Quorum quenching is responsible for the underestimated quorum sensing effects in biological wastewater treatment reactors

Quorum sensing mediated response mechanism of anammox consortia to anionic surfactant: Molecular simulation and molecular evidence

The widespread use of surfactants raise challenges to biological wastewater treatment. Anaerobic ammonium oxidation (anammox) process has the potential to treat wastewater containing anionic surfactants, but the response of anammox consortia at the molecular level under long-term exposure is unclear. Using high-throughput sequencing and gene quantification, combined with molecular docking, the effect of sodium dodecyl sulfonate (SDS) on anammox consortia were investigated. Levels of reactive oxygen species (ROS) might be lower than the threshold of oxidative damage, while the increase of lactate dehydrogenase (LDH) represented the cell membrane damage. Decreased abundance of functional genes (hdh, hzsA and nirS) indicated the decrease of the anammox bacterial abundance. Trace amounts of N-acyl homoserine lactone (AHL, C6-HSL, C8-HSL and C12-HSL) contained in influent could induce endogenous quorum sensing (QS), which could regulate the correlation between functional bacteria to optimize the microbial community and strengthen the resistance of anammox consortia to SDS. In addition, the proliferation of disinfectant resistance genes might increase the environmental pathogenicity of sewage discharge. This work highlights the potential response mechanism of anammox consortium to surfactants and provides a universal microbial-friendly bioenhancement strategy based on QS.

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.

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.

A method establishment and application for biofilm quorum quenching activity assay

The existence of quorum sensing (QS) and quorum quenching (QQ) plays important roles in biofilm formation. However, direct detection of QS ability is difficult due to the low concentrations of signal molecules inside the biofilm. Therefore, QQ activity is typically used to indicate the attribution of QS/QQ to the biofilm. Nevertheless, current detection methods of QQ activity based on biosensors present undesirable operability and accuracy. In this study, the 96-well plate assay based on a specific biosensor, Agrobacterium tumefaciens A136, and a colorimetric substance, X-gal was established. The reliable fitting results were obtained by standardizing the composition of the A136 X-gal assay solution and optimizing the operating conditions. This method improved the accuracy of QQ activity detection and reduced time and cost consumption. Finally, the 96-well plate assay was successfully applied to detect the QQ activities of biofilm samples and explore possible environmental influencing factors. In general, this study provided a new strategy for understanding the QQ effect in biofilm systems.

Research advances in application of mainstream anammox processes: Roles of quorum sensing and microbial metabolism

Anaerobic ammonium oxidation (anammox) is a low-carbon biological nitrogen removal process, that has been widely applied to treat high-strength wastewater. However, the practical application of mainstream anammox treatment is limited due to the slow growth rate of anammox bacteria (AnAOB). Therefore, it is important to provide a comprehensive summary of the potential impacts and regulatory strategies for system stability. This article systematically reviewed the effects of environmental fluctuations on anammox systems, summarizing the bacterial metabolisms and the relationship between metabolite and microbial functional effects. To address the shortcoming of mainstream anammox process, molecular strategies based on quorum sensing (QS) were proposed. Sludge granulation, gel encapsulation and carrier-based biofilm technologies were adopted to enhance the QS function in microbial aggregation and reduction of biomass loss. Furthermore, this article discussed the application and progress of anammox-coupled processes. Valuable insights were provided for the stable operation and development of mainstream anammox process from the perspectives of QS and microbial metabolism.

Role of bacterial quorum sensing and quenching mechanism in the efficient operation of microbial electrochemical technologies: A state-of-the-art review

Microbial electrochemical technologies (METs) are a group of innovative technologies that produce valuables like bioelectricity and biofuels with the simultaneous treatment of wastewater from microorganisms known as electroactive microorganisms. The electroactive microorganisms are capable of transferring electrons to the anode of a MET through various metabolic pathways such as direct (via cytochrome or pili) or indirect (through transporters) transfer. Though this technology is promising, the inferior yield of valuables and the high cost of reactor fabrication are presently impeding the large-scale application of this technology. Therefore, to overcome these major bottlenecks, a lot of research has been dedicated to the application of bacterial signalling, for instance, quorum sensing (QS) and quorum quenching (QQ) mechanisms in METs to improve its efficacy in order to achieve a higher power density and to make it more cost-effective. The QS circuit in bacteria produces auto-inducer signal molecules, which enhances the biofilm-forming ability and regulates the bacterial attachment on the electrode of METs. On the other hand, the QQ circuit can effectively function as an antifouling agent for the membranes used in METs and microbial membrane bioreactors, which is imperative for their stable long-term operation. This state-of-the-art review thus distinctly describes in detail the interaction between the QQ and QS systems in bacteria employed in METs to generate value-added by-products, antifouling strategies, and the recent applications of the signalling mechanisms in METs to improve their yield. Further, the article also throws some light on the recent advancements and the challenges faced while incorporating QS and QQ mechanisms in various types of METs. Thus, this review article will help budding researchers in upscaling METs with the integration of the QS signalling mechanism in METs.

Potential roles of acyl homoserine lactones (AHLs) in nitrifying bacteria survival under certain adverse circumstances

Potential roles of quorum sensing (QS) in nitrifying bacteria activity and ecology, particularly under adverse circumstances have been rarely reported. Herein, eight lab-scale nitrification sequencing batch reactors, with or without adding acyl homoserine lactones (AHLs) were operated under adverse circumstances respectively. The results indicated that the introduction of AHLs significantly enhanced the nitrogen removal efficiency in the presence of nitrification inhibitors (dicyandiamide, DCD), accelerated the low temperature (10 °C) group into stable stage, and improved the utilization efficiency of AHLs in these two groups. Community analysis and qPCR further confirmed that AHLs significantly increased the abundance of nitrifying bacteria in low temperature group and DCD group, especially AOB. For normal condition (28 °C, pH = 8) or low pH level (5.5), however, the AHLs had no significant effect. Canonical correspondence analysis showed that nitrifying bacteria positively responded to AHLs, indicating that adding AHLs was an effective strategy to regulate nitrification process. However, under acid conditions, the effect of this regulatory mechanism was not significant, indicating that the influence of pH on the system was greater than that of AHLs. This study demonstrated that exogenous AHLs could enhance the competitiveness of nitrifying bacteria to utilize more resource and occupy space under some adverse environmental conditions.

Quorum sensing unveils the sludge floccule-assisted stabilization of aerobic granules in granule-dominated sequencing batch reactors

Floccules are another major form of microbial aggregates in aerobic granular sludge systems. Previous studies mainly attributed the persistence of floccules to their relatively faster nutrient uptake and higher growth rate over aerobic granules; however, they failed to unravel the underlying mechanism of the long-term coexistence of these two aggregates. In this work, the existence and function of the floccules in an aerobic granule-dominated sequencing batch reactor were investigated from the view of quorum sensing (QS) and quorum quenching (QQ). The results showed that though the floccules were closely associated with the granules in terms of similar community structures (including the QS- and QQ-related ones), they exhibited a relatively higher QQ-related activity but a lower QS-related activity. A compatible proportion of floccules might be helpful to maintain the QS-related activity and keep the granules stable. In addition, the structure difference was demonstrated to diversify the QS- and QQ-related activities of the floccules and the aerobic granules. These findings could broaden our understanding of the interactions between the coexistent floccules and granules in aerobic granule-dominated systems and would be instructive for the development of the aerobic granular sludge process.

Exploring the Function of Quorum Sensing Regulated Biofilms in Biological Wastewater Treatment: A Review

Quorum sensing (QS), a type of bacterial cell–cell communication, produces autoinducers which help in biofilm formation in response to cell population density. In this review, biofilm formation, the role of QS in biofilm formation and development with reference to biological wastewater treatment are discussed. Autoinducers, for example, acyl-homoserine lactones (AHLs), auto-inducing oligo-peptides (AIPs) and autoinducer 2, present in both Gram-negative and Gram-positive bacteria, with their mechanism, are also explained. Over the years, wastewater treatment (WWT) by QS-regulated biofilms and their optimization for WWT have gained much attention. This article gives a comprehensive review of QS regulation methods, QS enrichment methods and QS inhibition methods in biological waste treatment systems. Typical QS enrichment methods comprise adding QS molecules, adding QS accelerants and cultivating QS bacteria, while typical QS inhibition methods consist of additions of quorum quenching (QQ) bacteria, QS-degrading enzymes, QS-degrading oxidants, and QS inhibitors. Potential applications of QS regulated biofilms for WWT have also been summarized. At last, the knowledge gaps present in current researches are analyzed, and future study requirements are proposed.

Transboundary intercellular communications between Penicillium and bacterial communities during sludge bulking: Inspirations on quenching fungal dominance

Fungal bulking is caused by the evolution toward a fungi-dominant unbalanced sludge system, which is indeed the phenomenon of fungi competing against bacterial cells. We hypothesized that the cross-kingdom intercellular communication between fungi and bacteria was internal driving force that stimulated fungal bulking. In this study, we identified three signal molecules related to Penicillium fungi bulking under low-pH stress in an activated sludge reactor, which inspired us to propose a sludge bulking prevention strategy using the quorum quenching theory. When pH dropped from 7.0 to 4.5, the abundance of Penicillium increased from 12.5% to 44.8%. However, some functional bacterial genera, such as Nitrosomonas and Sphingopyxis, were washed out from the sludge. The production of quorum-sensing (QS) molecules N-Heptanoyl-L-homoserine lactone (C7-HSL), N-Dodecanoyl-L-homoserine lactone (C12-HSL), and N-Tetradecanoyl-L-homoserine lactone (C14-HSL) was regulated with sludge bulking; especially the response of the latter two was significantly negative to Penicillium blooming (P < 0.05). To test their roles, trace commercial C12-HSL and C14-HSL were added to Penicillium culture, successfully causing 8.3% and 30.2% inhibition of mycelial formation, respectively. They also contributed to the improvement of activated sludge settleability by 6.1% and 39.7%, respectively (represented by sludge volume index). The transcriptome technique further revealed the regulation of the expression of genes in |logFC| >1, involving signal transduction, mycelium synthesis, and metabolic pathways. Our study provided an innovative strategy for controlling fungal bulking from the perspective of microbial transboundary informatics.

Exploiting Biofilm Characteristics to Enhance Biological Nutrient Removal in Wastewater Treatment Plants

Biological treatments are integral processes in wastewater treatment plants (WWTPs). They can be carried out using sludge or biofilm processes. Although the sludge process is effective for biological wastewater systems, it has some drawbacks that make it undesirable. Hence, biofilm processes have gained popularity, since they address the drawbacks of sludge treatments, such as the high rates of sludge production. Although biofilms have been reported to be essential for wastewater, few studies have reviewed the different ways in which the biofilm properties can be explored, especially for the benefit of wastewater treatment. Thus, this review explores the properties of biofilms that can be exploited to enhance biological wastewater systems. In this review, it is revealed that various biofilm properties, such as the extracellular polymeric substances (EPS), quorum sensing (Qs), and acylated homoserine lactones (AHLs), can be enhanced as a sustainable and cost-effective strategy to enhance the biofilm. Moreover, the exploitation of other biofilm properties such as the SOS, which is only reported in the medical field, with no literature reporting it in the context of wastewater treatment, is also recommended to improve the biofilm technology for wastewater treatment processes. Additionally, this review further elaborates on ways that these properties can be exploited to advance biofilm wastewater treatment systems. A special emphasis is placed on exploiting these properties in simultaneous nitrification and denitrification and biological phosphorus removal processes, which have been reported to be the most sensitive processes in biological wastewater treatment.

Colonization of biofilm in wastewater treatment: A review

The attachment and colonization process of microorganisms on a carrier is an interdisciplinary research field. Through a series of physical, chemical, and biological actions, the microorganisms can eventually reproduce on the carrier. This article introduces biofilm start-up and its applications, and explores the current issues to look forward to future development directions. Firstly, the mechanism of microbial film formation is analyzed from the microbial community colonization and reproduction process. Secondly, when analyzing the factors influencing microbial membrane formation, the effect of microbial properties (e.g., genes, proteins, lipids) and external conditions (i.e., carrier, operating environment, and regulation mechanism among microbial communities) were discussed in depth. Aimed at exploring the mechanisms and influencing factors of biofilm start-up, this article proposes the application measures to strengthen this process. Finally, the problems encountered and the future development direction of the technology are analyzed and prospected.

Quorum sensing - a promising tool for degradation of industrial waste containing persistent organic pollutants

Restoring an environment contaminated with persistent organic pollutants (POPs) is highly challenging. Biodegradation by biofilm-forming bacteria through quorum sensing (QS) is a promising treatment process to remove these pollutants and promotes eco-restoration. QS plays an important role in biofilm formation, solubilization, and biotransformation of pollutants. QS is a density-based communication between microbial cells via signalling molecules, which coordinates specific characters and helps bacteria to acclimatize against stress conditions. Genetic diversification of a biofilm offers excellent opportunities for horizontal gene transfer, improves resistance against stress, and provides a suitable environment for the metabolism of POPs. To develop this technology in industrial scale, it is important to understand the fundamentals and ubiquitous nature of QS bacteria and appreciate the role of QS in the degradation of POPs. Currently, there are knowledge gaps regarding the environmental niche, abundance, and population of QS bacteria in wastewater treatment systems. This review aims to present up-to-date and state-of-the-art information on the roles of QS and QS-mediated strategies in industrial waste treatment including biological treatments (such as activated sludge), highlighting their potentials using examples from the pulp and paper mill industry, hydrocarbon remediation and phytoremediation. The information will help to provide a throughout understanding of the potential of QS to degrade POPs and advance the use of this technology. Current knowledge of QS strategies is limited to laboratory studies, full-scale applications remain challenging and more research is need to explore QS gene expression and test in full-scale reactors for wastewater treatment.

Efficient biostimulants for bacterial quorum quenching to control fouling in MBR

Quorum quenching (QQ), which disrupts bacterial communication and biofilm formation, could alleviate biofouling in MBR. QQ bio-stimulus possessing similar conserved moiety as the signal molecule could promote indigenous QQ bacteria, and thus successfully alleviate biofouling in MBR. However, efficient biostimulant has been barely explored for QQ enhancement in activated sludge system. This study extensively enumerated the potential QQ bio-stimuli, and examined their efficacy on QQ promotion for activated sludge. Moreover, the effect of the QQ consortia on fouling mitigation was also investigated. The results indicated that gamma-caprolactone (GCL), d-xylonic acid-1,4-lactone (XAL), gamma-heptalactone (GHL), urea, and acetamide proved effective in promoting AHLs inactivating activity of activated sludge. GCL, XAL, and GHL intensified the lactonase activity, while urea and acetamide augmented acylase activity. While coupled with beads entrapment, GCL consortia beads, XAL consortia beads, and urea consortia beads effectively disrupted quorum sensing (QS) and controlled membrane fouling in MBR. This work found out several optional bio-stimuli valid for tuning QQ in activated sludge system, and provided easily available and economical alternatives for QQ biostimulation, meanwhile the proposed QQ-MBR approach through QQ biostimulation and consortia entrapment also proved effective and practical.

Effects of interactions between quorum sensing and quorum quenching on microbial aggregation characteristics in wastewater treatment: A review

Due to the increasingly urgent demand for effective wastewater denitrification and dephosphorization systems, there is a need to improve the performance of existing biological treatment technologies. As a bacteria-level communication mechanism, quorum sensing (QS) synchronizes gene expression in a density-dependent manner and regulates bacterial physiological behavior. On this basis, the QS-based bacterial communication mechanism and environmental factors affecting QS are discussed. This paper reviews the influence of QS on sludge granulation, biofilm formation, emerging contaminants (ECs) removal, and horizontal gene transfer in sewage treatment system. Furthermore, the QS inhibition strategies are compared. Based on the coexistence and balance of QQ and QS in the long-term operation system, QQ, as an effective tool to regulate the growth density of microorganisms, provides a promising exogenous regulation strategy for residual sludge reduction and biofilm pollution control. This paper reviews the potential of improving wastewater treatment efficiency based on QS theory and points out the feasibility and prospect of exogenous regulation strategy. PRACTITIONER POINTS: The mechanism of bacterial communication based on QS and the environmental factors affecting QS were discussed. The application of QS and QQ in improving the sludge performance of biological treatment systems was described. The significance of QS and QQ coexistence in sewage treatment process was described.

Realizing the role of N-acyl-homoserine lactone-mediated quorum sensing in nitrification and denitrification: A review

Nitrification and denitrification are crucial processes in the nitrogen cycle, a vital microbially driven biogeochemical cycle. N-acyl-homoserine lactone (AHL)-mediated quorum sensing (QS) is widespread in bacteria and plays a key role in their physiological status. Recently, there has been an increase in research into how the AHL-mediated QS system is involved in nitrification and denitrification. Consequentially, the AHL-mediated QS system has been considered a promising regulatory approach in nitrogen metabolism processes, with high potential for real-world applications. In this review, the universal presence of QS in nitrifiers and denitrifiers is summarized. Many microorganisms taking part in nitrification and denitrification harbor QS genes, and they may produce AHLs with different chain lengths. The phenotypes and processes affected by QS in real-world applications are also reviewed. In wastewater bioreactors, QS could affect nitrogen metabolism efficiency, granule aggregation, and biofilm formation. Furthermore, methods commonly used to identify the existence and functions of QS, including physiological tests, genetic manipulation and omics analyses are discussed.

Longer persistence of quorum quenching bacteria over quorum sensing bacteria in aerobic granules

Involvements of quorum sensing (QS) in the formation of aerobic granules for wastewater treatment have been well recognized. In previous studies the evolution of the QS-related activities and communities during bioreactor start-up period has been extensively studied, while the variation of QS in long-term reactor operation remains unrevealed. Furthermore, information about the roles of quorum quenching (QQ) in bioreactors is very limited. In this work, both QS and QQ during the start-up and successive long-term operation period of an aerobic granule bioreactor were explored. The QS activity and communities increased in the start-up but gradually decreased in the long-term operation, while the QQ activity and communities remained stable. These results indicate the longer persistence of QQ than QS in the granules and the minor contribution of QS in the long-term operation. This work provides a new insight into the roles of QQ and QS in wastewater treatment bioreactors.

Discrepant roles of a quorum quenching bacterium (Rhodococcus sp. BH4) in growing dual-species biofilms

Quorum quenching (QQ) is a promising alternative method for biofilm control. However, a largely unexplored issue is the mechanism through which QQ bacteria interact with biofilm-forming bacteria. Here, we explore inter-species interactions during biofilm development (using 96-well polystyrene plates in a static incubator) between the QQ bacterium Rhodococcus sp. BH4 and sludge bacteria. Experimental results revealed that strain BH4 provoked both competitive (76%) and cooperative (24%) interactions (P < 0.05) in dual-species biofilms after 24 h of incubation (mature biofilm), implying that signal destruction by strain BH4 was strain-dependent. Besides hike in the biofilm biomass (~21%), amount of extracellular polymeric substances (EPS) (25-30 times) and particle size (3.5 times) in the Serratia sp. JSB1 biofilm were increased by str. BH4. This suggests that strain BH4 may only have quenching effects against certain bacteria, and that such effects are overlooked at the community level. Taken together, present results imply that in a given biofilm community, not all QS-bacteria interact similarly with Rhodococcus sp. BH4, either because QS-bacteria are tolerant of strain BH4 or QS-bacteria have more than one mechanism for biofilm development. Overall, the QQ-strategy alone seems ineffective at controlling biofilm development, although it may be used in combination with other strategies.

Feasibility of isolated novel facultative quorum quenching consortiums for fouling control in an AnMBR

Anaerobic membrane bioreactor (AnMBR) technology is being recognized as an appealing strategy for wastewater treatment, however, severity of membrane fouling inhibits its widespread implementations. This study engineered novel facultative quorum quenching consortiums (FQQs) coping with membrane fouling in AnMBRs with preliminary analysis for their quorum quenching (QQ) performances. Herein, Acyl-homoserine lactones (AHLs)-based quorum sensing (QS) in a lab-scale AnMBR initially revealed that N-Hexanoyl-dl-homoserine lactone (C6-HSL), N-Octanoyl-dl-homoserine lactone (C8-HSL) and N-Decanoyl-dl-homoserine lactone (C10-HSL) were the dominant AHLs in AnMBRs in this study. Three FQQs, namely, FQQ-C6, FQQ-C8 and FQQ-C10, were harvested after anaerobic screening of aerobic QQ consortiums (AeQQs) which were isolated by enrichment culture, aiming to degrade C6-HSL, C8-HSL and C10-HSL, respectively. Growth of FQQ-C6 and FQQ-C10 using AHLs as carbon source under anaerobic condition was significantly faster than those using acetate, congruously suggesting that their QQ performance will not be compromised in AnMBRs. All FQQs degraded a wide range of AHLs pinpointing their extensive QQ ability. FQQ-C6, FQQ-C8 and FQQ-C10 remarkably alleviated extracellular polymeric substances (EPS) production in a lab-scale AnMBR by 72.46%, 35.89% and 65.88%, respectively, and FQQ-C6 retarded membrane fouling of the AnMBR by 2 times. Bioinformatics analysis indicated that there was a major shift in dominant species from AeQQs to FQQs where Comamonas sp., Klebsiella sp., Stenotrophomonas sp. and Ochrobactrum sp. survived after anaerobic screening and were the majority in FQQs. High growth rate utilizing AHLs under anaerobic condition and enormous EPS retardation efficiency in FQQ-C6 and FQQ-C10 could be attributed to Comamonas sp.. These findings demonstrated that FQQs could be leveraged for QQ under anaerobic systems. We believe that this was the first work proposing a bacterial pool of facultative QQ candidates holding biotechnological promises for membrane fouling control in AnMBRs.

A sustainable strategy for effective regulation of aerobic granulation: Augmentation of the signaling molecule content by cultivating AHL-producing strains

The positive roles of N-acyl homoserine lactone (AHL)-mediated quorum sensing (QS) in aerobic granular sludge (AGS) have been widely acknowledged. However, it is not feasible to manipulate granulation via direct addition of AHL chemicals or AHL-producing strains. Here, several strains with high AHL-producing capacity were successfully isolated from AGS. These QS strains were cultivated, mixed as a consortium, and then divided into two groups: AHLs supernatant and bacterial cells encapsulated in sodium alginate (CEBs). The potential of QS regulation, via doses of AHLs supernatant and CEBs, in accelerating granulation was evaluated. Results clearly indicated that short-term (days 21-70) addition of AHLs supernatant led to a rapid specific growth rate (0.08 d^-1), compact structure without filamentous bacteria overgrowth, excellent settlement performance (SVI₁₀ 37.2 mL/g), and a high integrity coefficient (4.4%) of the granules. Sustainable release of AHLs (mainly C₆- and C₈-HSL) was induced by exogenous AHLs, possibly attributed to the enrichment of the genera Aeromonas and Pseudomonas. Further, tryptophan and aromatic protein substances were produced to maintain structural stability, suggesting that short-term QS regulation had long-term positive effects on the characteristics of AGS. By comparison, the addition of CEBs posed negligible or negative impact on the granulation, as evidenced by the rupture of smaller aggregates and poor characteristics of AGS. Overall, augmentation of the signaling content via addition of AHLs supernatant from QS strains is an economical and feasible regulation strategy to accelerate granulation and sustain long-term structural stability.

The role of quorum sensing in granular sludge: Impact and future application: A review

Quorum sensing (QS) is a process widely exist in bacteria, which refers to the cell-cell communication through secretion and sensing the specific chemical signal molecules named autoinducers. This review demonstrated recent research progresses on the specific impacts of signal molecules in the granular sludge reactors, such corresponding exogenous strategies contained the addition of QS signal molecules, QS-related enzymes and bacteria associated with QS process. Accordingly, the correlation between QS signaling molecule content and sludge granulation (including the formation and stability) was assumed, the comprehensive conclusion elucidated that some QS signals (acyl-homoserine lactone and Autoinducer 2) can accelerate the growth of particle diameter, the production of extracellular polymeric substance (EPS), microbial adhesion and change the microbiome structure. But diffusable signal factor (DSF) acted as a significant disincentive to the formation and stability of GS. As a result, it deserved serious attention on the value and role of QS signals in the GS. This review attempts to illuminate the potential method for addressing the main bottleneck: to accelerate the formation of granules and keep the high stability of GS for a long-term reactor. Therefore, review discussed the possible trends of GS: QS and intercellular/intracellular signaling which can lay a theoretical foundation for mechanism of GS formation and stability, would be of practical significance for further application in the future.

Operational pattern affects nitritation, microbial community and quorum sensing in nitrifying wastewater treatment systems

Achievement of nitrite accumulation is critical for the application of advanced nitrogen removal processes. Two lab-scale sequencing batch biofilm reactors (SBBRs) and two sequencing batch reactors (SBRs) were operated under intermittent aeration with different feeding patterns. The nitrite accumulation ratio was 56.6% (pulse feeding) and 68.9% (constant feeding) in SBBRs with nitritation. Nitrate accounted for 98% of the effluent nitrogen in SBRs with complete nitrification. The dominant nitrifier was Nitrosomonas in SBBRs and Nitrospira in SBRs. Four types of acyl homoserine lactones (AHLs) were detected. N‑[(RS)‑3‑Hydroxybutyryl]‑l‑homoserine lactone and N‑octanoyl‑l‑homoserine lactone had a high concentration in the extracellular polymeric substance phase, and had an obvious relationship with nitrite accumulation and ammonia removal. Various microbial communities coexisted in nitrifying systems, with diverse microbial interactions. Microorganisms harboring AHLs-related genes had more interactions with each other, suggesting that nitritation could be regulated by AHLs based quorum sensing.

Identification of quorum sensing signal AHLs synthases in Candidatus Jettenia caeni and their roles in anammox activity

Acyl-homoserine lactone (AHL)-based quorum sensing (QS) in the anaerobic ammonium oxidizing (anammox) consortia has attracted increasing attention. However, AHL synthase in anammox bacteria and the relationship between AHL synthetic genes and anammox activity are still not clear because anammox bacteria have not been isolated from the consortia. Two novel synthases of AHLs (JqsI-1 and JqsI-2), which are HdtS-type rather than the widely studied LuxI-type, were identified in anammox bacteria Candidatus Jettenia caeni and synthesized four AHLs. There was a correlation between AHL concentration, in situ transcriptional expression of the AHL synthase genes (jqsI-1 and jqsI-2) and genetic marker of anammox activity (hydrazine synthase gene, hzsA). And AHL add-back studies demonstrated that AHL influence the expression of hzsA to regulate anammox bacterial activity. This study provides insight into the QS communication pathway of anammox bacteria for wastewater treatment.

Long- and short-chain AHLs affect AOA and AOB microbial community composition and ammonia oxidation rate in activated sludge

Quorum sensing (QS) regulation of the composition of ammonia-oxidising archaea (AOA) and ammonia-oxidising bacteria (AOB) communities and functions in wastewater treatment was investigated. Specifically, we explored the role of N-acyl-l-homoserine lactones (AHLs) in microbial community dynamics in activated sludge. On average, the specific ammonia-oxidising-rate increased from 1.6 to 2.8 mg NH₄⁺-N/g MLSS/hr after treatment with long-chain AHLs for 16 days, and the addition of AHLs to sludge resulted in an increased number of AOA/AOB amoA genes. Significant differences were observed in the AOA communities of control and AHL-treated cultures, but not the AOB community. Furthermore, the dominant functional AOA strains of the Crenarchaeota altered their ecological niche in response to AHL addition. These results provide evidence that AHLs play an important role in mediating AOA/AOB microbial community parameters and demonstrate the potential for application of QS to the regulation of nitrogen compound metabolism in wastewater treatment.

Roles of quorum sensing in biological wastewater treatment: A critical review

Quorum sensing (QS) and quorum quenching (QQ) are increasingly reported in biological wastewater treatment processes because of their inherent roles in biofilm development, bacterial aggregation, granulation, colonization, and biotransformation of pollutants. As such, the fundamentals and ubiquitous nature of QS bacteria are critical for fully understanding the process of the wastewater treatment system. In this article, the details of QS-based strategies related to community behaviors and phenotypes in wastewater treatment systems were reviewed. The molecular aspects and coexistence of QS and QQ bacteria were also mentioned, which provide evidence that future wastewater treatment will indispensably rely on QS-based strategies. In addition, recent attempts focusing on the use of QQ for biofilm or biofouling control were also summarized. Nevertheless, there are still several challenges and knowledge gaps that warrant future targeted research on the ecological niche, abundance, and community of QS- and QQ-bacteria in environmental settings or engineered systems.

Understanding of aerobic sludge granulation enhanced by sludge retention time in the aspect of quorum sensing

Aerobic granular sludge (AGS) reactors with different sludge retention times (SRTs) were established for enhanced functional microorganism enrichment and granular formation. Results showed that higher total nitrogen (TN) removal efficiency and compact granules were achieved in the 6-day-SRT reactor. Also, Xanthomonadaceae, Rhodobacteraceae and Hyphomonadaceae with AHL-producing and EPS-secreting functions also enriched under 6-day SRT. For investigating the enhanced mechanism of sludge granulation, typical quorum sensing signals of acylated-homoserine-lactones (AHLs) and extracellular polymeric substances (EPS) were analyzed. Tryptophan-and-protein-like substances were major EPS components in granules formed at 6-day SRT. Meanwhile, most detected AHLs, i.e. C8-HSL and 3OHC8-HSL, were correlated positively with contents of tryptophan-and-protein-like substances. According to AHLs add-back test, AHLs especially those with 8-carbon sidechains, played important roles in aerobic sludge granulation via secreting special extracellular proteins by functional microbes enrichment.

Membrane biofouling behaviors at cold temperatures in pilot-scale hollow fiber membrane bioreactors with quorum quenching

In this study, the seasonality of the biofouling behavior of pilot-scale membrane bioreactors (MBRs) run in parallel with vacant sheets and quorum quenching (QQ) sheets using real municipal wastewater was investigated. QQ media delayed fouling, but low temperatures caused severe biofouling. The greater amount of extracellular polymeric substances (EPSs) produced in cold weather was responsible for the faster biofouling of a membrane, even with QQ media. There were significant negative relationships between EPS levels and water temperature. Cold weather was detrimental to the degradation of quorum sensing signal molecules by QQ sheets, whose activity was restored with a higher dose of QQ bacteria. The QQ bacteria in the sheets experienced a slight loss in activity during the early stage of the field test, but survived in the pilot-scale MBR fed with real wastewater. There were no significant discrepancies in treatment efficiency among conventional, vacant, and QQ MBRs.

Potential roles of acyl homoserine lactone based quorum sensing in sequencing batch nitrifying biofilm reactors with or without the addition of organic carbon

Two lab-scale nitrifying sequencing batch biofilm reactors, with (SBBR_CN) or without the addition of organics (SBBR_N), were operated to investigate potential roles of acyl homoserine lactone (AHL) based quorum sensing. AHLs of N-[(RS)-3-Hydroxybutyryl]-L-homoserine lactone, N-hexanoyl-L-homoserine lactone (C₆-HSL) and N-octanoyl-L-homoserine lactone (C₈-HSL) were detected in both reactors. C₆-HSL and C₈-HSL were also detected in batch experiments, especially with stimulated nitrite oxidizing bacteria activities. Quorum sensing affected biofilm formation mainly through the regulation of extracellular protein production. By the metagenomics analysis, many identified genera and species could participate in quorum sensing, quorum quenching and extracellular polymeric substances (EPS) production. A high quorum quenching activity was obtained in SBBR_CN, whereas a high quorum sensing activity in SBBR_N. Nitrosomonas-like ammonia oxidizing bacteria, Nitrospira-like nitrite oxidizing bacteria and Comammox harbored genes for AHL synthesis and EPS production. Possible relationships among AHLs synthesis, biofilm formation and nitrifiers activity were proposed.

Determination of quorum-sensing signal substances in water and solid phases of activated sludge systems using liquid chromatography-mass spectrometry

The detection of acyl homoserine lactones (AHLs) in activated sludge is essential for clarifying their function in wastewater treatment processes. An LC-MS/MS method was developed for the detection of AHLs in both the aqueous and solid phases of activated sludge. In addition, the effects of proteases and extracellular polymeric substances (EPS) on the detection of AHLs were evaluated by adding protease inhibitors and extracting EPS, respectively. Recoveries of each AHL were improved by adding 50μL of protease inhibitor, and recoveries were also improved from 0 to 56.9% to 24.2%-105.8% by EPS extraction. Applying the developed method to determine the type and concentration of AHLs showed that C₄-HSL, C₆-HSL, C₈-HSL and 3-oxo-C₈-HSL were widely detected in a suspended activated sludge system. The dominant AHL was C₈-HSL, with a highest concentration of 304.3ng/L. C₄-HSL was mainly distributed in the aqueous phase, whereas C₆-HSL, C₈-HSL and 3-oxo-C₈-HSL were preferentially distributed in the sludge phase.

The diversity, distribution and function of N-acyl-homoserine lactone (AHL) in industrial anaerobic granular sludge

Although AHL-mediated quorum sensing (QS) signaling has been proved to be ecologically important in biofilm formation and aerobic granulation process, the biological role of AHL in anaerobic granule has not been experimentally investigated. In this paper, we explored the AHL level in 10 full-scale industrial anaerobic granular bioreactors and detected a total of 4 kinds of AHLs. C8-HSL and C10-HSL were indicated to be the potentially universal QS signal molecules in anaerobic granules and involved in extracellular polymeric substance (EPS) production and granulation process. The add-back experiments further supported the hypothesis that C8-HSL and C10-HSL might play an important role in facilitating anaerobic granulation through regulation of EPS synthesis. 12 Microorganisms might be regulated by AHL to play an important role in EPS production. This study provides a foundation for exploring the function of AHL in anaerobic granular systems, which may shed light on advanced AHL-based anaerobic granulation strategy.

A strategy to speed up formation and strengthen activity of biofilms at low temperature

The start-up period of biofilm reactors often takes a long time to obtain a mature and stable biofilm, especially at low temperature.

Acyl-homoserine lactone-based quorum sensing and quorum quenching hold promise to determine the performance of biological wastewater treatments: An overview

Quorum sensing (QS) is a communication process between cells, in which bacteria secrete and sense the specific chemicals, and regulate gene expression in response to population density. Quorum quenching (QQ) blocks QS system, and inhibits gene expression mediating bacterial behaviors. Given the extensive research of acyl-homoserine lactone (AHL) signals, existences and effects of AHL-based QS and QQ in biological wastewater treatments are being subject to high concern. This review summarizes AHL structure, synthesis mode, degradation mechanisms, analytical methods, environmental factors, AHL-based QS and QQ mechanisms. The existences and roles of AHL-based QS and QQ in biomembrane processes, activated sludge processes and membrane bioreactors are summarized and discussed, and corresponding exogenous regulation strategy by selective enhancement of AHL-based QS or QQ coexisting in biological wastewater treatments is suggested. Such strategies including the addition of AHL signals, AHL-producing bacteria as well as quorum quenching enzyme or bacteria can effectively improve wastewater treatment performance without killing or limiting bacterial survival and growth. This review will present the theoretical and practical cognition for bacterial AHL-based QS and QQ, suggest the feasibility of exogenous regulation strategies in biological wastewater treatments, and provide useful information to scientists and engineers who work in this field.

Biofilm activity and sludge characteristics affected by exogenous N-acyl homoserine lactones in biofilm reactors

This study verified the effect of N-acyl homoserine lactone (AHL) concentrations on mature biofilm systems. Three concentrations of an AHL mixture were used in the batch test. Introducing of 5nM AHLs significantly increased biofilm activity and increased sludge characteristics, which resulted in better pollutant removal performance, whereas exogenous 50nM and 500nM AHLs limited pollutant removal, especially COD and nitrogen removal. To further identify how exogenous signal molecular affects biofilm system nitrogen removal, analyzing of nitrifying bacteria through real-time polymerase chain reaction (RT-PCR) revealed that these additional signal molecules affect nitrifying to total bacteria ratio. In addition, the running state of the system was stable during 15days of operation without an AHL dose, which suggests that the changes in the system due to AHL are irreversible.

Effect of solid retention time on membrane fouling in membrane bioreactor: from the perspective of quorum sensing and quorum quenching

Solid retention time (SRT) is one of the most important operational parameters in membrane bioreactor (MBR), which significantly influences membrane fouling. It is widely recognized that SRT mainly changes biomass characteristics, and then, influences membrane fouling. Effect of SRT on quorum sensing (QS) in MBR, which could also influence fouling by coordinating biofilm formation, has not been reported. In this study, fouling, QS, soluble microbial products (SMP), and extracellular polymer substances (EPS) in MBRs operated under SRTs of 4, 10, and 40 days were investigated. The results showed that as SRT increased, the abundance of quorum quenching (QQ) bacteria increased, the quorum signal degradation activity of activated sludge increased, the concentrations of signal molecules in MBR decreased, the excretion of SMP and EPS decreased, and thus membrane biofouling was alleviated. Therefore, besides altering the biomass physiochemical properties, SRT also changed the balance between QS and QQ in MBR, and in this way, influenced membrane biofouling.

Exploiting Quorum Sensing Interfering Strategies in Gram-Negative Bacteria for the Enhancement of Environmental Applications

Quorum sensing (QS) is a widespread intercellular form of communication to coordinate physiological processes and cooperative activities of bacteria at the population level, and it depends on the production, secretion, and detection of small diffusible autoinducers, such as acyl-homoserine lactones (AHLs), auto-inducing oligo-peptides (AIPs) and autoinducer 2. In this review, the function of QS autoinducers of gram-negative bacteria in different aspects of wastewater treatment systems is examined. Based on research primarily performed over the past 10 years, QS involvement in the formation of biofilm and aerobic granules and changes of the microbial community and degradation/transformation pathways is discussed. In particular, the QS pathway in the role of bacterial infections and disease prevention in aquaculture is addressed. Interference of QS autoinducer-regulated pathways is considered potential treatment for a variety of environmentally related problems. This review is expected to serve as a stepping stone for further study and development strategies based on the mediation of QS-regulated pathways to enhance applications in both wastewater treatment systems and aquaculture.

Role of N-acyl-homoserine lactone (AHL) based quorum sensing on biofilm formation on packing media in wastewater treatment process

Quorum sensing (QS) signaling has been extensively studied in granules and single species populations.

Evidence for existence of quorum sensing in a bioaugmented system by acylated homoserine lactone-dependent quorum quenching

The introduction of a gene, strain, or microbial consortium into an indigenous bacterial population is known as bioaugmentation. This technique has been proposed as an effective strategy for accelerating and enhancing the removal of recalcitrant and toxic compounds during wastewater treatment. In this study, three types of reactors were used to test whether quorum sensing plays an important role in bioaugmented systems. Reverse transcriptase polymerase chain reaction showed that the inoculated strain, HF-1, successfully colonized in the bioaugmented reactor. Meanwhile, no HF-1 colonization was observed in the quorum-quenching and non-bioaugmented reactors. Removal of nicotine in the bioaugmented reactor was almost 100%, and removal of total organic carbon (TOC) was higher than 50%. However, less than 20% of nicotine and 30% of TOC was removed in quorum-quenching and non-bioaugmented reactors. Moreover, the release of acylated homoserine lactones reached the threshold for HF-1 biofilm formation in bioaugmented reactors but not in quorum-quenching or non-bioaugmented reactors. The addition of porcine kidney acylase I, a quenching reagent, to the quorum-quenching reactor hampered the colonization of HF-1. Together, these results demonstrate that quorum sensing plays an important role in HF-1 colonization of bioaugmented systems.