work sheds light on the mechanism of bidirectional EET of G. sulfurreducens biofilms and has implications in improving the performance of bioelectrochemical technology.

Characterization of two novel Fe(III)-reducing and electrogenic bacteria, Shewanella ferrihydritica sp. nov. and Shewanella electrica sp. nov., isolated from mangrove sediment.

Three novel strains in the genus Shewanella, designated A3AT, C31T and C32, were isolated from mangrove sediment samples. They were facultative anaerobic, Gram-stain-negative, rod-shaped, flagellum-harbouring, oxidase- and catalase-positive, electrogenic and capable of using Fe(III) as an electron acceptor during anaerobic growth. Results of phylogenetic analysis based on 16S rRNA gene and genomic sequences revealed that the strains should be assigned to the genus Shewanella. The 16S rRNA gene similarity, average nucleotide identity (ANI) and digital DNA-DNA hybridization (dDDH) values between the isolates and their closely related species were below the respective cut-off values for species differentiation. The 16S rRNA gene similarity, ANI and dDDH values between strains C31T and C32 were 99.7, 99.9 and 99.9 %, respectively, indicating that they should belong to the same genospecies. Based on polyphasic taxonomic approach, two novel species are proposed, Shewanella ferrihydritica sp. nov. with type strain A3AT (GDMCC 1.2732T=JCM 34899T) and Shewanella electrica sp. nov. with type strain C31T (GDMCC 1.2736T=JCM 34902T).

Sub-inhibitory concentrations of ampicillin affect microbial Fe(III) oxide reduction.

Antibiotics are ubiquitous in the iron-rich environments but their roles in microbial reduction of Fe(III) oxides are still unclear. Using ampicillin and Geobacter soli, this study investigated the underlying mechanism by which antibiotic regulated microbial reduction of Fe(III) oxides. Results showed that sub-minimal inhibitory concentrations (sub-MIC) of ampicillin significantly affected ferrihydrite reduction by G. soli, with a stimulatory effect at 1/64 and 1/32 MIC and an inhibitory effect at 1/8 MIC. Increasing ampicillin concentration resulted in increasing cell length and decreasing bacterial zeta potential that were beneficial for ferrihydrite reduction, and decreasing outer membrane permeability that was unfavorable for ferrihydrite reduction. The respiratory metabolism ability was enhanced by 1/64 and 1/32 MIC ampicillin and reduced by 1/8 MIC ampicillin, which was also responsible for regulation of ferrihydrite reduction by ampicillin. The ferrihydrite reduction showed a positive correlation with the redox activity of extracellular polymeric substances (EPS) which was tied to the cytochrome/polysaccharide ratio and the content of α-helices and ß-sheet in EPS. These results suggested that ampicillin regulated microbial Fe(III) oxide reduction through modulating the bacterial morphology, metabolism activity and extracellular electron transfer ability. Our findings provide new insights into the environmental factors regulating biogeochemical cycling of iron.

Geobacter benzoatilyticus sp. nov., a novel benzoate-oxidizing, iron-reducing bacterium isolated from petroleum contaminated soil.

A strictly anaerobic bacterial strain, designated Jerry-YXT, was isolated from petroleum-contaminated soil sampled in China. Strain Jerry-YXT was a Gram-stain-negative bacterium forming reddish colonies. It grew optimally at 30 °C and pH 7.0, and tolerated 1.0 % (w/v) NaCl. Strain Jerry-YXT was able to use fumarate, ferric citrate and ferrihydrite as electron acceptors, and ethanol, acetate and benzoate as electron donors. The major fatty acids of this strain were C16 : 0 and C16 : 1 ω7c/C16 : 1 ω6c (summed feature 3). The 16S rRNA gene sequence-based phylogenetic analysis placed this strain in the genus Geobacter, being most closely related to Geobacter metallireducens (98.2 % similarity), Geobacter hydrogenophilus (98.1 %) and Geobacter grbiciae (98.0 %). The DNA G+C content was 57.6 mol%. The average nucleotide identity and digital DNA-DNA hybridization values between the genomes of strain Jerry-YXT and G. metallireducens GS-15T were 81.8 and 35.4 %, respectively. The results of the polyphasic study allowed the genotypic and phenotypic differentiation of strain Jerry-YXT from its closest species, which suggested that strain Jerry-YXT represents a novel species of the genus Geobacter. The name for the proposed new species is Geobacter benzoatilyticus sp. nov. The type strain is Jerry-YXT (=MCCC 1K05659T=JCM 39190T).