Microbial immobilisation and adaptation to Cu2+ enhances microbial Fe2+ oxidation for bioleaching of printed circuit boards in the presence of mixed metal ions

A circular economy requires effective re-use of finite resources, such as metals from waste electrical and electronic equipment (WEEE). Bioleaching for extraction and recovery of base metals from printed circuit boards (PCBs) before recovering precious metals has potential to increase metal circularity. However, inhibition by base metals released from the PCBs and accumulated in PCB leachates on microbial Fe2+ oxidation, a critical bioleaching sub-process for Fe3+ regeneration, can limit this approach. Here, we explore the potential of microbial immobilisation on polyurethane foam (PUF) and adaptation to cupric ions to minimise inhibition by mixed metals released from PCBs, particularly zinc, nickel, and tin, and enhancing Fe2+ oxidation rates in PCB bioleaching systems. A mixed mesophilic culture dominant in Leptospirillum ferriphilum, Acidiplasma cupricumulans and Acidithiobacillus caldus was immobilised on PUF and adapted to 6 g/L Cu2+. Tolerance of Cu-adapted immobilised cells to the inhibitory metal ions Zn2+, Ni2+, and Sn2+, as individual (0-10 g/L) and mixed metal ions at concentrations typically leached from PCBs at solids loadings of 0-20% (mass/volume) was compared to that of non-adapted immobilised cells. Further, the impact of solutes from PCB leachates was evaluated. Inhibition by individual metal ions decreased in the order Sn2+ > Ni2+ > Zn2+. Inhibition of ferrous iron oxidation by mixed metal ions was synergistic with respect to individual metal ions. PCB leachates were more inhibitory than both mixed and individual metal ions even where metal concentration was low. Cu-adapted immobilised cells exhibited higher tolerance to increasing concentrations of inhibitory metal ions than non-adapted cells. These results are promising for the application of Cu-adapted cells in the bioleaching of PCBs and multi-metal concentrates.

Reactive and microbial inhibitory mechanisms depicting the panoramic view of pH stress effect on common biological nitrification

pH is a crucial factor of microbial nitrification, which often combines with high-strength ammonium to influence nitrogen removal pathway in wastewater treatment. However, the detailed inhibitory mechanisms of pH stress are not sufficiently disclosed yet. In this study, the pH stress effect on nitrification was comprehensively studied by a set of experiments which identified the reactivity of nitrification processes and activity of nitrifiers, the time dependence of inhibition effect and the hybrid pH stress effect with ammonium. The results revealed two distinct inhibitory mechanisms dominating in alkaline and acid ranges. In alkaline range (pH > 8), pH stress causes physiological damages on microorganisms which is named as microbial inhibition. It has the features of less recoverability of nitrifiers, time-dependent inhibition effect and low pH-tolerance of nitrite oxidation bacteria. Free ammonia enhanced microbial inhibition and greatly promoted nitrite accumulation. A novel reactive inhibition mechanism dominated in acid range (pH < 7) was disclosed. It only impedes ammonia oxidation process (AOP) but not impair microbial activity obviously and the effect is time-independent. The mechanism was clarified from H⁺ transport because AOP involved H⁺ production. The H⁺ transport was impeded under acid stress owing to the decrease of pH gradient across cell membrane. The two mechanisms formed a panoramic view of pH stress effect on nitrification advancing the understanding of nitrifier adaptability and nitritation regulation in wastewater treatment processes.

Antibacterial properties of Apis dorsata honey against some bacterial pathogens

Now-a-days, different bioproducts are being used extensively for the welfare of mankind. However, for proper utility of any bioproduct, the exact biotechnological potential of that product should be explored. Honey is produced in almost every country on the planet. It has long been used as a medicinal agent in addition to its broader use as a popular food throughout the human history. It can be used to treat various diseases without causing any negative side effects. In the present study, the antibacterial potential of honey produced by A. dorsata was investigated at its variable concentrations (25, 50, 75 and 100 %) against four pathogenic bacterial species. The highest antimicrobial action was seen against E. coli at 100 % concentration of the honey while showing zone of inhibition of 37.5 ± 3.5 mm. However, the lowest antibacterial action was observed against E. faecalis. The overall order of growth inhibition by the honey at its 100 % concentration for the implicated bacterial species appeared as: E. coli ˃ P. aeruginosa ˃ S. aureus ˃ E. faecalis. The honey couldn’t show antibacterial action at its 25 % concentration. Our findings of the present study will be helpful for utility of the honey as an alternative medicine for curing different complications caused by microbial pathogens.

Electrocatalytic deep dehalogenation of florfenicol using Fe-doped CoP nanotubes array for blocking resistance gene expression and microbial inhibition during biochemical treatment

Resistance gene expression and microbial inhibition by halogenated antibiotics is a major environmental concern. Although electrocatalytic dehalogenation can detoxify halogenated antibiotics, the effect of dehalogenation treatment on resistance gene expression and microbial inhibition is poorly understood. Herein, a novel electrocatalyst of Fe-doped CoP nanotubes array on nickel foam (Fe-CoP NTs/NiF) is prepared through a simple ultrasonication of Fe-doped CoP nanowires hydrothermally grown on NiF. The transformation from nanowires to nanotubes improves the crystallinity of CoP and fully exposes active sites, producing energetic atomic hydrogen for dehalogenation. Fe-CoP NTs/NiF exhibits a superior dehalogenation performance towards refractory florfenicol (FLO), achieving 100% removal within 20 min (‒1.2 V vs Ag/AgCl, C₀ = 20 mg L^‒1). The dechlorination ratio reaches nearly 100%, and the defluorination ratio achieves 36.8% within 50 min, showing the best electrocatalytic dehalogenation performance reported so far. Microbial community and correlation analysis show that Proteobacteria is the main potential host of FLO resistance gene. Electrocatalytic reductive dehalogenation pretreatment of FLO can reduce microbial inhibition, maintaining microbial richness and diversity in the subsequent biochemical treatment unit. The electrocatalytic reductive dehalogenation treatment can significantly reduce the relative abundance of FLO resistance gene, showing a reliable process for safe treatment of halogenated antibiotic containing wastewater.

Facile synthesis of Silver@Eggshell nanocomposite: A heterogeneous catalyst for the removal of heavy metal ions, toxic dyes and microbial contaminants from water

In this work, silver nanoparticles have been synthesized with an average particle size of 35 nm, within 90s, using microwave and Sapindus mukorossi extract as a stabilizing agent. The AgNps were surface immobilized on eggshells (ES) to obtain Ag@ES, which was characterized by UV-Vis, UV-DRS, FT-IR, ICP-OES, TGA-DSC, SEM-EDX, XRD and XPS. Its applicability as an environmental catalyst was evaluated by Cr (VI) adsorption, photocatalytic degradation of methyl orange, eriochrome black-T, methylene blue, rhodamine-B as model dyes and microbial inhibition against Staphylococcus aureus, Escherichia coli and Candida albicans. The results revealed that Ag@ES exhibited maximum adsorption capacity of 93 mg/g for Cr (VI) ion and degradation efficiency of ~90-98% for removing anionic and cationic dyes. Further, it showed a minimum inhibitory concentration of 15.6, 7.8 and 31.2 μg/mL for S. aureus, E. coli and C. albicans respectively. Moreover, the Ag@ES being a heterogeneous catalyst can be regenerated and reused without significant loss in its efficiency.

In-vitro evaluation of antimicrobial and insect repellent potential of supercritical-carbon dioxide (SCF-CO2) extracts of selected botanicals against stored product pests and foodborne pathogens

In the present study, the antimicrobial and the insect repellent activity of 16 botanical extracts obtained by supercritical CO₂ (SCF) extraction were evaluated. The present investigation was conducted as there is a necessity for exploration of natural botanical extracts that target both stored product insects and microbes. The antimicrobial activity was studied by disc diffusion and broth microdilution methods against ten microbial species, including Gram-positive bacteria (Staphylococcus aureus, Bacillus subtilis and Listeria monocytogenes), Gram-negative bacteria (Escherichia coli and Salmonella enterica), and fungi (Aspergillus flavus, Aspergillus paraciticus, Aspergillus ochraceous, Aspergillus niger and Penicillium verrucosum). Repellency assay was carried out by area preference method against three coleopteran insects (Tribolium castaneum, Rhyzopertha dominica and Sitophilus oryzae). Among all the extracts, thyme and ajwain were effective against all the tested bacteria with a minimum inhibition concentration (MIC) of 256-1024 µg/mL. Hop extract resulted in better antibacterial activity against all the tested Gram-positive bacteria with a MIC of 32-64 µg/mL. Oregano, thyme and ajwain extracts showed broad-spectrum antifungal activity against all the tested fungi with MIC of 128-1024 µg/mL. Most of the extracts exhibited class V (80.1-100%) repellency against T. castaneum. Extracts of hop, ajwain and thyme were found to have strong repellency against T. castaneum and R. dominica. Therefore, SCF extracts of ajwain and thyme can be explored further for the application of bio-extracts as a growth limiting factors in a microcosm where such consortia thrive.

Diazole and triazole inhibition of nitrification process in return activated sludge

Azoles are emerging contaminants that are resistant to biodegradation during wastewater treatment. Their presence has been widely reported in wastewater effluents and receiving waters. In this work, the potential inhibition of nitrification process by six different azole compounds in wastewater treatment plants was investigated in batch bioassays. The azoles studied included three diazoles: pyrazole (Pz); 1-methylpyrazole (MePz); 3,5-dimethylpyrazole (DMePz); and three triazoles: 1,2,4-triazole (Tz); benzotriazole (BTz); and 5-methyl benzotriazole (MeBTz). The concentration of azoles causing 50% inhibition (IC₅₀) increased (azoles became less inhibitory) in the following order (mg L^-1): BTz (1.99) < MeBTz (2.18) < Pz (2.69) < Tz (3.53) < DMePz (17.3) < MePz (49.6). No clear structure-inhibitory relationships were found using Log P and pKa as structural properties. The toxicity of any given azole may be related to the role of substituent groups on disabling/enabling binding to the active sites of metallo-enzymes in nitrifying microorganisms. This is exemplified by the low toxicity of MePz, which has a cyclic N blocked by a methyl group. The observed inhibition caused to nitrifying bacteria is more severe than their cytotoxicity to other target organisms (e.g., methanogens and heterotrophic bacteria), suggesting a specific inhibition to the copper-containing enzyme, ammonium monooxygenase, in ammonia oxidizing nitrifying microorganisms.

Inhibition of microorganisms involved in deterioration of an archaeological site by silver nanoparticles produced by a green synthesis method

The Citadel, part of the pre-Hispanic city of Teotihuacan and listed as a World Heritage Site, harbors irreplaceable archaeological walls and murals. This city was abandoned by the 7th century and its potential deterioration represents a noteworthy loss of the world's cultural heritage. This research consisted of isolation and identification of bacteria and fungi contributing to this deterioration from walls of a pre-Hispanic city. In addition, silver nanoparticles (AgNP) produced, using a green synthesis method, were tested as potential inhibitors of microbes. AgNP of different sizes and concentrations were tested using in situ assays. Leaf aqueous extracts from two plants species (Foeniculum vulgare and Tecoma stans) and two extraction procedures were used in the NP synthesis. The potential of AgNP as preventive/corrective treatments to protect stucco materials from biodeterioration, as well as the microbial inhibition on three stone materials (stucco, basalt and calcite) was analyzed. Twenty-three bacterial species belonging to eight genera and fourteen fungal species belonging to seven genera were isolated from colored stains, patinas and biofilms produced on the surfaces of archaeological walls from the pre-Hispanic city, Teotihuacan. AgNP from F. vulgare were more effective for in vitro microbial growth inhibition than those from T. stans. Bacteria were less sensitive to AgNP than fungi; however, sensitivity mainly depended on the microbial strain and the plant extract used to prepare AgNP. The use of AgNP as a preventive or corrective treatment to decrease microbial colonization in three kinds of stone used in historical walls was successful. Calcite was more colonized by Alternaria alternata, but less by Pectobacterium carotovorum. This is the first study at different scales (in vitro and tests on different stone types) of inhibition of biodeterioration-causing microorganisms isolated from an archaeological site by green synthesized AgNP.

Growth inhibitory properties of lactose fatty acid esters

Sugar esters are biodegradable, nonionic surfactants which have microbial inhibitory properties. The influence of the fatty acid chain length on the microbial inhibitory properties of lactose esters was investigated in this study. Specifically, lactose monooctanoate (LMO), lactose monodecanoate (LMD), lactose monolaurate (LML) and lactose monomyristate (LMM) were synthesized and dissolved in both dimethyl sulfoxide (DMSO) and ethanol. Minimum inhibitory concentrations (MIC) and minimum bactericidal concentrations (MBC) were determined in growth media. LML was the most effective ester, exhibiting MIC values of <0.05 to <5 mg/ml for each Gram-positive bacteria tested (Bacillus cereus, Mycobacterium KMS, Streptococcus suis, Listeria monocytogenes, Enterococcus faecalis, and Streptococcus mutans) and MBC values of <3 to <5 mg/ml for B. cereus, M. KMS, S. suis, and L. monocytogenes. LMD showed MIC and MBC values of <1 to <5 mg/ml for B. cereus, M. KMS, S. suis, L. monocytogenes, and E. faecalis, with greater inhibition when dissolved in ethanol. LMM showed MIC and MBC values of <1 to <5 mg/ml for B. cereus, M. KMS, and S. suis. LMO was the least effective showing a MBC value of <5 mg/ml for only B. cereus, though MIC values for S. suis and L. monocytogenes were observed when dissolved in DMSO. B. cereus and S. suis were the most susceptible to the lactose esters tested, while S. mutans and E. faecalis were the most resilient and no esters were effective on Escherichia coli O157:H7. This research showed that lactose esters esterified with decanoic and lauric acids exhibited greater microbial inhibitory properties than lactose esters of octanoate and myristate against Gram-positive bacteria.