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

Engineering a biomimicking strategy for discovering nonivamide-based quorum-sensing inhibitors for controlling bacterial infection

The overuse of antibiotics over an extended period has led to increasing antibiotic resistance in pathogenic bacteria, culminating in what is now considered a global health crisis. To tackle the escalating disaster caused by multidrug-resistant pathogens, the development of new bactericides with new action mechanism is highly necessary. In this study, using a biomimicking strategy, a series of new nonivamide derivatives that feature an isopropanolamine moiety [the structurally similar to the diffusible signal factor (DSF) of Xanthomonas spp.] were prepared for serving as potential quorum-sensing inhibitors (QSIs). After screening and investigation of their rationalizing structure-activity relationships (SARs), compound A26 was discovered as the most optimal active molecule, with EC50 values of 9.91 and 7.04 μg mL-1 against Xanthomonas oryzae pv oryzae (Xoo) and Xanthomonas axonopodis pv. citri (Xac). A docking study showed that compound A26 exhibited robust interactions with Glu A: 161 of RpfF, which was strongly evidenced by fluorescence titration assay (KA value for Xoo RpfF-A26 = 104.8709 M-1). Furthermore, various bioassays showed that compound A26 could inhibit various bacterial virulence factors, including biofilm formation, extracellular polysaccharides (EPS), extracellular enzyme activity, DSF production, and swimming motility. In addition, in vivo anti-Xoo results showed that compound A26 had excellent control efficiency (curative activity: 43.55 %; protective activity: 42.56 %), surpassing that of bismerthiazol and thiodiazole copper by approximately 8.0%-37.3 %. Overall, our findings highlight a new paradigm wherein nonivamide derivatives exhibit potential in combating pathogen resistance issues by inhibiting bacterial quorum sensing systems though attributing to their new molecular skeleton, novel mechanisms of action, and non-toxic features.

Phosphate recovery from urban sewage by the biofilm sequencing batch reactor process: Key factors in biofilm formation and related mechanisms

The biofilm sequencing batch reactor (BSBR) technique has been deployed in the laboratory to enrich phosphorus from simulated wastewater, but it is still not clear what its performance will be when real world sewage is used. In this work, the effluent from the multi-stage anoxic-oxic (AO) activated sludge process at a sewage plant was used as the feed water for a BSBR pilot system, which had three reactors operating at different levels of dissolved oxygen (DO). The phosphorus adsorption and release, the biofilm growth, and the extracellular polymeric substances (EPS) components and contents were examined. The microbial communities and the signaling molecules N-acyl-l-homoserine lactones (AHLs) were also analyzed. Gratifyingly, the BSBR process successfully processed the treated sewage, and the biofilm developed phosphorus accumulation capability within 40 days. After entering stable operation, the system concentrated phosphate from 2.59 ± 0.77 mg/L in the influent to as much as 81.64 mg/L in the recovery liquid. Sludge discharge had profound impacts on all aspects of BSBR, and it was carried out successfully when the phosphorus absorption capacity of the biofilm alone was comparable to that of the reactor containing the activated sludge. Shortly after the sludge discharge, the phosphate concentration of the recovery liquid surged from 50 to 140 mg/L, the biofilm thickness grew from 20.56 to 67.32 μm, and the diversity of the microbial population plunged. Sludge discharge stimulated Candidatus competibacter to produce a large amount of AHLs, which was key in culturing the biofilm. Among the AHLs, both C10-HSL and 3OC12-HSL were significantly positively correlated with EPS and the abundance of Candidatus competibacter. The current results demonstrated BSBR as a viable option to enrich phosphorus from real world sewage with low phosphorus content and fluctuating chemistry. The mechanistic explorations also provided theoretical guidance for cultivating phosphorus-accumulating biofilms.

The potential effects of N-Acyl homoserine lactones on aerobic sludge granulation during phenolic wastewater treatment

The formation of aerobic granular sludge (AGS) is relatively difficult during the treatment of refractory wastewater, which generally shows small granular sizes and poor stability. The formation of AGS is regulated by N-Acyl homoserine lactones (AHLs)-mediated quorum sensing (QS). However, the potential role of AHLs in AGS formation under the toxic stress of refractory pollutants and the heterogeneity in the distribution and function of AHLs across different aggregates are not well understood. This study investigated the potential effects of AHLs on the formation of AGS during phenolic wastewater treatment. The distribution and succession of AHLs across varying granular sizes and development stages of AGS were investigated. Results showed that AGS was successfully formed in 13 days with an average granular size of 335 ± 39 μm and phenol removal efficiency of >99%. The levels of AHLs initially increased and then decreased. C4-HSL and 3-oxo-C10-HSL were enriched in large granules, suggesting they may play a pivotal role in regulating the concentration and composition of extracellular polymeric substances (EPS). The content of EPS constantly increased to 149.4 mg/gVSS, and protein (PN) was enriched in small and large granules. Luteococcus was the dominant genus constituting up to 62% after the granulation process, and exhibited a strong association with C4-HSL. AHLs might also regulate the bacterial community responsible for EPS production, and pollutant removal, and facilitate the proliferation of slow-growing microorganisms, thereby enhancing the formation of AGS. The synthesis and dynamics of AHLs were mainly governed by AHLs-producing bacterial strains of Rhodobacter and Pseudomonas, and AHLs-quenching strains of Flavobacterium and Comamonas. C4-HSL and 3-oxo-C10-HSL might be the major contributors to promoting sludge granulation under phenol stress and play critical roles in large granules. These findings enhance our understanding of the roles that AHLs play in sludge granulation under toxic conditions.

An overview of quorum sensing in shaping activated sludge forms: Mechanisms, applications and challenges

Activated sludge method is an effective method for the wastewater treatment and has been widely applied. Activated sludge usually exists in various forms such as activated sludge floc, biofilm and granule. Due to the different character and function for each sludge type, the role and mechanism in the wastewater treatment process are also different, but all were crucial. The quorum sensing (QS) /quorum quenching (QQ) have been demonstrated and proved to regulate the group behavior by secreting signaling molecules among microorganisms and thus affect the manifestation of sludge. However, the complex mechanisms and regulatory strategies of QS/QQ in sludge forms have not been systematically summarized. This review provided an overview on the mechanism of QS/QQ shaping sludge forms from macro to micro (Explore it through signaling molecules, extracellular polymeric substances and microorganisms). In addition, the application and challenges of QS/QQ regulating sludge forms in various wastewater treatment processes including biofilm batch reactor, granule sludge and membrane bioreactor were discussed. Finally, some suggestions for further research and development of effective and economical QS/QQ strategies are put forward.

Biochar inoculated with Rhodococcus biphenylivorans altered microecological regulation by promoting quorum sensing and electron transfer: Up-regulation of related genes and enhancement of phenol and ammonia degradation

Bioaugmentation is an efficient method for improving the efficiency of coking wastewater removal. Nevertheless, how different immobilization approaches affect the efficiency of bioaugmentation remains unclear, as does the corresponding mechanism. With the assistance of immobilized bioaugmentation strain Rhodococcus biphenylivorans B403, the removal of synthetic coking wastewater was investigated (drying agent, alginate agent, and absorption agent). The reactor containing the absorption agent exhibited the highest average removal efficiency of phenol (99.74 %), chemical oxygen demand (93.09 %), and NH4+-N (98.18 %). Compared to other agents, the covered extracellular polymeric substance on the absorption agent surface enhanced electron transfer and quorum sensing, and the promoted quorum sensing benefited the activated sludge stability and microbial regulation. The phytotoxicity test revealed that the wastewater's toxicity was greatly decreased in the reactor with the absorption agent, especially under high phenol concentrations. These findings showed that the absorption agent was the most suitable for wastewater treatment bioaugmentation.

Comprehensive analysis of effects of magnetic nanoparticles on aerobic granulation and microbial community composition: From the perspective of acyl-homoserine lactones mediated communication

As dosing additives benefit for aerobic granular sludge (AGS) cultivation, effects of different concentrations (0, 10, 50 and 100 mg/L) of magnetic nanoparticles (Fe3O4 NPs) on aerobic granulation, contaminant removal and potential microbial community evolution related to acyl-homoserine lactones (AHLs) mediated bacterial communication were investigated with municipal wastewater. Results showed that the required time to achieve granulation ratio > 70 % was reduced by 60, 90 and 30 days in phase II with addition of 10, 50, 100 mg/L Fe3O4 NPs, respectively. 50 mg/L Fe3O4 NPs can improve contaminant removal efficiency. The promotion of relative abundance of AHLs-producing and AHLs-producing/quenching populations and AHLs-related functional genes accompanied with faster granulation. Iron-cycling-related bacteria were closely related with AHLs-related bacteria during AGS formation. Co-occurrence network analyses showed that AHLs-mediated communication may play an important role in coordinating microbial community composition and functional bacteria participating in nitrogen and polyphosphate metabolisms during aerobic granulation process.

Roles and regulation of quorum sensing in anaerobic granular sludge: Research status, challenges, and perspectives

Anaerobic granular sludge (AnGS) has a complex and important internal microbial communication system due to its unique microbial layered structure. As a concentration-dependent communication system between bacterial cells through signal molecules, QS (quorum sensing) is widespread in AnGS and exhibits great potential to regulate microbial behaviors. Therefore, the universal functions of QS in AnGS have been systematically summarized in this paper, including the influence on the metabolic activity, physicochemical properties, and microbial community of AnGS. Subsequently, the common QS-based AnGS regulation approaches are reviewed and analyzed comprehensively. The regulation mechanism of QS in AnGS is analyzed from two systems of single bacterium and mixed bacteria. This review can provide a comprehensive understanding of QS functions in AnGS systems, and promote the practical application of QS-based strategies in optimization of AnGS treatment process.

Granule formation mechanism, key influencing factors, and resource recycling in aerobic granular sludge (AGS) wastewater treatment: A review

The high-efficiency and additionally economic benefits generated from aerobic granular sludge (AGS) wastewater treatment have led to its increasing popularity among academics and industrial players. The AGS process can recycle high value-added biomaterials including extracellular polymeric substances (EPS), sodium alginate-like external polymer (ALE), polyhydroxyfatty acid (PHA), and phosphorus (P), etc., which can serve various fields including agriculture, construction, and chemical while removing pollutants from wastewaters. The effects of various key operation parameters on formation and structural stability of AGS are comprehensively summarized. The degradable metabolism of typical pollutants and corresponding microbial diversity and succession in the AGS wastewater treatment system are also discussed, especially with a focus on emerging contaminants removal. In addition, recent attempts for potentially effective production of high value-added biomaterials from AGS are proposed, particularly concerning improving the yield, quality, and application of these biomaterials. This review aims to provide a reference for in-depth research on the AGS process, suggesting a new alternative for wastewater treatment recycling.

A review of filamentous sludge bulking controls from conventional methods to emerging quorum quenching strategies

Filamentous bulking, which results from the overgrowth of filamentous microorganisms, is a common issue that frequently disrupts the stable operation of activated sludge processes. Recent literature has paid attention to the relationship between quorum sensing (QS) and filamentous bulking highlighting that the morphological transformations of filamentous microbes are regulated by functional signal molecules in the bulking sludge system. In response to this, a novel quorum quenching (QQ) technology has been developed to control sludge bulking effectively and precisely by disturbing QS-mediated filamentation behaviors. This paper presents a critical review on the limitations of classical bulking hypotheses and traditional control methods, and provides an overview of recent QS/QQ studies that aim to elucidate and control filamentous bulking, including the characterization of molecule structures, the elaboration of QS pathways, and the precise design of QQ molecules to mitigate filamentous bulking. Finally, suggestions for further research and development of QQ strategies for precise bulking control are put forward.

Celebrating 50 years of microbial granulation technologies: From canonical wastewater management to bio-product recovery

Microbial granulation technologies (MGT) in wastewater management are widely practised for more than fifty years. MGT can be considered a fine example of human innovativeness-driven nature wherein the manmade forces applied during operational controls in the biological process of wastewater treatment drive the microbial communities to modify their biofilms into granules. Mankind, over the past half a century, has been refining the knowledge of triggering biofilm into granules with some definite success. This review captures the journey of MGT from inception to maturation providing meaningful insights into the process development of MGT-based wastewater management. The full-scale application of MGT-based wastewater management is discussed with an understanding of functional microbial interactions within the granule. The molecular mechanism of granulation through the secretion of extracellular polymeric substances (EPS) and signal molecules is also highlighted in detail. The recent research interest in the recovery of useful bioproducts from the granular EPS is also emphasized.

Enhancing quorum quenching media with 3D robust electrospinning coating: A novel biofouling control strategy for membrane bioreactors

Bacterial quorum quenching (QQ) is an effective strategy for controlling biofouling in membrane bioreactor (MBR) by interfering the releasing and degradation of signal molecules during quorum sensing (QS) process. However, due to the framework feature of QQ media, the maintenance of QQ activity and the restriction of mass transfer threshold, it has been difficult to design a more stable and better performing structure in a long period of time. In this research, electrospun fiber coated hydrogel QQ beads (QQ-ECHB) were fabricated by using electrospun nanofiber coated hydrogel to strengthen layers of QQ carriers for the first time. The robust porous PVDF 3D nanofiber membrane was coated on the surface of millimeter-scale QQ hydrogel beads. Biocompatible hydrogel entrapping quorum quenching bacteria (sp.BH4) was employed as the core of the QQ-ECHB. In MBR with the addition of QQ-ECHB, the time to reach transmembrane pressure (TMP) of 40 kPa was 4 times longer than conventional MBR. The robust coating and porous microstructure of QQ-ECHB contributed to keeping a lasting QQ activity and stable physical washing effect at a very low dosage (10g beads/5L MBR). Physical stability and environmental-tolerance tests also verified that the carrier can maintain the structural strength and keep the core bacteria stable when suffering long-term cyclic compression and great fluctuations in sewage quality.

Novel Benzothiazole Derivatives as Potential Anti-Quorum Sensing Agents for Managing Plant Bacterial Diseases: Synthesis, Antibacterial Activity Assessment, and SAR Study

As quorum sensing (QS) regulates bacterial pathogenicity, antiquorum sensing agents have powerful application potential for controlling bacterial infections and overcoming pesticide/drug resistance. Identifying anti-QS agents thus represents a promising approach in agrochemical development. In this study, the anti-QS potency of 53 newly prepared benzothiazole derivatives containing an isopropanolamine moiety was analyzed, and structure-activity relationships were examined. Compound D₃ exhibited the strongest antibacterial activity, with an in vitro EC₅₀ of 1.54 μg mL^-1 against Xanthomonas oryzae pv oryzae (Xoo). Compound D₃ suppressed QS-regulated virulence factors (e.g., biofilm, extracellular polysaccharides, extracellular enzymes, and flagella) to inhibit bacterial infection. In vivo anti-Xoo assays indicated good control efficiency (curative activity, 47.8%; protective activity, 48.7%) at 200 μg mL^-1. Greater control efficiency was achieved with addition of 0.1% organic silicone or orange peel essential oil. The remarkable anti-QS potency of these benzothiazole derivatives could facilitate further novel bactericidal compound development.

Microbial population properties in the hierarchically structured aerobic granular sludge: Phenotype and genotype

Aerobic granular sludge (AGS) is a layered microbial aggregate formed by the ordered self-assembly of different microbial populations. In this study, the outer layer (OL), middle layer (ML), and the inner layer (IL) of matured AGS were obtained by circular cutting. The adhesion of microorganisms in IL was significantly higher than that in OL and ML during the famine period, while the adhesion of microorganisms in ML and OL was significantly higher than that in IL during the feast period, confirming that the formation of AGS started in the famine period, and the feast period promoted the increase of particle size. Microorganisms in the three-layer structure were highly diverse and rich in genes for cytochrome c oxidase synthesis with oxygen as the electron acceptor. G_Pseudoxanthomonas was the dominant bacterium in OL. Its spatial distribution increased gradually from the inside to the outside. G_Rhodanobacter was the dominant bacterium in IL. Its spatial distribution gradually decreased from the inside to the outside. The microorganisms in IL contained abundant pili genes. During the self-assembly process of particle formation, G_ Rhodanobaker adhered stronger than G_ Pseudoxanthomonas. The interface between aerobic and anoxic was about 0.6 mm away from the granule surface. Combined with the electron mediator properties of the extracellular polymeric substance (EPS) in granules, it was speculated that the degradation of organic substrates located in the anoxic layer relied on EPS as a mediator for long-range electron transfer, and finally transferred electrons to O₂. This study provides a new viewpoint on the formation mechanism of AGS from the perspective of the ordered self-assembly of microorganisms, offering a theoretical basis for the optimal selection of culture conditions and the application of AGS technology.

Regulating performance of CANON process via adding external quorum sensing signal molecules in membrane bioreactor

In this study, the regulation effect of the external quorum sensing signals, N-dodecanoyl homoserine lactone (C12-HSL) on CANON process were investigated in a membrane bioreactor. C12-HSL significantly enhanced the aerobic ammonia-oxidizing bacteria and improved the ammonia monooxygenase activity to 0.134 from 0.076 μg NO₂^--N mg^-1 protein min^-1, while suppressed anaerobic ammonia-oxidizing bacteria and limited the TN removal to 0.07 from 0.22 kg m^-3 d^-1. Key enzymes synthesis were enhanced during the operation without C12-HSL addition, enabling the resistance of CANON system to high C12-HSL. As a result, the hydroxylamine oxidoreductase and nitrite reductase activity reached 35.9 EU g^-1 SS and 1.28 μg NO₂^--N mg^-1 protein min^-1, respectively; Nitrosomonas and Candidatus Kuenenia, with the abundance as 12.5 % and 22.9 %, cooperatively contributed to the TN removal, which maintained at 0.19 kg m^-3 d^-1. C12-HSL was profitable for aerobic ammonia oxidation, which could be adopted for regulating the nitrite production rate.

New insights into exogenous N-acyl-homoserine lactone manipulation in biological nitrogen removal system against ZnO nanoparticle shock

The effects and mechanisms of three N-acyl-homoserine lactones (AHLs) (C₄-HSL, C₆-HSL, and C₁₀-HSL) on responses of biological nitrogen removal (BNR) systems to zinc oxide nanoparticle (NP) shock were investigated. All three AHLs improved the NP-impaired ammonia oxidation rates by up to 50.0 % but inhibited the denitrification process via regulating nitrogen metabolism-related enzyme activities. C₄-HSL accelerated the catalase activity by 13.2 %, while C₆-HSL and C₁₀-HSL promoted the superoxide dismutase activity by 26.6 % and 18.4 %, respectively, to reduce reactive oxygen species levels. Besides, the enhancements of tryptophan protein and humic acid levels in tightly-bound extracellular polymeric substance by AHLs were vital for NP toxicity attenuation. The metabonomic analysis demonstrated that all three AHLs up-regulated the levels of lipid- and antioxidation-related metabolites to advance the system's resistance to NP shock. The "dual character" of AHLs emphasized the concernment of legitimately employing AHLs to alleviate NP stress for BNR systems.

Anammox-based granulation cycle for sustainable granular sludge biotechnology from mechanisms to strategies: A critical review

Anaerobic ammonium oxidation (anammox) granular sludge is a promising biotechnological process for treating low-carbon nitrogenous wastewater, and is featured with low energy consumption and footprint. Previous theoretical and experimental research on anammox granular sludge processes mainly focused on granulation (flocs → granules), but pay little attention to the granulation cycle including granulation and regeneration. This work reviewed the previous studies from the perspective of anammox granules lifecycle and proposed various sustainable formation mechanisms of anammox granules. By reviewing the anaerobic, aerobic, and anammox granulation mechanisms, we summarize the mechanisms of thermodynamic theory, heterogeneous growth, extracellular polymeric substance (EPS)-based adhesion, quorum sensing (QS)-based regulation, biomineralization-based growth, and stratification of microorganisms to understand anammox granulation. In the regeneration process, the formation of precursors for re-granulation is explained by the mechanisms of physical crushing, quorum quenching and dispersion cue sensing. Based on the granulation cycle mechanism, the rebuilding of the normal regeneration process is considered essential to avoid granule floatation and the wash-out of granules. This comprehensive review indicates that future research on anammox granulation cycle should focus on the effects of filamentous bacteria in denitrification-anammox granulation cycle, the role of QS/ quorum quenching (QQ)-based autoinducers, development of diversified mechanisms to understand the cycle and the cycle mechanisms of stored granules.

Long-term impacts of polyethylene terephthalate (PET) microplastics in membrane bioreactor

Due to the mass production and daily use of plastic products, the potential toxicity of microplastics to the water environment has attracted worldwide attention. In this work, the effect of typical microplastics (PET) on the performance of activated sludge from membrane bioreactors (MBR) was evaluated. The impacts on biological removal efficiency were unconspicuous with continuous dosing of 60 particles/L. However, further investigations revealed that PET particle accumulation caused adverse impacts on settleability and dewaterability. The SVI value increased from 53.3 ml/g MLSS to 69.9 ml/g MLSS and the CST in the PET reactor increased by 22%. Nevertheless, hydrophobicity was reduced by 49.2%. Mechanism studies exposed that the PET microplastics accumulation improved extracellular polymeric substances (EPS) from 116.96 mg/L to 138.70 mg/L and caused cell membrane damage. The abundance and diversity of microbial community reduced in activated sludge in PET reactor compared with control reactor. These phenomena revealed a possible hypothesis that the microplastic particles increased EPS and cytotoxicity of activated sludge. However, the rate of transmembrane pressure (TMP) build-up was significantly mitigated in PET-MBR compared to that in a control-MBR (1.27 folds), which attributes that physical scour of particles may still alleviate membrane contamination in MBR.

The role of immobilized quorum sensing strain in promoting biofilm formation of Moving Bed Biofilm Reactor during long-term stable operation

Quorum sensing (QS) signaling plays a significant role in the natural regulation of biofilm formation. Multiple species QS systems in wastewater treatment processes have received significant attention in recent years and this study presents a long-term analysis of QS signaling, bacterial structures and extracellular polymeric substance (EPS) during biofilm formation, detachment and reformation processes. Six types of Acyl homoserine lactones (AHLs) were found to be closely related to different phases of biofilm development, with both QS and quorum quenching (QQ) strains being identified as drivers of various biofilm phases and 10 strains presenting a close relationship with AHLs (p < 0.05). Meanwhile, QS strain Sphingomonas rubra was immobilized and added into reactor systems, resulting in significant increase in AHL content, EPS production, and adhesion strength of biofilm (p < 0.05), which might promote biofilm formation processes during long-term stable operation. This study provides a potentially simple and economical way to improve activity and stability of MBBR in complex wastewater systems.

Insight of the bio-cathode biofilm construction in microbial electrolysis cell dealing with sulfate-containing wastewater

Signaling molecules are useful in biofilm formation, but the mechanism for biofilm construction still needs to be explored. In this study, a signaling molecule, N-butyryl-l-Homoserine lactone (C₄-HSL), was supplied to enhance the construction of the sulfate-reducing bacteria (SRB) bio-cathode biofilm in microbial electrolysis cell (MEC). The sulfate reduction efficiency was more than 90% in less time under the system with C₄-HSL addition. The analysis of SRB bio-cathode biofilms indicated that the activity, distribution, microbial population, and secretion of extracellular polymers prompted by C₄-HSL, which accelerate the sulfate reduction, in particular for the assimilatory sulfate reduction pathway. Specifically, the relative abundance of acidogenic fermentation bacteria increased, and Desulfovibrio was co-metabolized with acidogenic fermentation bacteria. This knowledge will help to reveal the potential of signaling molecules to enhance the SRB bio-cathode biofilm MEC construction and improve the performance of treating sulfate-containing wastewater.

New insight into filamentous sludge bulking: Potential role of AHL-mediated quorum sensing in deteriorating sludge floc stability and structure

The microcosmic mechanisms underlying filamentous bulking remain unclear. The role of extracellular polymeric substances (EPS) governed by quorum sensing (QS) in deteriorating sludge floc stability and structure during filamentous bulking and the feasibility of using quorum quenching (QQ) to maintain sludge floc stability and structure and sludge settling were investigated in this study. The results indicated that the concentration of C₆HSL increased from 22.08±3.22 ng/g VSS to 81.42±5.98 ng/g VSS during filamentous bulking. The filamentous bacteria gradually evolved the hdtS gene related to the synthesis of C₆HSL with increases in the population density. Triggered QS by filamentous bacteria proliferation induced variation in the composition and structure of EPS within the sludge flocs. The proteins (PN) content of the EPS increased evidently from 40.06 ± 2.41 mg/g VSS to 110.32 ± 4.32 mg/g VSS, and the polysaccharides (PS) content slightly increased during filamentous bulking. The upregulated proteins in the EPS led to a decrease in the relative hydrophobicity of the sludge and an increase in negative surface charge. The α-helix/(β-sheet+random coil) ratio evidently increased from 0.76 to 0.99 during filamentous bulking, revealing that the proteins were tightly structured, which prevented the exposure of inner hydrophobic groups. The total energy of the interaction (W_T) between bacteria increased during sludge bulking, which resulted in the weakening of sludge aggregation. Variation in the physicochemical properties of EPS induced by QS in the filamentous bacteria markedly restrained adhesion between the filamentous bacteria and floc-forming bacteria. The production of PN in the EPS and the expression of the hdtS gene were inhibited by vanillin, which served as a QS inhibitor. The W_T between bacteria with 50 mg/L of vanillin basically did not change. Filamentous bulking was significantly inhibited by the addition of vanillin. Therefore, QQ is a potential strategy for the prevention and control of filamentous bulking. This study provides new information regarding the microcosmic mechanisms of filamentous bulking.

Promoting the granulation process of aerobic sludge via a sustainable strategy of effluent reflux in view of AHLs-mediated quorum sensing

Aerobic granular sludge (AGS) has excellent performance in wastewater treatment. However, the formation and mechanism of AGS by effluent reflux are not fully understood in sequential batch reactors (SBRs). In this study, two reactors were constructed, among which R1 was the control group, and the R2 reactor refluxed one-fourth of the supernatant of the effluent to the influent water. In the reactor of R2, the granules had better COD and TN removal efficiencies and resistance to external shocks, and AGS produced more extracellular polymeric substances (EPS). Analysis of microbial community indicated that AHLs-mediated microbes, denitrifying microbes, and EPS producers were enriched. At the same time, the correlation between 3OC6-HSL, C8-HSL, C12-HSL and PN was 0.89*, 0.94** and 0.92* respectively, the possible mechanism of enhanced granulation was mainly the promotion of AHLs by effluent reflux. Therefore, the effluent reflux strategy could be an innovative and sustainable strategy that validates the function of AHLs-mediated QS to accelerate aerobic sludge granulation and maintain its structural stability.

Structure and Signal Regulation Mechanism of Interspecies and Interkingdom Quorum Sensing System Receptors

Quorum sensing (QS) is a signaling mechanism for cell-to-cell communication between bacteria, fungi, and even eukaryotic hosts such as plant and animal cells. Bacteria in real life do not exist as isolated organisms but are found in complex, dynamic, and microecological environments. The study of interspecies QS and interkingdom QS is a valuable approach for exploring bacteria-bacteria interactions and bacteria-host interaction mechanisms and has received considerable attention from researchers. The correct combination of QS signals and receptors is key to initiating the QS process. Compared with intraspecies QS, the signal regulation mechanism of interspecies QS and interkingdom QS is often more complicated, and the distribution of receptors is relatively wide. The present review focuses on the latest progress with respect to the distribution, structure, and signal transduction of interspecies and interkingdom QS receptors and provides a guide for the investigation of new QS receptors in the future.

Identification of partial denitrification granulation enhanced by low C/N ratio in the aspect of metabolomics and quorum sensing

Partial denitrification granular sludge (PDGS) and denitrification granular sludge (DGS) play an important role in nitrogen removal from wastewater. However, the inherent cause of aggregation capacity related to the ratio of COD to nitrogen (COD/N) is still unclear. In this study, metabolomics analysis was combined with microbiological analyses, granular performance and extracellular polymeric substances (EPS) structure to explore the granulation mechanism at different influent COD/N ratios. The results showed that the higher COD/N ratio selectively enhanced the gluconeogenesis pathway, purine and pyrimidine metabolism pathway, resulting in more extracellular polysaccharide (PS) excretion and floc sludge. The absence of carbon source weakened tricarboxylic acid cycle (TCA) reaction, resulting in NAD⁺ and ADP decrease, nitrite accumulation and change of microbial community structure. The amino acids biosynthesis pathway was enhanced under low COD/N ratio, which promoted the hydrophobicity of EPS. PDGS had stronger Acyl-homoserine lactones (AHLs)-based quorum sensing (QS) than DGS during the operational period. CO8-HSL, C8-HSL and C6-HSL, as the main form of AHLs, played a dominating role in DGS and PDGS. Batch tests illustrated that adding AHLs obviously improved the synthesis of the amino acids, threonine (Thr), tryptophan (Trp), methionine (Met) and glycine (Gly). Dosing AHLs regulated PS synthesis only at a high COD/N ratio. The glucose-6P, glycerate-3p and UDP-Glc were up-regulated only in DSG, which increased the hydrophilic groups in EPS. The results not only provided the new insights into the metabolism of denitrifying granular sludge, but also indicated the application potential of the technologies regarding start-up and operation of granule sludge.

Recovery of structure and activity of disintegrated aerobic granular sludge after long-term storage: Effect of exogenous N-acyl-homoserine lactones

Long-term storage of aerobic granular sludge (AGS) may lead to granule inactivation and disintegration. Granule recovery in both structure and activity is important for scale-up and stability of AGS, but information about the structure recovery of stored AGS is limited. In addition, whether short-term exogenous N-acyl-homoserine lactones (AHLs) regulations could accelerate the granule recovery and sustain positive effects on AGS is unknown. Herein, the recovery of 33-month stored AGS was performed in three reactors for 38 days (phase I) at different exogenous AHLs concentrations (0, 50 and 500 nM of AHL-mixtures in R0, R1 and R2, respectively) and for an extended 45 days without exogenous AHLs (phase II). Results demonstrated successful recovery of disintegrated AGS in all reactors, although it was relatively time-consuming in R0. The treatment performance was similar among the reactors and steady-state removal of COD (90%) and NH₄⁺-N (94%) could be recovered within 7 and 21 days, respectively. However, exogenous AHLs regulation (especially in R1) obviously accelerated bioactivity recovery of heterotrophs and nitrifiers and improved granule characteristics, including biomass, density, hydrophobicity and extracellular polymeric substance (EPS). During phase II, sustainable positive effects remained in R1, but granule characteristics deteriorated in R2. The abundance of functional genera Thauera, Nitrosomonas and Candidatus_Nitrotoga, contributed to the rapid recovery and helped maintain the structure and activity of AGS. The predictive functional profiling of bacterial communities also demonstrated sustainably higher activities of metabolism, growth and signal sensing under exogenous AHLs regulation at an appropriate content.

Effects of reflux ratio on the anaerobic sludge and microbial social behaviors in an expanded granular sludge bed reactor: From the perspective of acyl-homoserine lactones-mediated quorum sensing

Performance of anaerobic sludge and microbial social behaviors in an expanded granular sludge bed (EGSB) were evaluated by increasing reflux ratio from 50% to 500% stage by stage, with a constant influent chemical oxygen demand (COD) of 5500 mg/L at hydraulic retention time 12 h. The results indicated that the reflux ratio of 100% - 200% was more favorable for the EGSB with a methane production of 2.4 m³/m³·d. It was found that acyl-homoserine lactones (AHLs)-mediated quorum sensing (QS) could balance various microbial populations in the anaerobic digestion process. C₄-HSL and C₈-HSL were identified as the specific AHLs in enhancing granulation of anaerobic sludge by stimulating protein secretion into extracellular polymeric substances (EPS). 3-oxo-C₆-HSL and 3-oxo-C₁₄-HSL were verified for the enhancement of methanogenesis. The present study showed a novel perspective on the performance of EGSB with reflux ratios based on the AHLs-mediated QS.

Potential microbial functions and quorum sensing systems in partial nitritation and anammox processes

Diverse microbial communities coexist in the partial nitritation-anaerobic ammonium oxidation (PNA) process, in which nitrogen metabolism and information exchange are two important microbial interactions. In the PNA process, the existence of diverse microorganisms including nitrifiers, anammox bacteria, and heterotrophs makes it challenging to achieve a balanced relationship between anaerobic ammonium oxidation bacteria and ammonia oxidizing bacteria. In this study, potential microbial functions in nitrogen conversion and acyl-homoserine lactones (AHLs)-based quorum sensing (QS) in PNA processes were examined. Candidatus_Kuenenia and Nitrosomonas were the key functional bacteria responsible for PNA, while Nitrospira was detected as the dominant nitrite oxidizing bacteria (NOB). Heterotrophs containing nxr might play a similar function to NOB. The AHLs-QS system was an important microbial communication pathway in PNA systems. N-octanoyl-L-homoserine lactone, N-decanoyl homoserine lactone, and N-dodecanoyl homoserine lactone were the main AHLs, which might be synthesized by nitrogen converting microorganisms and heterotrophs. However, only heterotrophs had the potential to sense and degrade AHLs, such as Saccharophagus (sensing) and Leptospira (degradation). These results provide comprehensive information about the possible microbial functions and interactions in the PNA system and clues for system optimization from a microbial perspective. PRACTITIONER POINTS: ●Potential functions of anammox bacteria, nitrifiers, and heterotrophs were revealed. ●Diverse nitrogen conversion and AHLs-quorum sensing related genes were detected. ●Anammox bacteria and AOB played important roles in the AHLs synthesis process. ●Heterotrophs could sense and degrade AHLs during information exchange.

Bacterial assembly and succession in the start-up phase of an IFAS metacommunity: The role of AHL-driven quorum sensing

Quorum sensing (QS) plays an important role in biofilm formation and the start-up of biofilm-based reactors, while its involvement in bacterial assembly throughout biofilm development remains underexplored. We investigated the assembly and succession of the bacterial community in a full-scale integrated fixed-film activated sludge (IFAS) process, with emphasis on N-acylhomoserine lactone (AHL)-driven QS. Biofilm development could be divided into two major periods, (i) young biofilm formation phase and (ii) biofilm maturity and update phase. Mature biofilms exhibited lower levels of AHLs compared with young biofilms (p > 0.05). A structural equation model, constructed to assess the linkages between AHL level and bacterial community composition as well as environmental factors, indicated that pH significantly influenced both bacterial community composition and AHL content. Along with biofilm development, there was a negative correlation between AHL concentration and community composition variation (coefficients of -0.367 and -0.329). Regarding the lower AHL level in mature biofilms, these results were consistent with the phylogenetic molecular ecological networks (pMENs) analysis, indicating that quorum-quenching (QQ) bacteria occur in keystone taxa in mature biofilms. In addition, based on the pMENs results, the proportion of positive interactions increased from 77.64 to 82.39% in mature biofilms compared to young biofilms, indicating that bacterial cooperation was strengthened in mature biofilms. The QS bacteria were predominant in the keystone taxa of pMENs, with proportions being increased to 62% in mature biofilms, which is conducive for biofilm development. Overall, this study improves our understanding of the involvement of AHL-mediated QS in biofilm development.

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.

Characterization of Exopolysaccharides (EPSs) Obtained from Ligilactobacillus salivarius Strains and Investigation at the Prebiotic Potential as an Alternative to Plant Prebiotics at Poultry

In this study, it was aimed to reveal the potential of using exopolysaccharides (EPS) obtained from Ligilactobacillus salivarius as a prebiotic that regulates chicken intestinal microbiota. Characterization of EPS obtained from L. salivarius BIS312 (EPSBIS312) and BIS722 (EPSBIS722) strains was demonstrated by high performance liquid chromatography (HPLC), nuclear magnetic resonance (NMR), and size-exclusion chromatography (SEC) analyses. It was determined that the molecular weight of both EPS is in the range of 10⁴-10⁶ Daltons, and there are 4 types of monomers in their structure. Anti-biofilm and anti-quorum sensing effects of EPSBIS312 and EPSBIS722 were determined. EPSBIS312 and EPSBIS722 showed a strong anti-biofilm effect on Enterococcus faecalis ATCC 29212, Staphylococcus aureus EB-1, and Escherichia coli ATCC 11229. The anti-quorum sensing study revealed that the EPSBIS722 had a higher effect than the EPSBIS312. The effect of different concentrations of EPS (2.5%, 5%, 10%) on lactobacilli growth stimulator (LGS) was evaluated. The highest LGS was promoted at 10% concentration while the lowest LGS was promoted at 2.5% concentration by EPSBIS722. In addition, adhesion abilities of EPSBIS312 and EPSBIS722 in HT-29 colorectal adenocarcinoma cell line were tested. EPSs significantly increased the ability to adhere to HT-29 cells. The characterized EPSs may be an alternative to plant prebiotics such as inulin at poultry.

Function of quorum sensing and cell signaling in wastewater treatment systems

Quorum sensing (QS) is a communication mode between microorganisms to regulate bacteria ecological relations and physiological behaviors, thus achieve the physiological function that single bacteria cannot complete. This phenomenon plays important roles in the formation of biofilm and granular sludge, and may be related to enhancement of some functional bacteria activity in wastewater treatment systems. There is a need to better understand bacterial QS in engineered reactors, and to assess how designs and operations might improve the removal efficiency. This article reviewed the recent advances of QS in several environmental systems and mainly analyzed the regulation mechanism of QS-based strategies for biofilm, granular sludge, functional bacteria, and biofouling control. The co-existences of multiple signal molecules in wastewater treatment (WWT) processes were also summarized, which provide basis for the future research on the QS mechanism of multiple signal molecules' interaction in WWT. This review would present some prospects and suggestions which are of practical significance for further application.

Long-term antibacterial activity of a composite coating on titanium for dental implant application

Dental implants are the most innovative and superior treatment modality for tooth replacement. However, titanium implants still suffer from insufficient antibacterial capability and peri-implant diseases remain one of the most common and intractable complications. To prevent peri-implant diseases, a composite coating containing a new antibacterial agent, (Z-)-4-bromo-5-(bromomethylene)-2(5H)-furanone (BBF) was fabricated on titanium. This study was designed to investigate the antibacterial activity of the composite coating against two common peri-implant pathogens (Porphyromonas gingivalis and Actinobacillus actinomycetemcomitans). The morphology of the composite coating showed that BBF-loaded poly(L-lactic acid) nanospheres were well-distributed in the pores of the microarc oxidation coating, and cross-linked with each other and the wall pores by gelatin. A release study indicated that the antibacterial coating could sustain the release of BBF for 60 d, with a slight initial burst release occurring during the first 4 h. The antibacterial rate of the composite coating for adhering bacteria was the highest (over 97%) after 1 d and over 90% throughout a 30-day incubation period. The total fluorescence intensity of the composite coating was the lowest, and the vast majority of the fluorescence was red (dead bacteria). Moreover, real-time polymerase chain reaction analysis confirmed that the relative gene expression of the adherent bacteria on the composite coating was down-regulated. It was therefore concluded that the composite coating fabricated on titanium, which showed excellent and relatively long-term antibacterial activity against Porphyromonas gingivalis and Actinobacillus actinomycetemcomitans, is a potential and promising strategy to be applied on dental implants for the prevention of peri-implant diseases.

Investigating the influences of quorum quenching and nutrient conditions on activated sludge flocs at a short-time scale

Quorum sensing signals regulate various functions within activated sludge processes such as formation of microbial aggregates. Disturbance of this signaling system, known as quorum quenching (QQ), provides opportunities for eliminating some problems related to biological wastewater treatment (e.g., biofouling and excess sludge production). However, it is poorly understood how and to what extent QQ systems can affect the microbial aggregation processes and the following floc formation. In particular, an in-depth structural characterization at the scale of microbial aggregate while considering nutrient conditions in the reactor is still largely disregarded. Here, we evaluated the QQ effects at the short-term time scale (i.e., after 4 h for the exogenous period and 19 h for exogenous/endogenous period), by combining advanced techniques for microbial characterization (flow cytometry, CARD-FISH, and confocal laser scanning microscopy) and conventional physical-chemical assessments. The results indicated that by implementing QQ agents (immobilized Acylase I enzyme in porous alginate beads) the abundance of single cells and suspended microbial aggregates in the supernatant did not show significant changes during the exogenous period. Conversely, at the end of the exogenous/endogenous period a significant increase of single prokaryotic cells, small and large microbial aggregates favored the growth of grazers, including free-living nanoflagellates and ciliates. Flocs became looser and thinner than those in the control reactor, thus affecting the sludge settling behavior. Inability of microbial community in degradation of soluble protein during the endogenous period confirmed that the QQ agents are likely to inhibit the secretion of protease enzyme within microbial communities of activated sludge.