Quorum sensing and quenching in membrane bioreactors: Opportunities and challenges for biofouling control

Review of quorum-quenching probiotics: A promising non-antibiotic-based strategy for sustainable aquaculture

The emergence of antibiotic-resistant bacteria (ARBs) and genes (ARGs) in aquaculture underscores the urgent need for alternative veterinary strategies to combat antimicrobial resistance (AMR). These measures are vital to reduce the likelihood of entering a post-antibiotic era. Identifying environmentally friendly biotechnological solutions to prevent and treat bacterial diseases is crucial for the sustainability of aquaculture and for minimizing the use of antimicrobials, especially antibiotics. The development of probiotics with quorum-quenching (QQ) capabilities presents a promising non-antibiotic strategy for sustainable aquaculture. Recent research has demonstrated the effectiveness of QQ probiotics (QQPs) against a range of significant fish pathogens in aquaculture. QQ disrupts microbial communication (quorum sensing, QS) by inhibiting the production, replication, and detection of signalling molecules, thereby reducing bacterial virulence factors. With their targeted anti-virulence approach, QQPs have substantial promise as a potential alternative to antibiotics. The application of QQPs in aquaculture, however, is still in its early stages and requires additional research. Key challenges include determining the optimal dosage and treatment regimens, understanding the long-term effects, and integrating QQPs with other disease control methods in diverse aquaculture systems. This review scrutinizes the current literature on antibiotic usage, AMR prevalence in aquaculture, QQ mechanisms and the application of QQPs as a sustainable alternative to antibiotics.

Insights into the response of electroactive biofilm with petroleum hydrocarbons degradation ability to quorum sensing signals

Bioelectrochemical technologies based on electroactive biofilms (EAB) are promising for petroleum hydrocarbons (PHs) remediation as anode can serve as inexhaustible electron acceptor. However, the toxicity of PHs might inhibit the formation and function of EABs. Quorum sensing (QS) is ideal for boosting the performance of EABs, but its potential effects on reshaping microbial composition of EABs in treating PHs are poorly understood. Herein, two AHL signals, C4-HSL and C12-HSL, were employed to promote EABs for PHs degradation. The start-times of AHL-mediated EABs decreased by 18-26%, and maximum current densities increased by 28-63%. Meanwhile, the removal of total PHs increased to over 90%. AHLs facilitate thicker and more compact biofilm as well as higher viability. AHLs enhanced the electroactivity and direct electron transfer capability. The total abundance of PH-degrading bacteria increased from 52.05% to 75.33% and 72.02%, and the proportion of electroactive bacteria increased from 26.14% to 62.72% and 63.30% for MFC-C4 and MFC-C12. Microbial networks became more complex, aggregated, and stable with addition of AHLs. Furthermore, AHL-stimulated EABs showed higher abundance of genes related to PHs degradation. This work advanced our understanding of AHL-mediated QS in maintaining the stable function of microbial communities in the biodegradation process of petroleum hydrocarbons.

Low-biofouling membrane bioreactor: Effects of cis-2-Decenoic acid addition on EPS and biofouling mitigation

Biofouling is inevitable in the membrane process, particularly in membrane bioreactors (MBR) combined with activated sludge processes. Regulating microbial signaling systems with diffusible signal factors such as cis-2-Decenoic acid (CDA) can control biofilm formation without microbial death or growth inhibition. This study assessed the effectiveness of CDA in controlling biofouling in membrane bioreactors (MBRs), essential for wastewater treatment. By modulating microbial signaling, CDA mitigated biofilm formation without hindering microbial growth. Analysis using Confocal Laser Scanning Microscopy (CLSM) revealed structural alterations in the biofilm, reducing biomass and thickness upon CDA application. Moreover, examination of extracellular polymeric substances (EPS) highlighted a decrease in total EPS, particularly effective polysaccharides. In addition, the possibility of shifting from high molecular weight EPS to low molecular weight EPS was revealed through the change in dispersion activity. The 56% extension of MBR operational lifespan resulting from the reduction in EPS is anticipated to offer potential cost savings and improved performance. Despite these results, further investigation is crucial to validate any potential environmental risks associated with CDA and to comprehend its long-term effects at various conditions.

Membrane reciprocation and quorum quenching: An innovative combination for fouling control and energy saving in membrane bioreactors

Membrane bioreactors (MBRs) play a crucial role in wastewater treatment, but they face considerable challenges due to fouling. To tackle this issue, innovative strategies are needed. This study investigated the effectiveness of membrane reciprocation and quorum quenching (QQ) to control fouling in MBRs. The study compared MBRs using membrane reciprocation (30 rpm) and QQ (injecting media containing 100 or 200 mg/L BH4) with conventional MBRs employing different air-scouring intensities. The results demonstrated that combining membrane reciprocation (30 rpm) with QQ (200 mg/L BH4) significantly extended the service time of MBRs, making it approximately six times longer than conventional methods. Moreover, this approach reduced physically reversible resistance. The reduction in signal molecules related to biofouling due to QQ showcased its critical role in controlling biofouling, even under high shear caused by membrane reciprocation. However, the impact of QQ on microbial community structure appeared relatively insignificant when compared to factors such as operation time, aeration intensity, and membrane reciprocation. By combining membrane reciprocation and QQ, the study achieved a remarkable 81 % energy saving compared to extensive aeration (103 s-1 in velocity gradient), in addition to the extended service time. Importantly, this combined antifouling approach did not negatively affect microbial characteristics and wastewater treatment, emphasizing its effectiveness in MBRs. Overall, the findings of this study offer valuable insights for developing synergistic fouling control strategies in MBRs, significantly improving the energy efficiency of the wastewater treatment process.

Engineering extracellular electron transfer pathways of electroactive microorganisms by synthetic biology for energy and chemicals production

The excessive consumption of fossil fuels causes massive emission of CO2, leading to climate deterioration and environmental pollution. The development of substitutes and sustainable energy sources to replace fossil fuels has become a worldwide priority. Bio-electrochemical systems (BESs), employing redox reactions of electroactive microorganisms (EAMs) on electrodes to achieve a meritorious combination of biocatalysis and electrocatalysis, provide a green and sustainable alternative approach for bioremediation, CO2 fixation, and energy and chemicals production. EAMs, including exoelectrogens and electrotrophs, perform extracellular electron transfer (EET) (i.e., outward and inward EET), respectively, to exchange energy with the environment, whose rate determines the efficiency and performance of BESs. Therefore, we review the synthetic biology strategies developed in the last decade for engineering EAMs to enhance the EET rate in cell-electrode interfaces for facilitating the production of electricity energy and value-added chemicals, which include (1) progress in genetic manipulation and editing tools to achieve the efficient regulation of gene expression, knockout, and knockdown of EAMs; (2) synthetic biological engineering strategies to enhance the outward EET of exoelectrogens to anodes for electricity power production and anodic electro-fermentation (AEF) for chemicals production, including (i) broadening and strengthening substrate utilization, (ii) increasing the intracellular releasable reducing equivalents, (iii) optimizing c-type cytochrome (c-Cyts) expression and maturation, (iv) enhancing conductive nanowire biosynthesis and modification, (v) promoting electron shuttle biosynthesis, secretion, and immobilization, (vi) engineering global regulators to promote EET rate, (vii) facilitating biofilm formation, and (viii) constructing cell-material hybrids; (3) the mechanisms of inward EET, CO2 fixation pathway, and engineering strategies for improving the inward EET of electrotrophic cells for CO2 reduction and chemical production, including (i) programming metabolic pathways of electrotrophs, (ii) rewiring bioelectrical circuits for enhancing inward EET, and (iii) constructing microbial (photo)electrosynthesis by cell-material hybridization; (4) perspectives on future challenges and opportunities for engineering EET to develop highly efficient BESs for sustainable energy and chemical production. We expect that this review will provide a theoretical basis for the future development of BESs in energy harvesting, CO2 fixation, and chemical synthesis.

Treatment of volatile organic compounds and other waste gases using membrane biofilm reactors: A review on recent advancements and challenges

This article provides a comprehensive review of membrane biofilm reactors for waste gas (MBRWG) treatment, focusing on studies conducted since 2000. The first section discusses the membrane materials, structure, and mass transfer mechanism employed in MBRWG. The concept of a partial counter-diffusion biofilm in MBRWG is introduced, with identification of the most metabolically active region. Subsequently, the effectiveness of these biofilm reactors in treating single and mixed pollutants is examined. The phenomenon of membrane fouling in MBRWG is characterized, alongside an analysis of contributory factors. Furthermore, a comparison is made between membrane biofilm reactors and conventional biological treatment technologies, highlighting their respective advantages and disadvantages. It is evident that the treatment of hydrophobic gases and their resistance to volatility warrant further investigation. In addition, the emergence of the smart industry and its integration with other processes have opened up new opportunities for the utilization of MBRWG. Overcoming membrane fouling and developing stable and cost-effective membrane materials are essential factors for successful engineering applications of MBRWG. Moreover, it is worth exploring the mechanisms of co-metabolism in MBRWG and the potential for altering biofilm community structures.

Role of quorum sensing and quorum quenching in anaerobic digestion: A scoping review

Anaerobic digestion (AD) is a biological process that employs anaerobic microorganisms to degrade organic material, yielding biogas and biofertilizers. Understanding quorum sensing (QS) signaling in mixed microbial systems provides valuable insights into microbial behavior and functions. This review aims to examine recent studies on the roles of QS and QQ in the AD processes. A QS signal molecule, N-acyl homoserine lactone (AHL), induce the production of extraceluller polymers, promoting biofilm formation and bacterial aggregation, thereby the efficiency of AD process. QS-assisted granule formation fosters syntrophy between acetogens and methanogens, leading to increased organic removal and methane production. Specific AHLs were shown to be correlated with the abundance of hydrolytic bacteria and acidogens, further benefiting methane production. QQ was shown to effectively control membrane fouling in anaerobic membrane bioreactors, yet its impact on methane productivity remains unclear. This review shed lights on the existing literature gaps regarding the mechanisms of QS and QQ in AD systems, which will play a vital role in advancing AD applications in the future.

d-tyrosine enhances disoctyl dimethyl ammonium chloride on alleviating SRB corrosion

Microbiologically influenced corrosion (MIC) caused by sulfate reducing bacteria (SRB) is a serious challenge in many industries, but biofilm greatly decreases the toxicity of bactericides to cell inside. d-amino acids are potential enhancers for bactericides due to their excellent performance on biofilm inhibition. However, the mechanism of d-amino acid cooperating with bactericides for MIC inhibition is still unknown. In this study, d-tyrosine（D-Tyr）and disoctyl dimethyl ammonium chloride (DDAC) were selected as the typical d-amino acid and bactericide, respectively, to evaluate their synergetic inhibition on the corrosion caused by Desulfovibrio vulgaris. D-Tyr obviously enhanced the role of DDAC in inhibiting corrosion with high corrosion inhibition efficiency at 77.23 %. The attachment of EPS and live cells on the coupon surface decreased in the presence of D-Try, leading to more cells directly exposed to DDAC. Besides, D-Try decreased the amount of live cells on the surface and thus reduced the utilization of Fe by SRB and corrosion current. Moreover, dead cells settling to the coupon surface may form a protective lay to retard the contact between live SRB and Fe, leading to slow cathode reaction and less corrosion. Therefore, D-Tyr can reduce the coverage of biofilm, thereby reducing its protective effect on SRB and achieving better corrosion inhibition effect. This work provides a new strategy for improving bactericides and inhibiting MIC.

Biofouling mitigation and microbial community dynamics in the membrane bioreactor by the indigenous quorum quenching bacterium Delftia sp. JL5

The quorum quenching (QQ) strategy has attracted increasing attention in membrane bioreactor (MBR) fouling control. However, the applicable QQ strain remains limited. This study investigated the antibiofouling performance of a new indigenous QQ bacterium, Delftia sp. JL5 (JL5) in MBR. JL5 produces intracellular acylase that irreversibly degrades N-acylhomoserine lactones (AHL), inhibited biofilm formation of quorum-sensing bacteria from activated sludge. During 120 days of operation, immobilized JL5 substantially delayed MBR biofouling by 2.1 and 2.9 times, at a flux rate of 30 L/(m2·h) and 20 L/(m2·h), respectively. A slower flux rate was favorable for effective mitigation of JL5 biofouling. JL5 reduced the AHL and extracellular polymeric substances of biocake without affecting the efficiency of waste removal. The presence of JL5 significantly changed the microbial structure of the membrane biocake, but not the activated sludge. Collectively, high activity, durability, and acid tolerance credited JL5 as a promising strain for QQ-MBR.

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

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

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

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

Does quorum quenching matter to microbial community dynamics in long-term membrane bioreactor operation?

Quorum quenching (QQ) has effectively prevented biofouling in membrane bioreactors (MBRs) employing isolated QQ bacterial strains. However, the influence of QQ on the microbial population still needs to be fully understood. This research aims to analyze the microbial population in MBRs over an extended period (>250 days) under different conditions, such as varying aeration intensities and doses of QQ bacteria, QQ media, and types of feed. Results show that no significant changes occurred in the structure and diversity of the microbial community in the mixed liquor and biofilm due to QQ treatment. Canonical correspondence analysis did reveal that the microbial communities were strongly influenced by feed types and phases. The microbial community composition varied between bacterial habitats (i.e., mixed liquor and biofilm), showing the two dominant phyla Proteobacteria and Bacteroidota in the former and Proteobacteria and Chloroflexi in the latter. The co-occurrence network analysis indicated that the biofilm (with 163 edges) in the MBR fed with real wastewater exhibited a more intricate network than the biofilm (with 53 edges) in the MBR fed with synthetic wastewater. With QQ, the biofilm exhibited more positive edges than negative ones. The phylogenetic investigation of communities showed that QQ barely affects functional gene-related quorum sensing (e.g., bacterial chemotaxis, motility proteins, and secretion) in mixed liquor but in biofilms at relatively large QQ doses (> 75 mg/L BH4). This research sheds light on the bacterial QQ's role in reducing MBR biofouling and provides crucial insights into its underlying mechanisms.

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.

Effect of quorum quenching on biofouling control and microbial community in membrane bioreactors by Brucella sp. ZJ1

Quorum quenching (QQ) has been demonstrated to be a novel technique for controlling biofouling in membrane bioreactors (MBRs), as it can significantly inhibit biofilm formation by disrupting quorum sensing (QS). The exploration of new QQ bacterial strains and the evaluation of their performance in mitigating membrane fouling in MBR systems is significant. In this study, an efficient QQ strain, Brucella sp. ZJ1 was encapsulated in alginate beads and evaluated for its ability to mitigate biofouling. The findings revealed that MBR with QQ beads extended the operation time by 2-3 times without affecting pollutant degradation. QQ beads maintained approximately 50% QQ activity after more than 50 days operation, indicating a long-lasting and endurable QQ effect. The QQ effect reduced extracellular polymeric substance (EPS) production especially in terms of polysaccharide and protein by more than 40%. QQ beads in the MBR also reduced the cake resistance and the irreversible resistance of membrane biofouling. Metagenomic sequencing suggests that QQ beads suppressed the QS effect and increased the abundance of QQ enzyme genes, ultimately inducing efficient membrane biofouling control.

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.

Quorum sensing-mediated microbial interactions: Mechanisms, applications, challenges and perspectives

Microbial community in natural or artificial environments playes critical roles in substance cycles, products synthesis and species evolution. Although microbial community structures have been revealed via culture-dependent and culture-independent approaches, the hidden forces driving the microbial community are rarely systematically discussed. As a mode of cell-to-cell communication that modifies microbial interactions, quorum sensing can regulate biofilm formation, public goods secretion, and antimicrobial substances synthesis, directly or indirectly influencing microbial community to adapt to the changing environment. Therefore, the current review focuses on microbial community in the different habitats from the quorum sensing perspective. Firstly, the definition and classification of quorum sensing were simply introduced. Subsequently, the relationships between quorum sensing and microbial interactions were deeply explored. The latest progressives regarding the applications of quorum sensing in wastewater treatment, human health, food fermentation, and synthetic biology were summarized in detail. Finally, the bottlenecks and outlooks of quorum sensing driving microbial community were adequately discussed. To our knowledge, this current review is the first to reveal the driving force of microbial community from the quorum sensing perspective. Hopefully, this review provides a theoretical basis for developing effective and convenient approaches to control the microbial community with quorum sensing approaches.

Microbial community evolution and functional trade-offs of biofilm in odor treatment biofilters

Biofilters inoculated with activated sludge are widely used for odor control in WWTP. In this process, biofilm community evolution plays an important role in the function of reactor and is closely related to reactor performance. However, the trade-offs in biofilm community and bioreactor function during the operation are still unclear. Herein, an artificially constructed biofilter for odorous gas treatment was operated for 105 days to study the trade-offs in the biofilm community and function. Biofilm colonization was found to drive community evolution during the start-up phase (phase 1, days 0-25). Although the removal efficiency of the biofilter was unsatisfactory at this phase, the microbial genera related to quorum sensing and extracellular polymeric substance secretion led to the rapid accumulation of the biofilm (2.3 kg biomass/m³ filter bed /day). During the stable operation phase (phase 2, days 26-80), genera related to target-pollutant degradation showed increases in relative abundance, which accompanied a high removal efficiency and a stable accumulation of biofilm (1.1 kg biomass/m³ filter bed/day). At the clogging phase (phase 3, days 81-105), a sharp decline in the biofilm accumulation rate (0.5 kg biomass/m³ filter bed /day) and fluctuating removal efficiency were observed. The quorum quenching-related genera and quenching genes of signal molecules increased, and competition for resources among species drove the evolution of the community in this phase. The results of this study highlight the trade-offs in biofilm community and functions during the operation of bioreactors, which could help improve bioreactor performance from a biofilm community perspective.

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.

In-situ membrane fouling control and performance improvement by adding materials in anaerobic membrane bioreactor: A review

Anaerobic membrane bioreactor (AnMBR) is a promising treatment technique for various types of wastewaters, and is preferred over other conventional aerobic and anaerobic methods. However, membrane fouling is considered a bottleneck in AnMBR system, which technically blocks membrane pores by numerous inorganics, organics, and other microbial substances. Various materials can be added in AnMBR to control membrane fouling and improve anaerobic digestion, and studies reporting the materials addition for this purpose are hereby systematically reviewed. The mechanism of membrane fouling control including compositional changes in extracellular polymeric substances (EPSs) and soluble microbial products (SMPs), materials properties, stimulation of antifouling microbes and alteration in substrate properties by material addition are thoroughly discussed. Nonetheless, this study opens up new research prospects to control membrane fouling of AnMBR, engineered by material, including compositional changes of microbial products (EPS and SMP), replacement of quorum quenching (QQ) by materials, and overall improvement of reactor performance. Regardless of the great research progress achieved previously in membrane fouling control, there is still a long way to go for material-mediated AnMBR applications to be undertaken, particularly for materials coupling, real scale application and molecular based studies on EPSs and SMPs, which were proposed for future researches.

Biofilm inhibition based on controlling the transmembrane transport and extracellular accumulation of quorum sensing signals

The regulation of bacterial quorum sensing (QS) has been used to inhibit biofouling in wastewater treatment plants and the formation of biofilms. In contrast to traditional QS regulation strategies, this study aimed to obstruct the transmembrane transport process of QS signals to decrease their extracellular accumulation. Three phytochemicals, astragaloside IV, eugenol, and baicalin were selected, their effects on biofilm formation by Pseudomonas aeruginosa PA14 were studied, and the mechanisms determined. The inhibition efficiency of biofilm formation by 50 mg/L astragaloside IV, 1 mg/L eugenol, and 1 mg/L baicalin were 37%, 26%, and 26%, respectively. Confocal laser scanning microscopy and analysis of extracellular polymeric substances indicated that the three inhibitors affected the structure and composition of the biofilms. Furthermore, bacterial motility and a variety of QS-related virulence factors were suppressed by the inhibitor treatment due to changes in bacterial QS. Notably, the three inhibitors all decreased the extracellular concentration of the QS signaling molecule 3-oxo-C12-homoseine lactone by affecting the function of efflux pump MexAB-OprM. This indirectly interfered with the bacterial QS system and thus inhibited biofilm formation. In conclusion, this study revealed the inhibitory effects and inhibition mechanism of three phytochemicals on efflux pump and QS of P. aeruginosa and realized the inhibition on biofilm formation. We update the efflux pump inhibitor library and provide a new way for biofilm contamination control.

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.

Designing sustainable membrane-based water treatment via fouling control through membrane interface engineering and process developments

Membrane-based water treatment processes have been established as a powerful approach for clean water production. However, despite the significant advances made in terms of rejection and flux, provision of sustainable and energy-efficient water production is restricted by the inevitable issue of membrane fouling, known to be the major contributor to the elevated operating costs due to frequent chemical cleaning, increased transmembrane resistance, and deterioration of permeate flux. This review provides an overview of fouling control strategies in different membrane processes, such as microfiltration, ultrafiltration, membrane bioreactors, and desalination via reverse osmosis and forward osmosis. Insights into the recent advancements are discussed and efforts made in terms of membrane development, modules arrangement, process optimization, feed pretreatment, and fouling monitoring are highlighted to evaluate their overall impact in energy- and cost-effective water treatment. Major findings in four key aspects are presented, including membrane surface modification, modules design, process integration, and fouling monitoring. Among the above mentioned anti-fouling strategies, a large part of research has been focused on membrane surface modifications using a number of anti-fouling materials whereas much less research has been devoted to membrane module advancements and in-situ fouling monitoring and control. At the end, a critical analysis is provided for each anti-fouling strategy and a rationale framework is provided for design of efficient membranes and process for water treatment.

The relationship between quorum sensing dynamics and biological performances during anaerobic membrane bioreactor treatment

Anaerobic membrane bioreactors (AnMBRs) enhance carbon neutrality with biomethane recovery from wastewater; however, microbial signaling, which may affect biological performances, was poorly understood. Here, we thus evaluate quorum sensing (QS) dynamics while monitoring acyl-homoserine lactones (AHLs) and autoinducer-2 (AI-2) levels during long-term AnMBR operations after sludge inoculation. Significant organic removal and methane production were achieved with the reactor startup. Signal molecule levels varied with transient organic loading rates, depending on their types. A starving condition may cause an increase in short- and medium-chain AHLs and AI-2. Biopolymers, biosolids, volatile fatty acids, and alkalinity levels had positive correlations with short- and medium-chain AHLs and AI-2, whereas methane production had positive correlations with long-chain AHLs. The principal component analysis of QS signal composition and biological performance data explains their interconnectivity. The findings of this study help to understand that QS signals regulate metabolic pathways in addition to microbial group behaviors.

Microbial community regulation and performance enhancement in gas biofilters by interrupting bacterial communication

BACKGROUND

Controlling excess biomass accumulation and clogging is important for maintaining the performance of gas biofilters and reducing energy consumption. Interruption of bacterial communication (quorum quenching) can modulate gene expression and alter biofilm properties. However, whether the problem of excess biomass accumulation in gas biofilters can be addressed by interrupting bacterial communication remains unknown.

RESULTS

In this study, parallel laboratory-scale gas biofilters were operated with Rhodococcus sp. BH4 (QQBF) and without Rhodococcus sp. BH4 (BF) to explore the effects of quorum quenching (QQ) bacteria on biomass accumulation and clogging. QQBF showed lower biomass accumulation (109 kg/m³) and superior operational stability (85-96%) than BF (170 kg/m³; 63-92%) at the end of the operation. Compared to BF, the QQBF biofilm had lower adhesion strength and decreased extracellular polymeric substance production, leading to easier detachment of biomass from filler surface into the leachate. Meanwhile, the relative abundance of quorum sensing (QS)-related species was found to decrease from 67 (BF) to 56% (QQBF). The QS function genes were also found a lower relative abundance in QQBF, compared with BF. Moreover, although both biofilters presented aromatic compounds removal performance, the keystone species in QQBF played an important role in maintaining biofilm stability, while the keystone species in BF exhibited great potential for biofilm formation. Finally, the possible influencing mechanism of Rhodococcus sp. BH4 on biofilm adhesion was demonstrated. Overall, the results of this study achieved excess biomass control while maintaining stable biofiltration performance (without interrupting operation) and greatly promoted the use of QQ technology in bioreactors. Video Abstract.

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.

The exceptionally efficient quorum quenching enzyme LrsL suppresses Pseudomonas aeruginosa biofilm production

Quorum quenching (QQ) is the enzymatic degradation of molecules used by bacteria for synchronizing their behavior within communities. QQ has attracted wide attention due to its potential to inhibit biofilm formation and suppress the production of virulence factors. Through its capacity to limit biofouling and infections, QQ has applications in water treatment, aquaculture, and healthcare. Several different QQ enzymes have been described; however, they often lack the high stability and catalytic efficiency required for industrial applications. Previously, we identified genes from genome sequences of Red Sea sediment bacteria encoding potential QQ enzymes. In this study, we report that one of them, named LrsL, is a metallo-β-lactamase superfamily QQ enzyme with outstanding catalytic features. X-ray crystallography shows that LrsL is a zinc-binding dimer. LrsL has an unusually hydrophobic substrate binding pocket that can accommodate a broad range of acyl-homoserine lactones (AHLs) with exceptionally high affinity. In vitro, LrsL achieves the highest catalytic efficiency reported thus far for any QQ enzyme with a K_cat/K_M of 3 × 10⁷. LrsL effectively inhibited Pseudomonas aeruginosa biofilm formation without affecting bacterial growth. Furthermore, LrsL suppressed the production of exopolysaccharides required for biofilm production. These features, and its capacity to regain its function after prolonged heat denaturation, identify LrsL as a robust and unusually efficient QQ enzyme for clinical and industrial applications.

Core carbon fixation pathways associated with cake layer development in an anoxic-oxic biofilm-membrane bioreactor treating textile wastewater

Microbial carbon fixation pathways have not yet been adequately understood for their role in membrane case layer formation processes. Carbon fixation bacteria can play critical roles in either causing or enhancing cake layer formation in some autotrophic-prone anoxic conditions, such as sulfur-cycling conditions. Understanding the microbes capable of carbon fixation can potentially guide the design of membrane biofouling mitigation strategies in scientific ways. Thus, we used meta-omics methods to query carbon fixation pathways in the cake layers of a full-scale anoxic-oxic biofilm-MBR system treating textile wastewater in this study. Based on the wastewater constituents and other properties, such as anoxic conditions, sulfide-reducing and sulfur-oxidizing bacteria could co-exist in the membrane unit. In addition, low-light radiation conditions could also happen to the membrane unit. However, we could not quantify the light intensity or total energy input accurately because the whole experimental setup was a full-scale system. Potentially complete carbon fixation pathways in the cake layer included the Calvin-Benson-Bassham cycle, Wood-Ljungdahl pathway, and the 3-hydroxypropionate bicycle. We discovered that using aeration could effectively inhibit carbon fixation, which resulted in mitigating membrane cake layer development. However, the aeration resulted in the 3-hydroxypropionate bicycle pathway, presumably used by aerobic sulfur-oxidizing prokaryotes, to become a more abundant carbon fixation pathway in the cake layer under aerobic conditions.

Acceleration of start-up of moving bed biofilm reactor at low temperature by adding specialized quorum sensing bacteria

This study aims to accelerate biofilm formation and operational performance of moving bed biofilm reactor (MBBR) at 5 ℃ by adding specialized Quorum Sensing bacteria (sphingomonas rubra BH3^T). Results showed that bio augmented MBBR (R_S) achieved a higher chemical oxygen demand and NH₄⁺-N removal rate (93% and 75%), which in accordance with its increased biofilm thickness, higher biofilm activity, and nitrifying bacteria abundance (Nitrospira). The increased biofilm thickness (60.23 %) during the whole operating time, accompanied by more potent adhesion force (61.59 %), was related to increased polysaccharides and proteins in the biofilm. Pyrosequencing analysis indicated that BH3^T contributed to higher species richness and triggered the rapid growth of precursor microorganisms (Nakamurella, Micropruina, and Zoogloea) and the enrichment of multifunctional microorganisms (Pseudomonas, Aeromonas, Arcobacter, Dechloromonas, and Flavobacterium) at low temperatures. This study provides an economical and practical new insight into accelerating start-up of MBBR system at low temperature.

Gene-trait matching analysis reveals putative genes involved in Bifidobacterium spp. biofilm formation

Biofilm formation by bacteria represents an adaptation strategy to the environment, and some special genes may lead to a strong biofilm phenotype. In this study, we attempted to find functional genes associated with bifidobacterial biofilm formation. Firstly, we evaluated the biofilm formation ability of bifidobacterial strains from six species, which showed that Bifidobacterium longum, Bifidobacterium breve, Bifidobacterium animalis, Bifidobacterium adolescentis, and Bifidobacterium pseudocatenulatum had biofilm-forming and non-biofilm-forming strains, while all Bifidobacterium bifidum strains could form strong biofilms. Then 48 strains were selected for genome sequencing and comparative analysis. The gene-trait matching analysis revealed that B. bifidum biofilm formation phenotype may associate with their unique genes, involving in stress response, quorum sensing, two components, and peptide synthesis. B. pseudocatenulatum biofilm formation was positively correlated with the eps cluster (rfbX). While no genotype related to the biofilm phenotype was found in B. longum using this analysis, but all contain autoinducer-2 (AI-2) receptor genes. Moreover, luxS, rbsB, rfbX were selected for real-time qPCR analysis, suggesting that their expression are important to biofilm formation. These results indicated that strains carrying certain genes tend to form stronger biofilms than those formed by strains without these genes.

Photolytic quorum quenching effects on the microbial communities and functional gene expressions in membrane bioreactors

Photolytic quorum quenching by ultraviolet A (UVA) irradiation is an effective strategy for controlling membrane bioreactor (MBR) biofouling; however, its effects on MBR microbial communities and functional genes have not yet been explored. Here, we report on the effects of the UVA irradiation, which mitigates membrane biofouling, on the microbial community structures, alpha and beta diversities, and functional gene expressions in the MBR mixed liquor and biocake (membrane fouling layer) for the first time. The results show that the microbial communities become less diversified when alternating UVA is applied to the MBRs. The changes in the community structure are highly influenced by spatiotemporal factors, such as microbial habitats (mixed liquor and biocake) and reactor operation time, although UVA irradiation also has some impacts on the community. The relative abundance of the Sphingomonadaceae family, which can decompose the furan ring of autoinducer-2 (AI-2) signal molecules, becomes greater with continuous UVA irradiation. Xanthomonadaceae, which produces biofilm-degrading enzymes, is also more abundant with UVA photolysis than without it. Copies of monooxygenase and hydroxylase enzyme-related genes increase in the MBR with longer UVA exposures (i.e., continuous UVA). These enzymes seem to be inducible by UVA, enhancing the AI-2 inactivation. In conclusion, UVA irradiation alters the microbial community and the metabolism in the MBR, contributing to the membrane biofouling mitigation.

Layered Antibiofouling Composite Membrane for Quenching Bacterial Signaling

Bacterial quorum quenching (QQ) media with various structures (e.g., bead, cylinder, hollow cylinder, and sheet), which impart biofouling mitigation in membrane bioreactors (MBRs), have been reported. However, there has been a continuous demand for membranes with QQ capability. Thus, herein, we report a novel double-layered membrane comprising an outer layer containing a QQ bacterium (BH4 strain) on the polysulfone hollow fiber membrane. The double-layered composite membrane significantly inhibits biofilm formation (i.e., the biofilm density decreases by ~58%), biopolymer accumulation (e.g., polysaccharide), and signal molecule concentration (which decreases by ~38%) on the membrane surface. The transmembrane pressure buildup to 50 kPa of the BH4-embedded membrane (17.8 h ± 1.1) is delayed by more than thrice (p < 0.05) of the control with no BH4 in the membrane’s outer layer (5.5 h ± 0.8). This finding provides new insight into fabricating antibiofouling membranes with a self-regulating property against biofilm growth.

Dynamics of microbial community and their effects on membrane fouling in an anoxic-oxic gravity-driven membrane bioreactor under varying solid retention time: A pilot-scale study

Membrane fouling in a membrane bioreactor (MBR) is highly influenced by the characteristics of the influent, the mixed liquor microbial community and the operational parameters, all of which are environment specific. Therefore, we studied the dynamics of microbial community during the treatment of real municipal wastewater in a pilotscale anoxic-oxic (A/O) MBR equipped with a gravity-driven membrane filtration system. The MBR was operated at three different solid retention times (SRTs): 25, 40 and 10 days for a total period of 180 days in Nordic environmental conditions. Analysis of microbial community dynamics revealed a high diversity of microbial species at SRT of 40 days, whereas SRT of 25 days was superior with microbial richness. Production of soluble microbial products (SMP) and extracellular polymeric substances (EPS) was found to be intensely connected with the SRT and food to microorganism (F/M) ratio. Relatively longer operational period with the lowest rate of membrane fouling was observed at SRT of 25 days, which was resulted from the superior microbial community, lowest production of SMP and loosely bound EPS as well as the lower filtration resistance of larger sludge flocs. Abundance of quorum quenching (QQ) bacteria and granular floc forming bacterial genera at SRT of 25 days provided relatively lower membrane fouling tendency and larger floc formation, respectively. On the other hand, substantial amount of various surface colonizing and EPS producing bacteria was found at SRT of 10 days, which promoted more rapid membrane fouling compared with the fouling rate seen at other tested SRTs. To sum up, this research provides a realistic insight into the impact of SRT on microbial community dynamics and resulting characteristics of mixed liquor, floc size distribution and membrane fouling for improved MBR operation.

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.

Inhibition of AHL-mediated quorum sensing to control biofilm thickness in microbial fuel cell by using Rhodococcus sp. BH4

Anode biofilm thickness is a key point for high and sustainable power generation in microbial fuel cells (MFCs). Over time, the formation of a thicker biofilm on anode electrode hinders the power generation performance of MFC by causing a longer electron transfer path and the accumulation of undesirable components in anode biofilm. To overcome these limitations, we used a novel strategy named quorum quenching (QQ) for the first time in order to control the biofilm thickness on the anode surface by inactivation of signal molecules among microorganisms. For this purpose, the isolated QQ bacteria (Rhodococcus sp. BH4) were immobilized into alginate beads (20, 40, and 80 mg/10 ml sodium alginate) and added to the anode chamber of MFCs. The MFC exhibited the best electrochemical activity (1924 mW m^-2) with a biofilm thickness of 26 μm at 40 mg Rhodococcus sp. BH4/10 ml sodium alginate. The inhibition of signal molecules in anode chamber reduced the production of extracellular polymeric substance (EPS) by preventing microbial communication amonganode microorganisms. Microscopic observations revealed that anode biofilm thickness and the abundance of dead bacteria significantly decreased with an increase in Rhodococcus sp. BH4 concentration in MFCs. Microbiome diversity showed an apparent difference among the microbial community structures of anode biofilms in MFCs containing vacant and Rhodococcus sp. BH4 beads. The data revealed that the QQ strategy is an efficient application for improving MFC performance and may shed light on future studies.

Addition of Trichocladium canadense to an anaerobic membrane bioreactor: evaluation of the microbial composition and reactor performance

Membrane bioreactors are powerful systems for wastewater treatment and the removal of toxic compounds. However, membrane biofouling stands in the way of their widespread usage. In this study, the saprophytic fungus Trichocladium canadense was used as the bioaugmentor in an anaerobic membrane bioreactor (AnMBR) and its impact on membrane biofouling, biogas production, the microbial communities of the reactor and removal of the common antibiotics erythromycin (ERY), sulfamethoxazole (SMX) and tetracycline (TET) from synthetic wastewater was investigated. The results indicated that through bioaugmentation with 20% T. canadense, membrane biofouling was slowed by 25%, the chemical oxygen demand removal increased by 16% and a higher efficiency removal of ERY and SMX was achieved. The presence of T. canadense significantly increased the abundance and diversity of the biofilm archaeal community and the bacterial phylum Firmicutes, a known bio-foulant.

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.

Charting the landscape of RNA polymerases to unleash their potential in strain improvement

One major mission of microbial cell factory is overproduction of desired chemicals. To this end, it is necessary to orchestrate enzymes that affect metabolic fluxes. However, only modification of a small number of enzymes in most cases cannot maximize desired metabolites, and global regulation is required. Of myriad enzymes influencing global regulation, RNA polymerase (RNAP) may be the most versatile enzyme in biological realm because it not only serves as the workhorse of central dogma but also participates in a plethora of biochemical events. In fact, recent years have witnessed extensive exploitation of RNAPs for phenotypic engineering. While a few impressive reviews showcase the structures and functionalities of RNAPs, this review not only summarizes the state-of-the-art advance in the structures of RNAPs but also points out their enormous potentials in metabolic engineering and synthetic biology. This review aims to provide valuable insights for strain improvement.

QQ-PAC core-shell structured quorum quenching beads for potential membrane antifouling properties

Quorum quenching (QQ) has been proven to be an effective method to reduce MBR membrane biological contamination. In this paper, a novel and efficient QQ-PAC core-shell beads were prepared for mitigating the membrane contamination. The bead was composed of two parts: QQ bacteria embedded in the core and PAC in the shell. The microstructure of the bead was observed by scanning electron microscopy (SEM) and the functional groups were revealed by Fourier transform infrared spectroscopy (FTIR). Meanwhile, the mechanical strength, swelling property, penetration property and QQ activity of the core bead, the core shell-without PAC bead and the core shell-with PAC bead were compared. The core shell-with PAC structure improved the adsorption capacity under good mass transfer conditions. Besides, the combined effect of QQ bacteria and PAC enhanced the QQ effect and alleviated the process of MBR membrane biological contamination consequently. Therefore, the QQ-PAC core-shell beads have a potential possibility in MBR membrane fouling control as the immobilization technology of QQ bacteria.

Deciphering acyl-homoserine lactones-mediated quorum sensing on geotextile bio-clogging in municipal solid waste and bottom ash co-disposal landfills

Bottom ash co-disposed in landfills accelerates geotextile clogging and decreases landfill stability. As the main contributor to clogging, bio-clogging may be associated with quorum sensing (QS) in microbial communities. This study investigated the potential roles of acyl-homoserine lactones (AHLs)-mediated QS in geotextile bio-clogging under different landfill conditions, including municipal solid waste landfill and bottom ash co-disposal landfill. The unit area of geotextile bio-clogging mass in the municipal solid waste landfill (MSW_G) ranged from 5.2 × 10^-3 to 8.2 × 10^-3 g/cm²_, while it was in the range of 8.4 × 10^-3 to 1.2 × 10^-2 g/cm² in the bottom ash co-disposal landfill (BA_G). Two types of AHLs were detected and the total AHLs content in the MSW_G (1,616.9 ± 103.8 ng/g VSS) was half of that in the BA_G (3,233.0 ± 646.8 ng/g VSS). High contents of the AHLs could increase bio-clogging. The bio-clogging was also attributed to QS genes and extracellular polymeric substances (EPS). EPS aggregation was stimulated due to the higher Ca²⁺ and Mg²⁺ in the BA_G. These results suggested that the co-disposal of bottom ash could increase the AHLs content, resulting in accelerated bio-clogging.

Transcriptome Analysis Reveals the Genes Involved in Bifidobacterium Longum FGSZY16M3 Biofilm Formation

Biofilm formation has evolved as an adaptive strategy for bacteria to cope with harsh environmental conditions. Currently, little is known about the molecular mechanisms of biofilm formation in bifidobacteria. A time series transcriptome sequencing analysis of both biofilm and planktonic cells of Bifidobacterium longum FGSZY16M3 was performed to identify candidate genes involved in biofilm formation. Protein–protein interaction network analysis of 1296 differentially expressed genes during biofilm formation yielded 15 clusters of highly interconnected nodes, indicating that genes related to the SOS response (dnaK, groS, guaB, ruvA, recA, radA, recN, recF, pstA, and sufD) associated with the early stage of biofilm formation. Genes involved in extracellular polymeric substances were upregulated (epsH, epsK, efp, frr, pheT, rfbA, rfbJ, rfbP, rpmF, secY and yidC) in the stage of biofilm maturation. To further investigate the genes related to biofilm formation, weighted gene co-expression network analysis (WGCNA) was performed with 2032 transcript genes, leading to the identification of nine WGCNA modules and 133 genes associated with response to stress, regulation of gene expression, quorum sensing, and two-component system. These results indicate that biofilm formation in B. longum is a multifactorial process, involving stress response, structural development, and regulatory processes.

Nitrogen removal mechanism of marine anammox bacteria treating nitrogen-laden saline wastewater in response to ultraviolet (UV) irradiation: High UV tolerance and microbial community shift

Salt stress can be naturally overcome by marine anammox bacteria (MAB), while their low growth rate and sensitivity to operational conditions are still challenges for the application of anammox. To enhance the enrichment of MAB and decipher the effects of ultraviolet (UV) irradiation on MAB, UV was introduced in the nitrogen removal of MAB treating nitrogen-laden saline wastewater for the first time. The results indicated that MAB were resistant to a fairly high UV-C dose, 12000 mJ/cm². Their relative abundance was enhanced by 1.2 folds under 12000 mJ/cm² UV-C. However, the relative abundance of Actinobacteria, Acidobacteria, Chloroflexi and Marinicella were greatly dropped with enhanced UV-C dose. The tolerance mechanism was diversified, e.g. excessive extracellular polymeric substances, special structure of MAB and interspecific competition/cooperation. Although further study was still needed, the findings shed a light on MAB enrichment and exploited great potentials of MAB in nitrogen-laden saline wastewater treatment.

Potential biofilm control strategies for extended spaceflight missions

Biofilms, surface-adherent microbial communities, are associated with microbial fouling and corrosion in terrestrial water-distribution systems. Biofilms are also present in human spaceflight, particularly in the Water Recovery System (WRS) on the International Space Station (ISS). The WRS is comprised of the Urine Processor Assembly (UPA) and the Water Processor Assembly (WPA) which together recycles wastewater from human urine and recovered humidity from the ISS atmosphere. These wastewaters and various process streams are continually inoculated with microorganisms primarily arising from the space crew microbiome. Biofilm-related fouling has been encountered and addressed in spacecraft in low Earth orbit, including ISS and the Russian Mir Space Station. However, planned future missions beyond low Earth orbit to the Moon and Mars present additional challenges, as resupplying spare parts or support materials would be impractical and the mission timeline would be in the order of years in the case of a mission to Mars. In addition, future missions are expected to include a period of dormancy in which the WRS would be unused for an extended duration. The concepts developed in this review arose from a workshop including NASA personnel and representatives with biofilm expertise from a wide range of industrial and academic backgrounds. Here, we address current strategies that are employed on Earth for biofilm control, including antifouling coatings and biocides and mechanisms for mitigating biofilm growth and damage. These ideas are presented in the context of their applicability to spaceflight and identify proposed new topics of biofilm control that need to be addressed in order to facilitate future extended, crewed, spaceflight missions.

Regulatory Mechanisms and Promising Applications of Quorum Sensing-Inhibiting Agents in Control of Bacterial Biofilm Formation

A biofilm is an assemblage of microbial cells attached to a surface and encapsulated in an extracellular polymeric substance (EPS) matrix. The formation of a biofilm is one of the important mechanisms of bacterial resistance, which not only leads to hard-to-control bacterial infections in humans and animals but also enables bacteria to be a major problem in various fields, such as food processing, wastewater treatment and metalworking. Quorum sensing (QS) is a bacterial cell-to-cell communication process that depends on the bacterial population density and is mediated by small diffusible signaling molecules called autoinducers (AIs). Bacteria use QS to regulate diverse arrays of functions, including virulence and biofilm formation. Therefore, the interference with QS by using QS inhibiting agents, including QS inhibitors (QSIs) and quorum quenching (QQ) enzymes, to reduce or even completely repress the biofilm formation of pathogenic bacteria appears to be a promising approach to control bacterial infections. In this review, we summarize the mechanisms of QS-regulating biofilm formation and QS-inhibiting agents that control bacterial biofilm formation, strategies for the discovery of new QS inhibiting agents, and the current applications of QS-inhibiting agents in several fields to provide insight into the development of effective drugs to control pathogenic bacteria.