Quorum quenching bacteria isolated from industrial wastewater sludge to control membrane biofouling

Quorum Quenching in Membrane Bioreactors for Fouling Retardation: Complexity Provides Opportunities

The occurrence of biofouling restricts the widespread application of membrane bioreactors (MBRs) in wastewater treatment. Regulation of quorum sensing (QS) is a promising approach to control biofouling in MBRs, yet the underlying mechanisms are complex and remain to be illustrated. A fundamental understanding of the relationship between QS and membrane biofouling in MBRs is lacking, which hampers the development and application of quorum quenching (QQ) techniques in MBRs (QQMBRs). While many QQ microorganisms have been isolated thus far, critical criteria for selecting desirable QQ microorganisms are still missing. Furthermore, there are inconsistent results regarding the QQ lifecycle and the effects of QQ on the physicochemical characteristics and microbial communities of the mixed liquor and biofouling assemblages in QQMBRs, which might result in unreliable and inefficient QQ applications. This review aims to comprehensively summarize timely QQ research and highlight the important yet often ignored perspectives of QQ for biofouling control in MBRs. We consider what this "information" can and cannot tell us and explore its values in addressing specific and important questions in QQMBRs. Herein, we first examine current analytical methods of QS signals and discuss the critical roles of QS in fouling-forming microorganisms in MBRs, which are the cornerstones for the development of QQ technologies. To achieve targeting QQ strategies in MBRs, we propose the substrate specificity and degradation capability of isolated QQ microorganisms and the surface area and pore structures of QQ media as the critical criteria to select desirable functional microbes and media, respectively. To validate the biofouling retardation efficiency, we further specify the QQ effects on the physicochemical properties, microbial community composition, and succession of mixed liquor and biofouling assemblages in MBRs. Finally, we provide scale-up considerations of QQMBRs in terms of the debated QQ lifecycle, practical synergistic strategies, and the potential cost savings of MBRs. This review presents the limitations of classic QS/QQ hypotheses in MBRs, advances the understanding of the role of QS/QQ in biofouling development/retardation in MBRs, and builds a bridge between the fundamental understandings and practical applications of QQ technology.

Integrated Biofilm Modification and Transcriptional Analysis for Improving Fengycin Production in Bacillus amyloliquefaciens

Although fengycin exhibits broad-spectrum antifungal properties, its application is hindered due to its low biosynthesis level and the co-existence of iturin A and surfactin in Bacillus amyloliquefaciens HM618, a probiotic strain. In this study, transcriptome analysis and gene editing were used to explore the potential mechanisms regulating fengycin production in B. amyloliquefaciens. The fengycin level of B. amyloliquefacien HM-3 (∆itu-ΔsrfAA) was 88.41 mg/L after simultaneously inhibiting the biosyntheses of iturin A and surfactin. The knockout of gene eps associated with biofilm formation significantly increased the fengycin level of the strain HM618, whereas the fengycin level decreased 32.05% after knocking out sinI, a regulator of biofilm formation. Transcriptome analysis revealed that the differentially expressed genes, involved in pathways of amino acid and fatty acid syntheses, were significantly down-regulated in the recombinant strains, which is likely associated with a decrease of fengycin production. The knockout of gene comQXPA and subsequent transcriptome analysis revealed that the ComQXPA quorum sensing system played a positive regulatory role in fengycin production. Through targeted genetic modifications and fermentation optimization, the fengycin production of the engineered strain HM-12 (∆itu-ΔsrfAA-ΔyvbJ) in a 5-L fermenter reached 1.172 g/L, a 12.26-fold increase compared to the fengycin level in the strain HM-3 (∆itu-ΔsrfAA) in the Erlenmeyer flask. Taken together, these results reveal the underlying metabolic mechanisms associated with fengycin synthesis and provide a potential strategy for improving fengycin production in B. amyloliquefaciens.

Carbon metabolism characteristics of quorum quenching bacteria Rhodococcus sp. BH4 determine the bioaugmentation efficiency under different carbon source conditions

Carbon sources are critical factors influencing bacterial bioaugmentation, however, the underlying mechanisms, particularly the metabolic characteristics of bioaugmented bacteria remain poorly understood. The bioaugmented bacterium Rhodococcus sp. BH4 secretes the quorum quenching (QQ) enzyme QsdA to disrupt the quorum sensing (QS) in the activated sludge (AS) process, reducing AS yield in-situ. This study investigated the carbon metabolic characteristics of BH4 and explored the effects on bioaugmentation with different influent carbon sources. Because of the absence of glucose-specific phosphoenol phosphotransferase system (PTS), BH4 prefers sodium acetate to glucose. However, the lactones produced during extracellular glucose metabolism enhance BH4 qsdA expression. Moreover, BH4 possess carbon catabolite repression (CCR), acetate inhibits glucose utilization. BH4 microbeads were added to reactors with different carbon sources (R1: sodium acetate; R2: glucose; R3: a mixture of sodium acetate and glucose) for in-situ AS yield reduction. During operation, AS reduction efficiency decreased in the following order: R1 > R3 > R2. R2 and R3 microbeads exhibited similar QQ activity to R1, with less BH4 biomass at 5 d. 13C labeling and Michaelis-Menten equation showed that, due to differences in the competitiveness of carbon sources, R1 BH4 obtained the most carbon, whereas R2 BH4 obtained the least carbon. Moreover, acetate inhibited glucose utilization of R3 BH4. Transcriptome analysis showed that R1 BH4 qsdA expression was the lowest, R2 BH4 was the most serious form of programmed cell death, and the R3 BH4 PTS pathway was inhibited. At 10 d, R1 BH4 biomass and microbead QQ activity were higher than that in R3, and the R2 BH4 lost viability and QQ activity. This study provides new insights into bioaugmentation from the perspectives of carbon source competitiveness, carbon metabolism pathways, and CCR.

Isolation and application of Bacillus thuringiensis LZX01: Efficient membrane biofouling mitigation function and anti-toxicity potential

Significant progress has been made in mitigating membrane biofouling by microbial quorum quenching (QQ). More efficient and survivable QQ strains need to be discovered. A new strain named Bacillus thuringiensis LZX01 was isolated in this study using a low carbon source concentration "starving" method from a membrane bioreactor (MBR). LZX01 secreted intracellular lactonase to enable QQ behavior and was capable of degrading 90 % of C8-HSL (200 ng/mL) within 30 min, which effectively delayed biofouling by inhibiting the growth of bacteria associated with biofouling and improving the hydrophilicity of bound extracellular polymeric substances. As a result, the membrane biofouling rate of MBR adding LZX01 was four times slower than that of the control MBR. Importantly, LZX01 maintains its QQ activity even in environments contaminated with typical toxic pollutants. Therefore, with high efficiency, toxicity resistance, and easy culture, LZX01 holds great potential and significant promise for biofouling control applications.

Inhibiting Biofilm Formation via Simultaneous Application of Nitric Oxide and Quorum Quenching Bacteria

Membrane biofouling is an inevitable challenge in membrane-based water treatment systems such as membrane bioreactors. Recent studies have shown that biological approaches based on bacterial signaling can effectively control biofilm formation. Quorum quenching (QQ) is known to inhibit biofilm growth by disrupting quorum sensing (QS) signaling, while nitric oxide (NO) signaling helps to disperse biofilms. In this study, batch biofilm experiments were conducted to investigate the impact of simultaneously applying NO signaling and QQ for biofilm control using Pseudomonas aeruginosa PAO1 as a model microorganism. The NO treatment involved the injection of NONOates (NO donor compounds) into mature biofilms, while QQ was implemented by immobilizing QQ bacteria (Escherichia coli TOP10-AiiO or Rhodococcus sp. BH4) in alginate or polyvinyl alcohol/alginate beads to preserve the QQ activity. When QQ beads were applied together with (Z)-1-[N-(3-aminopropyl)-N-(n-propyl) amino]diazen-1-ium-1,2-diolate (PAPA NONOate), they achieved a 39.0% to 71.3% reduction in biofilm formation, which was substantially higher compared to their individual applications (16.0% to 54.4%). These findings highlight the significant potential of combining QQ and NO technologies for effective biofilm control across a variety of processes that require enhanced biofilm inhibition.

Isolation and characterization of quorum quenching bacteria from municipal solid waste and bottom ash co-disposal landfills

Co-landfilling of bottom ash (BA) accelerates the clogging of leachate collection systems (LCSs) and increases the risk of landfill failure. The clogging was mainly associated with bio-clogging, which may be reduced by quorum quenching (QQ) strategies. This communication reports on a study of how isolated facultative QQ bacterial strains from municipal solid waste (MSW) landfills and BA co-disposal landfills. In MSW landfills, two novel QQ strains (Brevibacillus agri and Lysinibacillus sp. YS11) can degrade the signal molecule hexanoyl-l-homoserine lactone (C6-HSL) and octanoyl-l-homoserine lactone (C8-HSL), respectively. Pseudomonas aeruginosa could degrade C6-HSL and C8-HSL in BA co-disposal landfills. Moreover, P. aeruginosa (0.98) was observed with a higher growth rate (OD600) compared to that of B. agri (0.27) and Lysinibacillus sp. YS11 (0.53). These results indicated that the QQ bacterial strains were associated with leachate characteristics and signal molecules and could be used for controlling bio-clogging in landfills.

Biofouling in Membrane Bioreactors-Mitigation and Current Status: a Review

Biological fouling as termed biofouling is caused by varied living organisms and is difficult to eliminate from the environment thus becoming a major issue during membrane bioreactors. Biofouling in membrane bioreactors (MBRs) is a crucial problem in increasing liquid pressure due to reduced pore diameter, clogging of the membrane pores, and alteration of the chemical composition of the water which greatly limits the growth of MBRs. Thus, membrane biofouling and/or microbial biofilms is a hot research topic to improve the market competitiveness of the MBR technology. Though several antibiofouling strategies (addition of bioflocculant or sponge into MBRs) came to light, biological approaches are sustainable and more practicable. Among the biological approaches, quorum sensing-based biofouling control (so-called quorum quenching) is an interesting and promising tool in combating biofouling issues in the MBRs. Several review articles have been published in the area of membrane biofouling and mitigation approaches. However, there is no single source of information about biofouling and/or biofilm formation in different environmental settings and respective problems, antibiofilm strategies and current status, quorum quenching, and its futurity. Thus, the objectives of the present review were to provide latest insights on mechanism of membrane biofouling, quorum sensing molecules, biofilm-associated problems in different environmental setting and antibiofilm strategies, special emphasis on quorum quenching, and its futurity in the biofilm/biofouling control. We believe that these insights greatly help in the better understanding of biofouling and aid in the development of sustainable antibiofouling strategies.

Biofilm-based technology for industrial wastewater treatment: current technology, applications and future perspectives

The microbial community in biofilm is safeguarded from the action of toxic chemicals, antimicrobial compounds, and harsh/stressful environmental circumstances. Therefore, biofilm-based technology has nowadays become a successful alternative for treating industrial wastewater as compared to suspended growth-based technologies. In biofilm reactors, microbial cells are attached to static or free-moving materials to form a biofilm which facilitates the process of liquid and solid separation in biofilm-mediated operations. This paper aims to review the state-of-the-art of recent research on bacterial biofilm in industrial wastewater treatment including biofilm fundamentals, possible applications and problems, and factors to regulate biofilm formation. We discussed in detail the treatment efficiencies of fluidized bed biofilm reactor (FBBR), trickling filter reactor (TFR), rotating biological contactor (RBC), membrane biofilm reactor (MBfR), and moving bed biofilm reactor (MBBR) for different types of industrial wastewater treatment. Besides, biofilms have many applications in food and agriculture, biofuel and bioenergy production, power generation, and plastic degradation. Furthermore, key factors for regulating biofilm formation were also emphasized. In conclusion, industrial applications make evident that biofilm-based treatment technology is impactful for pollutant removal. Future research to address and improve the limitations of biofilm-based technology in wastewater treatment is also discussed.

Scavenging of reactive oxygen species effectively reduces Pseudomonas aeruginosa biofilms through disrupting policing

Biofilm formation is likely to contribute greatly to antibiotic resistance in bacteria and therefore the efficient removal of bacterial biofilms needs addressing urgently. Here, we reported that the supplement of non-inhibitory concentration of N-acetyl-L-cysteine (NAC), a common reactive oxygen species (ROS) scavenger, can significantly reduce the biomass of mature Pseudomonas aeruginosa biofilms (corroborated by crystal violet assay and laser scanning confocal microscopy). 1 mM NAC increased the cheater (ΔlasR mutant) frequency to 89.4 ± 1.5% in the evolved PAO1 after the 15-day treatment. Scavenging of ROS by NAC induced the collapse of P. aeruginosa biofilms, but it did not alter quorum sensing-regulated genes expression (e.g., hcnC and cioAB) and hydrogen cyanide production. The replenishment of public good protease contributed to the recovery of biofilm biomass, indicating the role of disrupting policing in biofilm inhibition. Furthermore, 7 typical ROS scavengers (e.g., superoxide dismutase, catalase and peroxidase, etc.) also effectively inhibited mature P. aeruginosa biofilms. This study demonstrates that scavenging of ROS can promote the selective control of P. aeruginosa biofilms through policing disruption as a targeted biofilm control strategy in complex water environments.

Quorum-quenching potential of recombinant PvdQ-engineered bacteria for biofilm formation

Quorum sensing (QS) is a core mechanism for bacteria to regulate biofilm formation, and therefore, QS inhibition or quorum quenching (QQ) is used as an effective and economically feasible strategy against biofilms. In this study, the PvdQ gene encoding AHL acylase was introduced into Escherichia coli (DE3), and a PvdQ-engineered bacterium with highly efficient QQ activity was obtained and used to inhibit biofilm formation. Gene sequencing and western blot analysis showed that the recombinant pET-PvdQ strain was successfully constructed. The color reaction of Agrobacterium tumefaciens A136 indicated that PvdQ engineering bacteria had shown strong AHL signal molecule quenching activity and significantly inhibited the adhesion (motility) of Pseudomonas aeruginosa and biofilm formation of activated sludge bacteria in Membrane Bio-Reactor (MBR; inhibition rate 51-85%, p < 0.05). In addition, qRT-PCR testing revealed that recombinant PvdQ acylase significantly reduced the transcription level of QS biofilm formation-related genes (cdrA, pqsA, and lasR; p < 0.05). In this study, QQ genetically engineered bacteria enhanced by genetic engineering could effectively inhibit the QS signal transduction mechanism and have the potential to control biofilm formation of pathogenic bacteria in the aquaculture environment, providing an environmentally friendly and alternative antibiotic strategy to suppress biofilm contamination.

Innovative microbial disease biocontrol strategies mediated by quorum quenching and their multifaceted applications: A review

With the increasing resistance exhibited by undesirable bacteria to traditional antibiotics, the need to discover alternative (or, at least, supplementary) treatments to combat chemically resistant bacteria is becoming urgent. Quorum sensing (QS) refers to a novel bacterial communication system for monitoring cell density and regulation of a network of gene expression that is mediated by a group of signaling molecules called autoinducers (AIs). QS-regulated multicellular behaviors include biofilm formation, horizontal gene transfer, and antibiotic synthesis, which are demonstrating increasing pathogenicity to plants and aquacultural animals as well as contamination of wastewater treatment devices. To inhibit QS-regulated microbial behaviors, the strategy of quorum quenching (QQ) has been developed. Different quorum quenchers interfere with QS through different mechanisms, such as competitively inhibiting AI perception (e.g., by QS inhibitors) and AI degradation (e.g., by QQ enzymes). In this review, we first introduce different signaling molecules, including diffusible signal factor (DSF) and acyl homoserine lactones (AHLs) for Gram-negative bacteria, AIPs for Gram-positive bacteria, and AI-2 for interspecies communication, thus demonstrating the mode of action of the QS system. We next exemplify the QQ mechanisms of various quorum quenchers, such as chemical QS inhibitors, and the physical/enzymatic degradation of QS signals. We devote special attention to AHL-degrading enzymes, which are categorized in detail according to their diverse catalytic mechanisms and enzymatic properties. In the final part, the applications and advantages of quorum quenchers (especially QQ enzymes and bacteria) are summarized in the context of agricultural/aquacultural pathogen biocontrol, membrane bioreactors for wastewater treatment, and the attenuation of human pathogenic bacteria. Taken together, we present the state-of-the-art in research considering QS and QQ, providing theoretical evidence and support for wider application of this promising environmentally friendly biocontrol strategy.

Reclaimed wastewater reuse in irrigation: Role of biofilms in the fate of antibiotics and spread of antimicrobial resistance

Reclaimed wastewater associated biofilms are made up from diverse class of microbial communities that are continuously exposed to antibiotic residues. The presence of antibiotic resistance bacteria (ARB) and their associated antibiotic resistance genes (ARGs) ensures also a continuous selection pressure on biofilms that could be seen as hotspots for antibiotic resistance dissemination but can also play a role in antibiotic degradation. In this study, the antibiotic degradation and the abundance of four ARGs (qnrS, sul1, blaTEM, ermB), and two mobile genetic elements (MGEs) including IS613 and intl1, were followed in reclaimed wastewater and biofilm samples collected at the beginning and after 2 weeks of six antibiotics exposure (10 µg L^-1). Antibiotics were partially degraded and remained above lowest minimum inhibitory concentration (MIC) for environmental samples described in the literature. The most abundant genes detected both in biofilms and reclaimed wastewater were sul1, ermB, and intl1. The relative abundance of these genes in biofilms increased during the 2 weeks of exposure but the highest values were found in control samples (without antibiotics pressure), suggesting that bacterial community composition and diversity are the driven forces for resistance selection and propagation in biofilms, rather than exposure to antibiotics. Planktonic and biofilm bacterial communities were characterized. Planktonic cells are classically defined "as free flowing bacteria in suspension" as opposed to the sessile state (the so-called biofilm): "a structured community of bacterial cells enclosed in a self-produced polymeric matrix and adherent to an inert or living. surface" as stated by Costerton et al. (1999). The abundance of some genera known to harbor ARG such as Streptococcus, Exiguobacterium, Acholeplasma, Methylophylaceae and Porphyromonadaceae increased in reclaimed wastewater containing antibiotics. The presence of biofilm lowered the level of these genera in wastewater but, at the opposite, could also serve as a reservoir of these bacteria to re-colonize low-diversity wastewater. It seems that maintaining a high diversity is important to limit the dissemination of antimicrobial resistance among planktonic bacteria. Antibiotics had no influence on the biofilm development monitored with optical coherence tomography (OCT). Further research is needed in order to clarify the role of inter-species communication in biofilm on antibiotic degradation and resistance development and spreading.