Natural defence mechanisms of electrochemically active biofilms: From the perspective of microbial adaptation, survival strategies and antibiotic resistance.

Electrochemically active biofilms (EABs) play an ever-growingly critical role in the biological treatment of wastewater due to its low carbon footprint and sustainability. However, how the multispecies biofilms adapt, survive and become tolerant under acute and chronic toxicity such as antibiotic stress still remains well un-recognized. Here, the stress responses of EABs to tetracycline concentrations (CTC) and different operation schemes were comprehensively investigated. Results show that EABs can quickly adapt (start-up time is barely affected) to low CTC (≤ 5 µM) exposure while the adaptation time of EABs increases and the bioelectrocatalytic activity decreases at CTC ≥ 10 µM. EABs exhibit a good resilience and high anti-shocking capacity under chronic and acute TC stress, respectively. But chronic effects negatively affect the metabolic activity and extracellular electron transfer, and simultaneously change the spatial morphology and microbial community structure of EABs. Particularly, the typical exoelectrogens Geobacter anodireducens can be selectively enriched under chronic TC stress with relative abundance increasing from 45.11% to 85.96%, showing stronger TC tolerance than methanogens. This may be attributed to the effective survival strategies of EABs in response to TC stress, including antibiotic efflux regulated by tet(C) at the molecular level and the secretion of more extracellular proteins in the macro scale, as the C=O bond in amide I of aromatic amino acids plays a critical role in alleviating the damage of TC to cells. Overall, this study highlights the versatile defences of EABs in terms of microbial adaptation, survival strategies, and antibiotic resistance, and deepens the understanding of microbial communities' evolution of EABs in response to acute and chronic TC stress.

Additional polypyrrole as conductive medium in artificial electrochemically active biofilm (EAB) to increase the sensitivity of EAB based biosensor in water quality early-warning.

Researchers believe that adding conductive mediums in electrochemically active biofilms (EABs) would improve the sensitivity of EAB-based biosensor for real-time water quality early-warning through facilitating the extracellular electron transfer (EET), which has been hardly evidenced mostly because naturally formed EABs employed in previous biosensor studies were recognized distinct and incapable of delivering comparable electrical signals. By preparing artificial EABs where Shewanella oneidensis MR-1 was encapsulated in sodium alginate (SA), this study solved how polypyrrole (PPy) as conductive medium would affect the sensitivity of EAB-based biosensor, as well as mass transfer of toxicant during this process. Different mass ratios (0.125:1, 0.25:1 and 1:1) of PPy over SA were tested, and the sensitivity promoted by 20%, 15% and 6%, respectively. Results indicated that a small amount of PPy addition (PPy: SA = 0.125: 1 in mass ratio) was more effective to increase the biosensor's sensitivity compared to larger amount of PPy employed in EAB. This was when improved conductivity introduced by PPy would dominate in affecting the sensitivity over contrarily weakened mass transfer in the meantime.

Electrochemical and microbial community responses of electrochemically active biofilms to copper ions in bioelectrochemical systems.

Heavy metals play an important role in the conductivity of solution, power generation and activity of microorganisms in bioelectrochemical systems (BESs). However, effect of heavy metal on the process of exoelectrogenesis metabolism and extracellular electron transfer of electrochemically active biofilms (EABs) was poorly understood. Herein, we investigated the impact of Cu2+ at gradually increasing concentration on the morphological and electrochemical performance and bacterial communities of anodic biofilms in mixed-culture BESs. The voltage output decreased continuously and dropped to zero at 10 mg L-1, which was attributed to the toxic inhibition that cased anodic biofilm damage and decreased secretion of outer membrane cytochromes. When stopping the introduction of Cu2+ to anodic chamber, the maximum voltage production recovered 75.1% of the voltage produced from BES and coulombic efficiency was higher but acetate removal rate was lower than that before Cu2+ addition, demonstrating the recovery capability of EABs was higher compared to nonelectroactive bacteria. Moreover, SEM-EDS and XPS suggested that most of Cu2+ was adsorbed by the anode electrode and reduced by EABs on anode. Compared to the open-circuit BES, the flow of electrons through a circuit could improve the reduction of copper. Community analysis showed a decrease in Geobacter accompanied by an increase in Stenotrophomonas in response to Cu2+ shock in anodic chamber.

Enhanced catalytic capability of electroactive biofilm modified with different kinds of carbon nanotubes.

In this study two methods including coating carbon nanotubes (CNTs) layers on the electrode surface and adding CNTs-suspension during electrochemically active biofilms (EABs) growth were used, respectively, to develop CNTs hybrid EABs for enhancing electricity generation capability of EABs. EABs growth on the CNTs with functional groups of hydroxyl (CNTs-OH) or carboxyl (CNTs-COOH) and pristine CNTs without functionalization (P-CNTs) modified electrode was investigated. The maximum current densities of EABs growth on the P-CNTs, CNTs-OH and CNTs-COOH coated electrode were respective 1300 ±â€¯117, 1082 ±â€¯54 and 1124 ±â€¯78 µA cm-2, which were much higher than unmodified electrode (663 µA cm-2). Meanwhile, EABs growth in doping CNTs-COOH or CNTs-OH suspensions system also produced twice higher current density than that on unmodified electrode. These results indicated that the current production of EABs can be significantly enhanced by coating P-CNTs, CNTs-OH, CNTs-COOH layers on the electrode surface or doping CNTs-OH and CNTs-COOH suspension into EABs. Furthermore, morphology analysis of as-obtained EABs had also been studied. It was found that there was no significant difference of the morphological characteristic for EABs growth on different types CNTs coated electrode surface. By comparison, a nano-hybrid porous structure of CNTs and EABs was observed when CNTs-COOH or CNTs-OH suspension was added into the medium during EABs growth, which will be responsible for high current generation.

Inhibitory effect of cadmium(II) ion on anodic electrochemically active biofilms performance in bioelectrochemical systems.

Cadmium(II) ion can affect the anode performance of bioelectrochemical systems (BES); however, how the presence of Cd2+ affect the extracellular electron transfer of anodic electrochemically active biofilms (EABs), the microbial viability and species composition of microorganism on the anode remain poorly understood. Here, we investigated the inhibitory effect of Cd2+ at different concentrations on the electrochemical performance and the biofilm community in mixed-culture enriched BES. The electrochemical performance of the BES was not inhibited at 2 mg L-1 Cd2+, while higher concentrations of 5-20 mg L-1 resulted in the decrease in the maximum power density, with 0.34 ±â€¯0.01 W m-2 at 5 mg L-1, 0.28 ±â€¯0.01 W m-2 at 10 mg L-1, and 0.17 ±â€¯0 W m-2 at 20 mg L-1, respectively. When adding 30 mg/L Cd2+, there was almost no power output. The decline of the power output was possibly ascribed to the suppressed viability and the change of species richness as evident from confocal laser scanning microscopy and microbial community analysis. Cyclic voltammogram and electrochemical impedance spectroscopy revealed that high concentration of Cd2+ exceeding 5 mg L-1 can inhibit the secretion of outer membrane cytochromes, thus reducing the electron transfer between the EABs and the anode surface. Analysis of bacterial structures showed a decrease in Geobacter accompanied by an increase in Stenotrophomonas and Azospira in response to Cd2+ at 10 and 20 mg L-1. This study added to the in-depth analysis of the inhibition of Cd2+ on EABs, and provided new insights into the removing Cd2+ and organics simultaneously in BES.

Gold-modified indium tin oxide as a transparent window in optoelectronic diagnostics of electrochemically active biofilms.

Microbial electrochemical technologies (METs) are one of the emerging green bioenergy domains that are utilizing microorganisms for wastewater treatment or electrosynthesis. Real-time monitoring of bioprocess during operation is a prerequisite for understanding and further improving bioenergy harvesting. Optical methods are powerful tools for this, but require transparent, highly conductive and biocompatible electrodes. Whereas indium tin oxide (ITO) is a well-known transparent conductive oxide, it is a non-ideal platform for biofilm growth. Here, a straightforward approach of surface modification of ITO anodes with gold (Au) is demonstrated, to enhance direct microbial biofilm cultivation on their surface and to improve the produced current densities. The trade-off between the electrode transmittance (critical for the underlying integrated sensors) and the enhanced growth of biofilms (crucial for direct monitoring) is studied. Au-modified ITO electrodes show a faster and reproducible biofilm growth with three times higher maximum current densities and about 6.9 times thicker biofilms compared to their unmodified ITO counterparts. The electrochemical analysis confirms the enhanced performance and the reversibility of the ITO/Au electrodes. The catalytic effect of Au on the ITO surface seems to be the key factor of the observed performance improvement since the changes in the electrode conductivity and their surface wettability are relatively small and in the range of ITO. An integrated platform for the ITO/Au transparent electrode with light-emitting diodes was fabricated and its feasibility for optical biofilm thickness monitoring is demonstrated. Such transparent electrodes with embedded catalytic metals can serve as multifunctional windows for biofilm diagnostic microchips.