Sorption characteristics of N-acyl homserine lactones as signal molecules in natural soils based on the analysis of kinetics and isotherms

Simultaneous regulation of biocathodic γ-HCH dechlorination and CH4 production by tailoring the structure and function of biofilms based on quorum sensing

Dechlorination of chlorinated organic pollutants and methanogenesis are attractive biocathode reductions in microbial electrolysis cells (MECs). Quorum sensing (QS) is applied to regulate microbial communications. However, how acyl-homoserine lactones (AHLs)-dependent QS organize the assembly of the biocathode microbial community, and then regulate multiple biocathode reductions remains unclear. By applying N-butanoyl homoserine lactone (C4-HSL), N-hexanoyl homoserine lactone (C6-HSL) and 3-oxo-hexanoyl homoserine lactone (3OC6-HSL) in γ-hexachlorocyclohexane (γ-HCH) contaminated MECs, this study investigated the changes of biofilm microbial structure and function and the mechanisms of AHLs-QS on γ-HCH dechlorination and CH4 production. Exogenous C4-HSL and 3OC6-HSL increased cytochrome c production and enriched dechlorinators, electroactive bacteria but not methanogens to accelerate γ-HCH dechlorination and inhibit CH4 production. C6-HSL facilitated dechlorination and CH4 production by enhancing biofilm electroactivity and increasing membrane transportation. Besides, exogenous C6-HSL restored the electron transfer capacity that was damaged by the concurrent addition of acylase, an endogenous AHL quencher. From the perspective of microbial assembly, this study sheds insights into and provides an efficient strategy to selectively accelerate dechlorination and CH4 production by harnessing microbial structure based on QS systems to meet various environmental demands.

Dichotomous Role of Humic Substances in Modulating Transformation of Antibiotic Resistance Genes in Mineral Systems

Widespread antibiotic resistance genes (ARGs) have emerged as a focus of attention for public health. Transformation is essential for ARGs dissemination in soils and associated environments; however, the mechanisms of how soil components contribute to the transformation of ARGs remain elusive. Here we demonstrate that three representative mineral-humic acid (HA) composites exert contrasting influence on the transformation of plasmid-borne ARGs in Bacillus subtilis. Mineral surface-bound HA facilitated transformation in kaolinite and montmorillonite systems, while an inhibitory effect of HA was observed for goethite. The elevated transformation by HA-coated kaolinite was mainly attributed to the enhanced activity of competence-stimulating factor (CSF), while increased transformation by montmorillonite-HA composites was assigned to the weakened adsorption affinity of DNA and enhanced gene expression induced by flagella-driven cell motility. In goethite system, HA played an overriding role in suppressing transformation via alleviation of cell membrane damage. The results obtained offer insights into the divergent mechanisms of humic substances in modulating bacterial transformation by soil minerals. Our findings would help for a better understanding on the fate of ARGs in soil systems and provide potentials for the utilization of soil components, particularly organic matter, to mitigate the spread of ARGs in a range of settings.

Feasibility of implementing quorum quenching technology to mitigate membrane fouling in MBRs treating phenol-rich pharmaceutical wastewater: Application of Rhodococcus sp. BH4 and quorum quenching consortium

This study aimed to implement quorum quenching (QQ) to mitigate membrane fouling in membrane bioreactors (MBRs) treating phenol-rich pharmaceutical wastewater using Rhodococcus sp. BH4 and isolated QQ consortium (QQcs) from activated sludge. Neither BH4 nor QQcs impacted the removal efficiency of chemical oxygen demand (COD) (>94%), phenol (>99%), and ammonium (>99%), indicating that QQ did not have adverse impact on treatment performance. In addition, both BH4 and QQcs effectively retarded membrane fouling, which could be attributed to the reduction of soluble microbial products (SMP). Interestingly, the TMP increase was delayed 68.7% by Rhodococcus sp. BH4, while 31.3% was achieved by QQcs. This difference may be due to the relatively higher degradation for short- and medium-chain N-acyl-homoserine lactones (AHLs) by BH4 compared to the QQcs. Furthermore, the possible presence of quorum sensing (QS) bacteria within QQcs also could have contributed to the less effective fouling control than that of BH4.

Artificial Soils Reveal Individual Factor Controls on Microbial Processes

Soil matrix properties influence microbial behaviors that underlie nutrient cycling, greenhouse gas production, and soil formation. However, the dynamic and heterogeneous nature of soils makes it challenging to untangle the effects of different matrix properties on microbial behaviors. To address this challenge, we developed a tunable artificial soil recipe and used these materials to study the abiotic mechanisms driving soil microbial growth and communication. When we used standardized matrices with varying textures to culture gas-reporting biosensors, we found that a Gram-negative bacterium (Escherichia coli) grew best in synthetic silt soils, remaining active over a wide range of soil matric potentials, while a Gram-positive bacterium (Bacillus subtilis) preferred sandy soils, sporulating at low water potentials. Soil texture, mineralogy, and alkalinity all attenuated the bioavailability of an acyl-homoserine lactone (AHL) signaling molecule that controls community-level microbial behaviors. Texture controlled the timing of AHL sensing, while AHL bioavailability was decreased ~10⁵-fold by mineralogy and ~10³-fold by alkalinity. Finally, we built artificial soils with a range of complexities that converge on the properties of one Mollisol. As artificial soil complexity increased to more closely resemble the Mollisol, microbial behaviors approached those occurring in the natural soil, with the notable exception of organic matter. IMPORTANCE Understanding environmental controls on soil microbes is difficult because many abiotic parameters vary simultaneously and uncontrollably when different natural soils are compared, preventing mechanistic determination of any individual soil parameter's effect on microbial behaviors. We describe how soil texture, mineralogy, pH, and organic matter content can be varied individually within artificial soils to study their effects on soil microbes. Using microbial biosensors that report by producing a rare indicator gas, we identify soil properties that control microbial growth and attenuate the bioavailability of a diffusible chemical used to control community-level behaviors. We find that artificial soils differentially affect signal bioavailability and the growth of Gram-negative (Escherichia coli) and Gram-positive (Bacillus subtilis) microbes. These artificial soils are useful for studying the mechanisms that underlie soil controls on microbial fitness, signaling, and gene transfer.

Sorption of N-acyl homoserine lactones on maize straw derived biochars: Characterization, kinetics and isotherm analysis

Soil amendment with biochar may trigger a series of positive and negative biological effects, partly because it interferes quorum sensing (QS) signals synthesized by microorganisms for communication. However, the mechanisms through which biochar interacts with these QS signals remain elusive. This study explored the mechanisms of interactions between N-acyl homoserine lactones (AHLs) and two maize straw-derived biochars (MBs) with different pyrolysis temperature. Pseudo-second-order equation model best depicted AHLs sorption kinetics on MBs. The intra-particle diffusion model revealed that AHLs sorption onto MBs consists of several stages. The sorption isotherms data of AHLs on MBs were in well agreement with both Langmuir and Freundlich models, indicating the occurrence of energetic distribution of active sites on the heterogeneous biochar with multilayer sorption. However, the AHLs sorption capacity on MBs varied, with biochar pyrolyzed at 600 °C displaying a higher AHLs sorption capacity compared with biochar pyrolyzed at 300 °C. It may be attributed to a variety of physiochemical interactions such as pore filling, functional groups complexation, hydrogen bond, and hydrophobic action. The adsorption/partitioning model results and thermodynamic parameters of Gibbs free energy (ΔG) confirmed that physical and chemical sorption occurred concurrently throughout the whole AHLs sorption process, with physical partitioning playing a greater role than surface sorption. The findings suggest that soil amendment with biochar may have a variety of effects on intra/inter-cellular communication, further implying biochar can be specially prepared to mediate soil processes related to microbial communication, like pollutant biodegradation, and carbon/nitrogen cycling.

Red fluorescent ultra-small gold nanoclusters functionalized with signal molecules to probe specificity in quorum sensing receptors in gram-negative bacteria

Ultra-small (size < 2 nm) gold nanoclusters (AuNCs) are used as fluorescent probes which have excellent applications in bioimaging and sensing due to their emission in visible and NIR spectral region. Here, this property is exploited for understanding the quorum sensing phenomenon in bacteria which is regulated by signal molecules which are specific to various species. AuNCs are then functionalized with the signal molecules, Acyl Homoserine Lactones (AHL) of varying carbon chain length, C-6, C-8, and C-12 without 3rd C modification, to sense different strains of gram-negative bacteria i.e., Escherichia coli, Cronobacter sakazakii and Pseudomonas aeruginosa. In the concentration employed, selectivity to a limited extent is observed between the three Gram-negative bacteria tested. E. coli showed emission with all the AHL conjugates and P. aeruginosa did not interact with any of the three conjugates, whereas C. sakazakii showed specificity to C-8AHL. This is probably due to selectivity for cognate AHL molecules of appropriate concentrations.

Analysis of N-Acy-L-homoserine lactones (AHLs) in wastewater treatment systems using SPE-LLE with LC-MS/MS

The occurrence and distribution of N-acyl homoserine lactones (AHLs) in membrane bioreactors (MBRs) treating wastewater has garnered much attention as they have been shown to play critical role in biofouling. There is a need to develop a single method capable of analysing AHLs in various wastewater with comparable and reliable performance. A novel and robust method was proposed for trace analysis of 11 AHLs in wastewater treatment systems treating domestic and industrial wastewater. This method utilised solid phase extraction (SPE) to extract AHLs from wastewater followed by liquid-liquid extraction (LLE) to extract AHLs from the SPE eluant, and used N-heptanoyl-dl-homoserine lactone (C7-HSL) as an internal standard. There was no need to prepare matrix-matched calibration curve for accurate quantification of AHLs in the liquid chromatography tandem mass chromatography (LC-MS/MS) analysis. The developed method was validated with six different types of domestic and industrial wastewater with regard to AHLs recoveries and matrix effects. For treated domestic and industrial wastewater, the relative recoveries ranged from 75% to 130% and the matrix effects ranged from 89% to 122%. This method exhibited remarkable improvement compared with single SPE. The results also indicated that inclusion of LLE after SPE could effectively alleviate matrix effects, which may be because of the removal of relatively hydrophilic interferences by using dichloromethane to extract AHLs from the SPE eluant composing of methanol and water. The limits of detection of the AHLs were all below 5 ng/L for the tested wastewater samples. The developed method of SPE-LLE with LC-MS/MS was applied to analyse AHLs in four lab-scale and one pilot-scale wastewater treatment systems. Wide spectrum of AHLs with alkanoyl chains ranging from C4 to C14 were detected with concentrations ranging from 2.7 to 299.2 ng/L. This method is capable of identifying and quantifying trace levels of AHLs in various wastewater treatment systems and can help us better understand the mechanisms of AHL-mediated quroum sensing in various wastewater treatment systems.

Highly Efficient and Sustainable Spent Mushroom Waste Adsorbent Based on Surfactant Modification for the Removal of Toxic Dyes

The treatment of wastewater always demands eco-friendly and cost-efficient adsorbents. In this paper, spent mushroom waste (SMW) was modified by a cationic surfactant (cetyltrimethylammonium bromide, CTAB) to eliminate toxic dyes. A characterization of adsorbents confirmed that CTAB was successfully embedded into the SMW structure. The spent mushroom waste, modified by CTAB (SMWC), exhibited an excellent adsorption capacity of 249.57 mg·g−1, 338.67 mg·g−1, and 265.01 mg·g−1 for the Direct red 5B (DR5B), Direct blue 71 (DB71), and Reactive black (RB5) dyes, respectively. Batch experiments indicated that the dye adsorption of SMWC depended mainly on pH, dye concentration, temperature, and ionic strength. The adsorption isotherm could be fitted to the Langmuir model and described by the pseudo-second-order kinetic model. The dye adsorption mechanism was dominated mostly by the chemosorption of the dyes and the SMWC surface. Thermodynamic parameters showed that the adsorption was endothermic and spontaneous. SMWC could successfully remove over 90% of dyes from various water samples. This can be considered a feasible waste resource utility, since it meets both the ecological and the economic requirements for auspicious industrial applications.

Removal of methylene blue from aqueous solution by cattle manure-derived low temperature biochar

Biochar is a low cost and renewable adsorbent which can be used to remove dye from wastewater. Cattle manure-derived low temperature biochar (CMB) was studied to remove methylene blue (MB) from aqueous solution in this paper. The effect of factors including initial concentration of MB, dosage, contact time, and pH on the adsorption properties of MB onto biochar were studied. Characterization of the CMB and MB adsorbed on CMB was performed using techniques including BET, FTIR and SEM. The adsorption isotherm, kinetics, thermodynamics and mechanism were also studied. The results showed the equilibrium data were well fitted to the Langmuir isotherm model, and the saturation adsorption capacity of CMB₂₀₀ was 241.99 mg g⁻¹. Pseudo-second order kinetics was the most suitable model for describing the adsorption of MB onto biochar. The adsorption thermodynamics of MB on biochar showed that the adsorption was a spontaneous and endothermic process. Through zeta potential measurement, Boehm titration, cation exchange, deashing and esterification experiments, the importance of ash to adsorption was verified, as well as the adsorption mechanism. The adsorption mechanism of MB on CMB₂₀₀ involved cation exchange, electrostatic interaction, hydrogen bonding, physical effects and others. This work shows that CMB₂₀₀ holds promise to act as an effective adsorbent to remove MB in wastewater.

Biochar is a kind of low cost and renewable adsorbents which can be used to remove dye from wastewater. The mechanism between MB and CMB involved cation exchange, electrostatic interaction, hydrogen bonding, physical function and others.