Electron transfer in Gram-positive bacteria: enhancement strategies for bioelectrochemical applications

Genomic and proteomic analyses reveal the reduction mechanism of hexavalent chromium by the culturing supernatant of strain Pediococcus acidilactici 13-7

This study used lactic acid bacteria with high antioxidative properties to screen for strains capable of reducing hexavalent chromium [Cr (VI)] in their culturing supernatants. The strain Pediococcus acidilactici 13-7 exhibited potent Cr (VI)-reducing capability and remarkable resistance to Cr (VI) even at concentration as high as 24 mM. Comparative genomics analysis revealed a unique gene, ChrR, associated with Cr (VI) reduction in this strain, distinguishing it from four reference strains of P. acidilactici. The proteomic investigation identified proteins linked to the ChrR gene, such as nqo1, frdA, and gshR, indicating significant enrichment in redox-related functions and oxidative phosphorylation pathways. These findings suggest that P. acidilactici 13-7 possesses superior electron transfer capacity compared to other strains, making it more adaptable under highly oxidative conditions by modulating the external environment to mitigate oxidative stress. Collectively, the results demonstrated the potential application of this lactic acid bacterial strain for bioremediation of heavy metals by its ability to reduce Cr (VI), and shed light on the molecular mechanisms underlying Cr (VI) reduction of the strain P. acidilactici 13-7.

Screen-printed biosensor based on electro-polymerization of bio-composite for nitrate detection in aqueous media

Bacillus sp. possessing a periplasmic nitrate reductase was used as a recognition element to develop a nitrate biosensor. The bacteria was embedded within a polyaniline (PANI) electro-conductive matrix via electro-polymerization on miniaturized carbon screen-printed electrodes (SPE) at 100 mV/s and scan rate from -0.35 V to + 1.7 V. Surface medication of SPE was verified via Fourier Transform Infrared spectroscopy (FTIR) and Scanning Electron Microscopy (SEM). The optimal bacterial density was OD600 1.2. To enhance the biosensors performance, Bacillus sp. was (1) grown in riboflavin (RF) inducing media as an endogenous redox mediator and (2) exposed to different gamma radiation doses as a physical method to increase electron transfer. Results show a link between exposing cells to gamma irradiation stress, this was evident by electron spin resonance (ESR) and changes in FTIR spectrum, in addition to the increase in catalase enzyme. The nitrate limit of detection (LOD) was 0.5-25 mg/L for non-irradiated RF induced immobilized cells and LOD was 0.5-75 mg/L nitrate for 2 kGy gamma irradiated cells. The prepared biosensor showed acceptable reproducibility and multiple usages after storage at 4°C over 3 months. Low cost and simple preparation allow the biosensor to be mass-produced as a disposable device. Bacillus sp. and its endogenous redox mediator immobilized within polyaniline are good candidates for the improvement of amperometric biosensors for the quantification of nitrate in aqueous solutions.

Filamentous marine Gram-positive Nocardiopsis dassonvillei biofilm as biocathode and its electron transfer mechanism

This study investigated electrochemical characteristics of Gram-positive, Nocardiopsis dassonvillei B17 facultative bacterium in bioelectrochemical systems. The results demonstrated that anodic and cathodic reaction rates were catalyzed by this bacterium, especially by utilization of aluminium alloy as a substrate. Cyclic voltammogram results depicted an increase of peak current and surface area through biofilm development, confirming its importance on catalysis of redox reactions. Phenazine derivatives were detected and their electron mediating behavior was evaluated exogenously. A symmetrical redox peak in the range of -59 to -159 mV/SHE was observed in cyclic voltammogram of bacterial solution supplemented with 12 μM phenazine, a result consistent with cyclic voltammogram of a 5-d biofilm, confirming its importance as an electron mediator in extracellular electron transfer. Furthermore, the dependency of bacterial catalysis and polarization potential were studied. These results suggested that B17 biofilm behaved as a biocathode and transferred electrons to bacterial cells through a mechanism associated with electron mediators.

Shewanella chilikensis MG22 isolated from tannery site for malachite green decolorization in microbial fuel cell: a proposed solution for recirculating aquaculture system (RAS)

Malachite Green (MG) dye of the triphenylmethane group is a toxic compound used in the aquaculture industry as an antifungal agent, however, it can accumulate in fish and pose toxicity. The present work aims to remove MG in Microbial Fuel Cell (MFC) as a sustainable and eco-friendly solution. Out of six samples, the highest malachite green degradation was obtained by a sample obtained from Robiki tannery site in agar plates in 24 h at 37 °C. Robiki sample was used to inoculate the anodic chamber in Microbial Fuel cell, the resulting average electricity production was 195.76 mV for two weeks. The decolorization average was almost 88%. The predominant bacteria responsible for MG decolorization and electricity production were identified using 16S rRNA as Shewanella chilikensis strain MG22 (Accession no. OP795826) and formed a heavy biofilm on the anode. At the end of the decolorization process, MG was added again for re-use of water. The results showed efficiency for re-use 3 times. To ensure the sterility of treated water for re-use, both UV and filter sterilization were used, the latter proved more efficient. The obtained results are promising, MFC can be used as recirculating aquaculture system (RAS). The same aquaculture water can be treated multiple times which provides a sustainable solution for water conservation.

Extracellular Fe(III) reductase structure reveals a modular organization enabling S-layer insertion and electron transfer to insoluble substrates

The thermophilic anaerobic Gram-positive bacterium Carboxydothermus ferrireducens utilizes insoluble Fe(III) oxides as electron acceptors in respiratory processes using an extracellular 11-heme cytochrome c OmhA as a terminal reductase. OmhA is able to transfer electrons to soluble and insoluble Fe(III) compounds, substrates of multiheme oxidoreductases, and soluble electron shuttles. The crystal structure of OmhA at 2.5 Å resolution shows that it consists of two functionally distinct parts: the cytochrome с electron transfer and the S-layer binding domains. Nonaheme C-terminal subdomain of the cytochrome с domain is structurally similar to the extracellular multiheme cytochrome OcwA from the metal-reducing Gram-positive bacterium "Thermincola potens." S-layer binding domain of OmhA is responsible for interaction with the S-layer that surrounds the Carboxydothermus ferrireducens cell envelope. The structural foundations enabling the embedding of extracellular multiheme cytochromes to the S-layer of a Gram-positive-type cell wall and putative electron transfer pathways to insoluble minerals are discussed.

Enterococcus faecalis NADH Peroxidase-Defective Mutants Stain Falsely in Colony Zymogram Assay for Extracellular Electron Transfer to Ferric Ions

Enterococcus faecalis cells can reduce ferric ions and other electron acceptors by extracellular electron transfer (EET). To find mutants with enhanced or defective EET, strain OG1RF with random transposon insertions in the chromosome was screened for ferric reductase activity by colony zymogram staining using the chromogenic ferrous-chelating compound Ferrozine. The screen revealed npr, eetB, and ndh3 mutants. The aberrant ferric reductase phenotype of Npr (NADH peroxidase)-defective mutants was found to be a property of colonies and not apparent with washed cells grown in liquid culture. EetB- and Ndh3-defective mutants, in contrast, consistently showed low ferric reductase activity. It is concluded that colony zymogram staining for ferric reductase activity using Ferrozine can be misleading, especially through false negative results. It is suggested that hydrogen peroxide produced in the colony quenches the zymogram staining. In addition, it is demonstrated that the negative effect of heme on EET to ferric ion in E. faecalis is relieved by cytochrome bd deficiency. The findings can help to identify bacteria with EET ability and contribute to our understanding of EET in Gram-positive bacteria and the physiology of E. faecalis.

Mg-, Zn-, and Fe-Based Alloys With Antibacterial Properties as Orthopedic Implant Materials

Implant-associated infection (IAI) is one of the major challenges in orthopedic surgery. The development of implants with inherent antibacterial properties is an effective strategy to resolve this issue. In recent years, biodegradable alloy materials have received considerable attention because of their superior comprehensive performance in the field of orthopedic implants. Studies on biodegradable alloy orthopedic implants with antibacterial properties have gradually increased. This review summarizes the recent advances in biodegradable magnesium- (Mg-), iron- (Fe-), and zinc- (Zn-) based alloys with antibacterial properties as orthopedic implant materials. The antibacterial mechanisms of these alloy materials are also outlined, thus providing more basis and insights on the design and application of biodegradable alloys with antibacterial properties as orthopedic implants.