Novel Biochemical Properties and Physiological Role of the Flavin Mononucleotide Oxidoreductase YhdA from Bacillus subtilis

Single-Chamber Microbial Fuel Cell with an Innovative Sensing Component for Real-Time Continual Monitoring of a Wide Range of Cr(VI) Concentrations in Wastewater

Hexavalent chromium (Cr(VI)) is toxic, carcinogenic, and harmful to biological systems. Common detection methods, such as colorimetry, atomic absorption spectrometry, ion chromatography, and biological systems, can only be used in the laboratory and do not provide real-time feedback. To address these limitations, the current study cloned the ChrB gene, which exhibits high specificity in detecting Cr(VI), and the ChrA gene, which exhibits high Cr(VI) tolerance, into Escherichia coli. This recombinant strain, ChrA-ChrB-E. coli, was integrated into a single-chamber microbial fuel cell for accurate continual monitoring over a wide range of Cr(VI) concentrations. ChrA-ChrB-E. coli thrived in temperatures from 25 °C to 45 °C and pH levels between 5 and 8. Its ability to reduce Cr(VI) remained consistent across Cr(VI) forms, carbon sources, and oxyanions. Cyclic voltammetry was employed to verify the electrical activity of the biosensor. The biosensor exhibited a detection limit of 0.0075 mg/L. Under conditions simulating the regulatory emission limit for Cr(VI) of 0.5 mg/L in industrial wastewater, the biosensor achieved a response time of 20 s during continual operation. When tested with synthetic wastewater containing Cr(VI) concentrations from 0.02 to 150 mg/L, the system exhibited high adaptability and facilitated stable monitoring (relative standard deviation ≤ 2.7%). Additionally, the biosensor's accuracy (-1.73% to 2.5%) matched that of traditional batch methods, highlighting its suitability for real-time Cr(VI) monitoring in aquatic environments.

Unveiling the chromate stress response in the marine bacterium Bacillus enclensis AGM_Cr8: a multifaceted investigation

In this study, we introduce Bacillus enclensis AGM_Cr8, a gram-positive marine bacterium isolated from the chronically polluted Versova Creek in Mumbai, India. AGM_Cr8 exhibits robust tolerance to chromate stress, thriving in marine agar media containing up to 3200 mg/l of hexavalent chromium [Cr(VI)], with the Minimum Inhibitory Concentration (MIC) established at 5000 mg/l. Notably, AGM_Cr8 also displays tolerance to other heavy metals, including Lead [Pb (II)] (1200 mg/l), Arsenic [As (III)] (400 mg/l), Cadmium [Cd(II)] (100 mg/l), and Nickel [Ni(II)] (100 mg/l). Scanning Electron Microscopy (SEM) reveals the presence of Cr(VI) on the bacterial surface, accompanied by the secretion of extracellular polymeric substances (EPSs) facilitating Cr(VI) sequestration. This observation is validated through Energy Dispersive Spectroscopy (EDS). Transmission Electron Microscopy (TEM) and Scanning Transmission Electron Microscopy-Energy Dispersive Spectroscopy (STEM-EDS) confirm internal bioaccumulation of Cr(VI). X-ray photoelectron spectroscopy (XPS) identifies distinct peaks around 579 and 576 eV, indicating the coexistence of Cr(VI) and Cr(III), implying a bioreduction mechanism. De novo genome sequencing identifies twenty-two chromate-responsive genes, including putative chromate transporters (srpC1 and srpC2), suggesting an efflux mechanism. Other identified genes encode NAD(P)H-dependent FMN-containing oxidoreductase, NADH quinone reductase, ornithine aminotransferase, transporter genes (natA, natB, ytrB), and genes related to DNA replication and repair (recF), DNA mismatch repair (mutH), and superoxide dismutase. We therefore, propose a chromate detoxification pathway that involves an interplay of chromate transporters, enzymatic reduction of Cr(VI) to Cr(III), DNA repair and role of antioxidants in response to chromate stress. We have highlighted the potential of AGM_Cr8 for bioremediation in chromium-contaminated environments, given its robust tolerance and elucidated molecular mechanisms for detoxification.

Cr(VI) bioreduction enhanced by the electron transfer between flavin reductase and persistent free radicals

Persistent free radicals (PFRs) in biochar are an important electron shuttle for mediating electron transfer, which has significant impact on the biogeochemical redox reactions. Although the influence of biochar on the extracellular electron transfer (EET) for redox cycle has been extensively studied, the molecular mechanism for promoting the EET with PFRs remains poorly understood. This study investigated the oxygen-centered PFRs-mediated Cr(VI) reduction by Shewanella oneidensis MR-1 (MR-1) and exhibited the molecular mechanism of electron transfer between flavin substances and PFRs. Results showed that the Cr(VI) bioreduction rate by MR-1 increased from 31% to 70% with the addition of biochar. Electrochemical results illustrated that biochar increased biocurrent generation in the Cr(VI) bioreduction process. 3D-EEM and LC/MS spectra indicated that MR-1 secreted the flavin mononucleotide (FMN) reductase that relied on the [H] to provide the electrons. Electron paramagnetic resonance spectra illustrated that PFRs in biochar accepted the electrons from FMN reductase and stored those bioelectrons. Because of the oxidation of FMN, the electron transfer from FMN reductase to PFRs would increase the intracellular reactive oxygen species, which further produced the extracellular ·O2-. The reduced PFRs released the bioelectrons, accelerating the Cr(VI) reduction by ·O2-. Together with the results of the mutant strains experiment, it was found that the EET by c-cytochrome and free radicals contributed to the Cr(VI) bioreduction by 7.1% and 92.9%, respectively. These findings revealed that the PFRs could participate in the EET process and promote the redox reactions, providing a new approach for enhancing the remediation of heavy metal pollution by microorganisms and suggesting the important role of PFRs in the electron transfer process.

Exploration on the Cr(VI) resistance mechanism of a novel thermophilic Cr(VI)-reducing bacteria Anoxybacillus flavithermus ABF1 isolated from Tengchong geothermal region, China

Hexavalent chromium resistance and reduction mechanisms of microorganism provide a critical guidance for Cr(VI) bioremediation. However, related researches are limited in mesophiles and deficient for thermophiles. In this work, a novel alkaline Cr(VI)-reducing thermophile Anoxybacillus flavithermus ABF1 was isolated from geothermal region. The mechanisms of Cr(VI) resistance and reduction were investigated. The results demonstrated that A. flavithermus ABF1 could survive in a wide temperature range from 50°C to 70°C and in pH range of 7.0-9.0. Strain ABF1 showed excellent growth activity and Cr(VI) removal performance when initial Cr(VI) concentration was lower than 200 mg L^-1 . 93.71% of Cr(VI) was removed at initial concentration of 20 mg L^-1 after 72 h. The majority of Cr(VI) was found to be reduced extracellularly by enzymes secreted by cells. XPS and Raman analysis further manifested that Cr₂ O₃ was the product of Cr(VI) reduction. Moreover, the Cr(VI) transportation-related gene cysP and Cr(VI) reduction-related gene azoR of A. flavithermus ABF1 played key roles in inhibiting Cr(VI) entering cells and promoting extracellular Cr(VI) reduction respectively. This work provides novel insight into the mechanisms of Cr(VI) resistance and detoxication of thermophiles, which leads to a promising alternative strategy for heavy metal bioremediation in areas with elevated temperature.

SifR is an Rrf2-family quinone sensor associated with catechol iron uptake in Streptococcus pneumoniae D39

Streptococcus pneumoniae (pneumococcus) is a Gram-positive commensal and human respiratory pathogen. How this bacterium satisfies its nutritional iron (Fe) requirement in the context of endogenously produced hydrogen peroxide is not well understood. Here, we characterize a novel virulence-associated Rrf2-family transcriptional repressor that we term SifR (streptococcal IscR-like family transcriptional repressor) encoded by spd_1448 and conserved in Streptococci. Global transcriptomic analysis of a ΔsifR strain defines the SifR regulon as genes encoding a candidate catechol dioxygenase CatE, an uncharacterized oxidoreductase YwnB, a candidate flavin-dependent ferric reductase YhdA, a candidate heme-based ferric reductase domain-containing protein and the Piu (pneumococcus iron uptake) Fe transporter (piuBCDA). Previous work established that membrane-anchored PiuA binds Fe^III-bis-catechol or monocatechol complexes with high affinity, including the human catecholamine stress hormone, norepinephrine. We demonstrate that SifR senses quinone via a single conserved cysteine that represses its regulon when in the reduced form. Upon reaction with catechol-derived quinones, we show that SifR dissociates from the DNA leading to regulon derepression, allowing the pneumococcus to access a catechol-derived source of Fe while minimizing reactive electrophile stress induced by quinones. Consistent with this model, we show that CatE is an Fe^II-dependent 2,3-catechol dioxygenase with broad substrate specificity, YwnB is an NAD(P)H-dependent quinone reductase capable of reducing the oxidized and cyclized norepinephrine, adrenochrome, and YhdA is capable of reducing a number of Fe^III complexes, including PiuA-binding transport substrates. These findings are consistent with a model where Fe^III-catechol complexes serve as significant nutritional Fe sources in the host.

Pseudocitrobacter anthropi reduces heavy metal uptake and improves phytohormones and antioxidant system in Glycine max L

Heavy metal contamination due to anthropogenic activities is a great threat to modern humanity. A novel and natural technique of bioremediation using microbes for detoxification of heavy metals while improving plants' growth is the call of the day. In this study, exposing soybean plants to different concentrations (i.e., 10 and 50 ppm) of chromium and arsenic showed a severe reduction in agronomic attributes, higher reactive oxygen species production, and disruption in the antioxidant system. Contrarily, rhizobacterial isolate C18 inoculation not only rescued host growth, but also improved the production of nonenzymatic antioxidants (i.e., flavonoids, phenolic, and proline contents) and enzymatic antioxidants i.e., catalases, ascorbic acid oxidase, peroxidase activity, and 1,1-diphenyl-2-picrylhydrazyl, lower reactive oxygen species accumulation in leaves. Thereby, lowering secondary oxidative stress and subsequent damage. The strain was identified using 16 S rDNA sequencing and was identified as Pseudocitrobacter anthropi. Additionally, the strain can endure metals up to 1200 ppm and efficient in detoxifying the effect of chromium and arsenic by regulating phytohormones (IAA 59.02 µg/mL and GA 101.88 nM/mL) and solubilizing inorganic phosphates, making them excellent phytostimulant, biofertilizers, and heavy metal bio-remediating agent.