Exopolysaccharides matrix affects the process of extracellular electron transfer in electroactive biofilm

Microbial Biofilms: Features of Formation and Potential for Use in Bioelectrochemical Devices

Microbial biofilms present one of the most widespread forms of life on Earth. The formation of microbial communities on various surfaces presents a major challenge in a variety of fields, including medicine, the food industry, shipping, etc. At the same time, this process can also be used for the benefit of humans-in bioremediation, wastewater treatment, and various biotechnological processes. The main direction of using electroactive microbial biofilms is their incorporation into the composition of biosensor and biofuel cells This review examines the fundamental knowledge acquired about the structure and formation of biofilms, the properties they have when used in bioelectrochemical devices, and the characteristics of the formation of these structures on different surfaces. Special attention is given to the potential of applying the latest advances in genetic engineering in order to improve the performance of microbial biofilm-based devices and to regulate the processes that take place within them. Finally, we highlight possible ways of dealing with the drawbacks of using biofilms in the creation of highly efficient biosensors and biofuel cells.

Significance of a minor pilin PilV in biofilm cohesion of Geobacter sulfurreducens

Bacterial adhesion plays a vital role in forming and shaping the structure of electroactive biofilms that are essential for the performance of bioelectrochemical systems (BESs). Type IV pili are known to mediate cell adhesion in many Gram-negative bacteria, but the mechanism of pili-mediated cell adhesion of Geobacter species on anode surface remains unclear. Herein, a minor pilin PilV2 was found to be essential for cell adhesion ability of Geobacter sulfurreducens since the lack of pilV2 gene depressed the cell adhesion capability by 81.2% in microplate and the anodic biofilm density by 23.1 % at -0.1 V and 37.7 % at -0.3 V in BESs. The less cohesiveness of mutant biofilms increased the charge transfer resistance and biofilm resistance, which correspondingly lowered current generation of the pilV2-deficient strain by up to 63.2 % compared with that of the wild-type strain in BESs. The deletion of pilV2 posed an insignificant effect on the production of extracellular polysaccharides, pili, extracellular cytochromes and electron shuttles that are involved in biofilm formation or extracellular electron transfer (EET) process. This study demonstrated the significance of pilV2 gene in cell adhesion and biofilm formation of G. sulfurreducens, as well as the importance of pili-mediated adhesion for EET of electroactive biofilm.

Influence of support materials on the electroactive behavior, structure and gene expression of wild type and GSU1771-deficient mutant of Geobacter sulfurreducens biofilms

Geobacter sulfurreducens DL1 is a metal-reducing dissimilatory bacterium frequently used to produce electricity in bioelectrochemical systems (BES). The biofilm formed on electrodes is one of the most important factors for efficient electron transfer; this is possible due to the production of type IV pili and c-type cytochromes that allow it to carry out extracellular electron transfer (EET) to final acceptors. In this study, we analyzed the biofilm formed on different support materials (glass, hematite (Fe2O3) on glass, fluorine-doped tin oxide (FTO) semiconductor glass, Fe2O3 on FTO, graphite, and stainless steel) by G. sulfurreducens DL1 (WT) and GSU1771-deficient strain mutant (Δgsu1771). GSU1771 is a transcriptional regulator that controls the expression of several genes involved in electron transfer. Different approaches and experimental tests were carried out with the biofilms grown on the different support materials including structure analysis by confocal laser scanning microscopy (CLSM), characterization of electrochemical activity, and quantification of relative gene expression by RT-qPCR. The gene expression of selected genes involved in EET was analyzed, observing an overexpression of pgcA, omcS, omcM, and omcF from Δgsu1771 biofilms compared to those from WT, also the overexpression of the epsH gene, which is involved in exopolysaccharide synthesis. Although we observed that for the Δgsu1771 mutant strain, the associated redox processes are similar to the WT strain, and more current is produced, we think that this could be associated with a higher relative expression of certain genes involved in EET and in the production of exopolysaccharides despite the chemical environment where the biofilm develops. This study supports that G. sulfurreducens is capable of adapting to the electrochemical environment where it grows.

Biotransformation characteristics of tetracycline by strain Serratia marcescens MSM2304 and its mechanism evaluation based on products analysis and genomics

Microbial biotransformation is a recommended and reliable method in face of formidable tetracycline (TC) with broad-spectrum antibacterial activity. Herein, comprehensive characteristics of a newfound strain and its molecular mechanism in process of TC bioremediation were involved in this study. Specifically, Serratia marcescens MSM2304 isolated from pig manure sludge grew well in presence of TC and achieved optimal removal efficiency of 61% under conditions of initial TC concentration of 10 mg/L, pH of 7.0, cell inoculation amount of 5%, and tryptone of 10 g/L as additional carbon. The pathways of biotransformation include EPS biosorption, cell surface biosorption and biodegradation, which enzymatic processes of biodegradation were occurred through TC adsorbed by biofilms was firstly broken down by extracellular enzymes and part of TC migrated towards biofilm interior and degraded by intracellular enzymes. Wherein extracellular polysaccharides in extracellular polymeric substances (EPS) from biofilm of strain MSM2304 mainly performed extracellular adsorption, and changes in position and intensity of CO, =CH and C-O-C/C-O of EPS possible further implied TC adsorption by it. Biodegradation accounting for 79.07% played a key role in TC biotransformation and could be fitted well by first-order model that manifesting rapid and thorough removal. Potential biodegradation pathway including demethylation, dihydroxylation, oxygenation, and ring opening possibly involved in TC disposal process of MSM2304, TC-degrading metabolites exhibited lower toxicity to indicator bacteria relative to parent TC. Whole genome sequencing as underlying molecular evidence revealed that TC resistance genes, dehydrogenases-encoding genes, monooxygenase-encoding genes, and methyltransferase-encoding genes of strain MSM2304 were positively related to TC biodegradation. Collectively, these results favored a theoretical evaluation for Serratia marcescens MSM2304 as a promising TC-control agent in environmental bioremediation processes.

Electroactive properties of EABs in response to long-term exposure to polystyrene microplastics/nanoplastics and the underlying adaptive mechanisms

Given widespread presence of polystyrene (PS) microplastics/nanoplastics (MPs/NPs), the electroactive responses and adaptation mechanisms of electroactive biofilms (EABs) exposed long-term to PS-containing aquatic environments remain unclear. Therefore, this study investigated the impacts of PS MPs/NPs on electroactivity of EABs. Results found that EABs exhibited delayed formation upon initially exposure but displayed an increased maximum current density (Imax) after subsequent exposure for up to 55 days. Notably, EABs exposure to NH2PS NPs (EAB-NH2PSNPs) demonstrated a 50% higher Imax than the control, along with a 17.84% increase in viability and a 58.10% increase in biomass. The cytochrome c (c-Cyts) content in EAB-NH2PSNPs rose by 178.35%, benefiting the extracellular electron transfer (EET) of EABs. Moreover, bacterial community assembly indicated the relative abundance of electroactive bacteria increased to 87.56% in EAB-NH2PSNPs. The adaptability mechanisms of EABs under prolonged exposure to PS MPs/NPs predominantly operate by adjusting viability, EET, and bacterial community assembly, which were further confirmed a positive correlation with Imax through structural equation model. These findings provide deeper insights into long-term effects and mechanisms of MPs/NPs on the electroactive properties of EABs and even functional microorganisms in aquatic ecosystems.

Engineering extracellular electron transfer pathways of electroactive microorganisms by synthetic biology for energy and chemicals production

The excessive consumption of fossil fuels causes massive emission of CO2, leading to climate deterioration and environmental pollution. The development of substitutes and sustainable energy sources to replace fossil fuels has become a worldwide priority. Bio-electrochemical systems (BESs), employing redox reactions of electroactive microorganisms (EAMs) on electrodes to achieve a meritorious combination of biocatalysis and electrocatalysis, provide a green and sustainable alternative approach for bioremediation, CO2 fixation, and energy and chemicals production. EAMs, including exoelectrogens and electrotrophs, perform extracellular electron transfer (EET) (i.e., outward and inward EET), respectively, to exchange energy with the environment, whose rate determines the efficiency and performance of BESs. Therefore, we review the synthetic biology strategies developed in the last decade for engineering EAMs to enhance the EET rate in cell-electrode interfaces for facilitating the production of electricity energy and value-added chemicals, which include (1) progress in genetic manipulation and editing tools to achieve the efficient regulation of gene expression, knockout, and knockdown of EAMs; (2) synthetic biological engineering strategies to enhance the outward EET of exoelectrogens to anodes for electricity power production and anodic electro-fermentation (AEF) for chemicals production, including (i) broadening and strengthening substrate utilization, (ii) increasing the intracellular releasable reducing equivalents, (iii) optimizing c-type cytochrome (c-Cyts) expression and maturation, (iv) enhancing conductive nanowire biosynthesis and modification, (v) promoting electron shuttle biosynthesis, secretion, and immobilization, (vi) engineering global regulators to promote EET rate, (vii) facilitating biofilm formation, and (viii) constructing cell-material hybrids; (3) the mechanisms of inward EET, CO2 fixation pathway, and engineering strategies for improving the inward EET of electrotrophic cells for CO2 reduction and chemical production, including (i) programming metabolic pathways of electrotrophs, (ii) rewiring bioelectrical circuits for enhancing inward EET, and (iii) constructing microbial (photo)electrosynthesis by cell-material hybridization; (4) perspectives on future challenges and opportunities for engineering EET to develop highly efficient BESs for sustainable energy and chemical production. We expect that this review will provide a theoretical basis for the future development of BESs in energy harvesting, CO2 fixation, and chemical synthesis.

Global transcriptional analysis of Geobacter sulfurreducens gsu1771 mutant biofilm grown on two different support structures

Electroactive biofilms formation by the metal-reducing bacterium Geobacter sulfurreducens is a step crucial for bioelectricity generation and bioremediation. The transcriptional regulator GSU1771 controls the expression of essential genes involved in electron transfer and biofilm formation in G. sulfurreducens, with GSU1771-deficient producing thicker and more electroactive biofilms. Here, RNA-seq analyses were conducted to compare the global gene expression patterns of wild-type and Δgsu1771 mutant biofilms grown on non-conductive (glass) and conductive (graphite electrode) materials. The Δgsu1771 biofilm grown on the glass surface exhibited 467 differentially expressed (DE) genes (167 upregulated and 300 downregulated) versus the wild-type biofilm. In contrast, the Δgsu1771 biofilm grown on the graphite electrode exhibited 119 DE genes (79 upregulated and 40 downregulated) versus the wild-type biofilm. Among these DE genes, 67 were also differentially expressed in the Δgsu1771 biofilm grown on glass (56 with the same regulation and 11 exhibiting counter-regulation). Among the upregulated genes in the Δgsu1771 biofilms, we identified potential target genes involved in exopolysaccharide synthesis (gsu1961-63, gsu1959, gsu1972-73, gsu1976-77). RT-qPCR analyses were then conducted to confirm the differential expression of a selection of genes of interest. DNA-protein binding assays demonstrated the direct binding of the GSU1771 regulator to the promoter region of pgcA, pulF, relA, and gsu3356. Furthermore, heme-staining and western blotting revealed an increase in c-type cytochromes including OmcS and OmcZ in Δgsu1771 biofilms. Collectively, our findings demonstrated that GSU1771 is a global regulator that controls extracellular electron transfer and exopolysaccharide synthesis in G. sulfurreducens, which is crucial for electroconductive biofilm development.

Revealing the roles of chemical communication in restoring the formation and electroactivity of electrogenic biofilm under electrical signaling disruption

Electrogenic biofilms in microbial electrochemical systems have played significant roles in simultaneous wastewater treatment and energy recovery owing to their unique extracellular electron transfer. Their formation has been shown to be regulated by electrical and chemical communication, but the interaction between these signal communication pathways has not been studied. This research investigated the coordination between intracellular c-di-GMP signaling and reinforced quorum sensing with or without exogenous HSL (a common quorum sensing molecule), on the formation of mixed-cultured electrogenic biofilm under electrical signaling disruption by tetraethylammonium (TEA, a broad-range potassium channel blocker). Intracellular c-di-GMP was spontaneously reinforced in response to TEA stress, and metagenomic analysis revealed that the dominant DGC (the genes for producing c-di-GMP) induced the eventual biofilm formation by mediating exopolysaccharide synthesis. Meanwhile, reinforced quorum sensing by exogenous HSL could also benefit the biofilm restoration, however, it alleviated the TEA-induced communication stress, resulting in the weakening of c-di-GMP dominance. Interestingly, suppressing electrical communication with or without HSL addition both induced selective enrichment of Geobacter of 85.5% or 30.1% respectively. Functional contribution analysis revealed the significant roles of Geobacter and Thauera in c-di-GMP signaling, especially Thauera in resistance to TEA stress. This study proposed a potential strategy for electrogenic biofilm regulation from the perspectives of cell-to-cell communication.

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 β-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.

Moving towards the enhancement of extracellular electron transfer in electrogens

Electrogens are very common in nature and becoming a contemporary theme for research as they can be exploited for extracellular electron transfer. Extracellular electron transfer is the key mechanism behind bioelectricity generation and bioremediation of pollutants via microbes. Extracellular electron transfer mechanisms for electrogens other than Shewanella and Geobacter are less explored. An efficient extracellular electron transfer system is crucial for the sustainable future of bioelectrochemical systems. At present, the poor extracellular electron transfer efficiency remains a decisive factor in limiting the development of efficient bioelectrochemical systems. In this review article, the EET mechanisms in different electrogens (bacteria and yeast) have been focused. Apart from the well-known electron transfer mechanisms of Shewanella oneidensis and Geobacter metallireducens, a brief introduction of the EET pathway in Rhodopseudomonas palustris TIE-1, Sideroxydans lithotrophicus ES-1, Thermincola potens JR, Lysinibacillus varians GY32, Carboxydothermus ferrireducens, Enterococcus faecalis and Saccharomyces cerevisiae have been included. In addition to this, the article discusses the several approaches to anode modification and genetic engineering that may be used in order to increase the rate of extracellular electron transfer. In the side lines, this review includes the engagement of the electrogens for different applications followed by the future perspective of efficient extracellular electron transfer.

Isolation and Characterisation of Electrogenic Bacteria from Mud Samples

To develop efficient microbial fuel cell systems for green energy production using different waste products, establishing characterised bacterial consortia is necessary. In this study, bacteria with electrogenic potentials were isolated from mud samples and examined to determine biofilm-formation capacities and macromolecule degradation. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry identifications have revealed that isolates represented 18 known and 4 unknown genuses. They all had the capacities to reduce the Reactive Black 5 stain in the agar medium, and 48 of them were positive in the wolfram nanorod reduction assay. The isolates formed biofilm to different extents on the surfaces of both adhesive and non-adhesive 96-well polystyrene plates and glass. Scanning electron microscopy images revealed the different adhesion potentials of isolates to the surface of carbon tissue fibres. Eight of them (15%) were able to form massive amounts of biofilm in three days at 23 °C. A total of 70% of the isolates produced proteases, while lipase and amylase production was lower, at 38% and 27% respectively. All of the macromolecule-degrading enzymes were produced by 11 isolates, and two isolates of them had the capacity to form a strong biofilm on the carbon tissue one of the most used anodic materials in MFC systems. This study discusses the potential of the isolates for future MFC development applications.

Potassium channel mediates electroactive biofilm formation via recruiting planktonic Geobacter cells

Potassium (K⁺)-channel-based electrical signaling can coordinate microbial actions at a distance that provides an evolutionary advantage to cell communities. Electroactive cells are usually cultured surrounded by an electric field which provided stronger electrical signaling than the K⁺-mediated electrical signaling. Whether the K⁺ signaling also plays a role in coordinating the behavior of electroactive microorganisms has not been accurately demonstrated. Thus, we constructed a K⁺-channel-deficient strain ΔgsuK of Geobacter sulfurreducens to directly investigate roles of K⁺ signaling in electroactive biofilm formation for the first time. The ΔgsuK strain exhibited significantly inferior biofilm formation (i.e., biomass, thickness and component) and consequently showed weaker electrical performance (i.e., start-up time, current output, electrochemical catalytic behavior and charge transfer resistance) than the wild-type strain. Individual electric generation capacity and the expression of genes involved in biofilm formation and electrical performance in the single cell did not significantly change with the deletion of gsuK, indicating that K⁺ signaling indeed influenced the recruiting behavior of planktonic cell but not the functioning of the single cell related to biofilm formation or electric generation. This study is intended to provide an in-depth understanding of electroactive biofilm formation and serve as a basis for optimizing its electrical performance via strengthening the recruitment behavior.

Insight into biofilm-forming patterns: biofilm-forming conditions and dynamic changes in extracellular polymer substances

The microbial biofilm adheres to the surface of the carrier, which protects the pollutant-degrading bacteria and resists harsh environments; thus, research on biofilm-forming patterns will help promote the application of biofilms in wastewater treatment. Herein, univariate analysis and response surface methodology (RSM) confirmed that glucose and mannose at 3-5 g/L promoted biofilm formation. Notably, the microplate method demonstrated that compared to trivalent cations, divalent cations could more greatly enhance the activity (especially magnesium) of the biofilm matrix, and the period of biofilm formation in the three strains was divided into the following stages: initial attachment (0-10 h), microcolony (10-24 h), maturation (24-48 h), and dispersion (36-72 h). During maturation, large amounts of extracellular polysaccharides (EPs) and extracellular DNA (eDNA) were distributed in the extracellular and intracellular spaces, respectively, as observed by super-resolution structured illumination microscopy (SR-SIM). This study enhances the understanding of the characteristics and patterns of biofilm formation and can facilitate the application of biofilms in wastewater treatment.

Biochar facilitates ferrihydrite reduction by Shewanella oneidensis MR-1 through stimulating the secretion of extracellular polymeric substances

Biochar can mediate extracellular electron transfer (EET) of Shewanella oneidensis MR-1 and subsequently facilitate dissimilatory reduction of iron(III) minerals. Previous studies mainly focused on the interaction of biochar and membrane cytochrome complexes to reveal the mediating mechanisms between biochar and S. oneidensis MR-1. However, the influence of biochar on the production and activity of extracellular polymeric substances (EPS) has long been neglected, despite the fact that EPS are commonly exudated by S. oneidensis MR-1 and can participate in a variety of electron transfer processes due to their redox activity. Here, we performed a series of microbial ferrihydrite reduction experiments in combination with electrochemical voltametric and impedance analyses to investigate the role of biochar in the formation and transformation of cell EPS during EET. Results showed that the added biochar not only functioned as an electron shuttle facilitating electron transfer, but also induced the secretion of five times more EPS by S. oneidensis MR-1, leading to a 1.4-fold faster ferrihydrite reduction in comparison with biochar-free setups. We further extracted the secreted EPS and found that the proportion of redox-active exoproteins was significantly (p < 0.05) increased in the EPS and resulted in a higher electron exchange capacity in secreted EPS. Such increased exoprotein content also induced a higher ratio of exoprotein to exopolysaccharide, which largely dropped diffusion and electron transfer impedance of EPS to 1.1 and 18 Ω, respectively, and accelerated the EET and thus the ferrihydrite reduction. Overall, our findings revealed the interactions between biochar and EPS matrices, which could potentially play a critical role in EET processes in both environmental or biotechnological systems.

Potassium channel blocker selectively enriched Geobacter from mixed-cultured electroactive biofilm: Insights from microbial community, functional prediction and gene expressions

This study investigated the effects of electrical signaling disruption induced by adding tetraethylammonium (TEA, a potassium channel blocker) on the formation of mixed-cultured electroactive biofilms, especially the relative abundance of Geobacter over time. Results showed that TEA addition decelerated the biofilm formation, but selectively enriched Geobacter over time (45.8% on Day 32, 67.7% on Day 60 and 78.1% on Day 90), thus resulting in higher final extracellular electron transfer (EET) efficiency. Redundancy analysis (RDA) confirmed that TEA and operation time were significant factors for the selective enrichment of Geobacter. Moreover, increase in cellular processes and signal processing by PICRUSt analysis indicated adaptive responses of electrogenic biofilms to electrical signaling disruption. Furthermore, qRT-PCR indicated the compensatory roles of key cytochromes and pilA in electrochemical communication, which induced Geobacter enrichment. This work provided a broader understanding of electroactive biofilm regulation and potential applications for electricity generation and biosensor in the future.

Deciphering the role of extracellular polymeric substances in the regulation of microbial extracellular electron transfer under low concentrations of tetracycline exposure: Insights from transcriptomic analysis

Low concentrations of antibiotics can regulate the formation of electroactive biofilms, however, the underlying mechanisms, especially the composition and spatial distribution of extracellular polymeric substances (EPS) and their effects on extracellular electron transfer (EET) process, have not been fully deciphered. Here, the response of EPS of Geobacter sulfurreducens biofilm to low concentrations of tetracycline (μg L^-1 to mg L^-1) was explored, and the impact of such EPS variations on EET efficiency was further elucidated by transcriptomic analysis. Results showed that 0.05 mg L^-1 of tetracycline achieved both beneficial quantitative and spatial regulation of redox-active proteins and non-conducting exopolysaccharides in EPS, while higher concentrations induced negative effects. Moreover, 1 mg L^-1 of tetracycline upregulated multiple exopolysaccharide biosynthesis-related genes, indicating a stress response for cell-protection, while 0.05 mg L^-1 of tetracycline upregulated most direct EET-related gene expressions, resulting in the promoted EET efficiency. Furthermore, 0.05 mg L^-1 of tetracycline selectively enriched Geobacter (45.55% vs 19.55% in control, respectively) from mixed inoculum. This research provides a new insight of how antibiotics at low concentrations regulated EET process through modulation of EPS.

Electroactive Biofilms of Activated Sludge Microorganisms on a Nanostructured Surface as the Basis for a Highly Sensitive Biochemical Oxygen Demand Biosensor

The possibility of the developing a biochemical oxygen demand (BOD) biosensor based on electroactive biofilms of activated sludge grown on the surface of a graphite-paste electrode modified with carbon nanotubes was studied. A complex of microscopic methods controlled biofilm formation: optical microscopy with phase contrast, scanning electron microscopy, and laser confocal microscopy. The features of charge transfer in the obtained electroactive biofilms were studied using the methods of cyclic voltammetry and electrochemical impedance spectroscopy. The rate constant of the interaction of microorganisms with the extracellular electron carrier (0.79 ± 0.03 dm³(g s)^-1) and the heterogeneous rate constant of electron transfer (0.34 ± 0.02 cm s^-1) were determined using the cyclic voltammetry method. These results revealed that the modification of the carbon nanotubes' (CNT) electrode surface makes it possible to create electroactive biofilms. An analysis of the metrological and analytical characteristics of the created biosensors showed that the lower limit of the biosensor based on an electroactive biofilm of activated sludge is 0.41 mgO₂/dm³, which makes it possible to analyze almost any water sample. Analysis of 12 surface water samples showed a high correlation (R² = 0.99) with the results of the standard method for determining biochemical oxygen demand.

The Role of Exopolysaccharides in Direct Interspecies Electron Transfer

Direct interspecies electron transfer (DIET) is an effective mechanism for microbial species to exchange electrons cooperatively during syntrophic metabolism. It is generally accepted that DIET is mainly mediated by electrically conductive pili and outer surface c-type cytochromes (c-Cyts). However, as an extracellular matrix is ubiquitous and abundant on the surface of microorganisms, the effect and mechanism of exopolysaccharides on DIET are still unclear. This study constructed a co-culture of exopolysaccharides-deficient Geobacter sulfurreducens with Geobacter metallireducens to explore the role of exopolysaccharides in DIET. Results revealed that the deficiency of exopolysaccharides extended the metabolic period of the co-culture by 44.4% and changed the proportions of each species in the co-culture. The exopolysaccharides-deficient co-culture failed to form large, tight spherical aggregates and the expression of c-Cyts and pili was decreased. The addition of magnetite and granular activated carbon (GAC), respectively, might compensate for the functions of c-Cyts and pili in the first generation of co-culture, but the stimulatory effect on the metabolic stable period co-culture was fairly limited. These findings demonstrate that non-conductive exopolysaccharides are an important component of DIET aggregates and an extracellular matrix for DIET-required c-Cyts.

Lysozyme regulates the extracellular polymer of activated sludge and promotes the formation of electroactive biofilm

The formation of electroactive biofilm from activated sludge on electrode surface is a key step to construct a bio-electrochemical system, yet it is greatly limited by the poor affinity between the bacteria and the electrode interface. Herein, we report a new method to promote the formation of electroactive biofilm by regulating the extracellular polymeric substance (EPS) content in activated sludge with lysozyme. The investigation of the effect of lysozyme treatment on the content of extracellular polymers and the biofilm formation of electroactive bacteria suggests that lysozyme can improve the permeability of the positive bacterial cell membrane and thus increase the EPS content in the activated sludge. The characterizations of electrochemical activity, surface morphology and community structure of the anode biofilm indicate that increasing EPS content promotes the adhesion of the mixed bacteria in the activated sludge on the electrode and results in denser biofilms with better conductivities. The microbial fuel cell (MFC) inoculated with the sludge of high EPS content exhibits the power density up to 2.195 W/m², much higher than that inoculated with the untreated sludge (1.545 W/m²). The strategy of adjusting EPS content in activated sludge with a biological enzyme can effectively enhance the ability of the bacterial community to form biofilms and exhibits great application potentials in the construction of high efficiency bio-electrochemical systems.

"Electroactive biofilms: how microbial electron transfer enables bioelectrochemical applications"

Microbial biofilms are ubiquitous. In marine and freshwater ecosystems, microbe–mineral interactions sustain biogeochemical cycles, while biofilms found on plants and animals can range from pathogens to commensals. Moreover, biofouling and biocorrosion represent significant challenges to industry. Bioprocessing is an opportunity to take advantage of biofilms and harness their utility as a chassis for biocommodity production. Electrochemical bioreactors have numerous potential applications, including wastewater treatment and commodity production. The literature examining these applications has demonstrated that the cell–surface interface is vital to facilitating these processes. Therefore, it is necessary to understand the state of knowledge regarding biofilms’ role in bioprocessing. This mini-review discusses bacterial biofilm formation, cell–surface redox interactions, and the role of microbial electron transfer in bioprocesses. It also highlights some current goals and challenges with respect to microbe-mediated bioprocessing and future perspectives.