AHLs-mediated quorum sensing threshold and its response towards initial adhesion of wastewater biofilms

Methanogenic partner influences cell aggregation and signalling of Syntrophobacterium fumaroxidans

For several decades, the formation of microbial self-aggregates, known as granules, has been extensively documented in the context of anaerobic digestion. However, current understanding of the underlying microbial-associated mechanisms responsible for this phenomenon remains limited. This study examined morphological and biochemical changes associated with cell aggregation in model co-cultures of the syntrophic propionate oxidizing bacterium Syntrophobacterium fumaroxidans and hydrogenotrophic methanogens, Methanospirillum hungatei or Methanobacterium formicicum. Formerly, we observed that when syntrophs grow for long periods with methanogens, cultures tend to form aggregates visible to the eye. In this study, we maintained syntrophic co-cultures of S. fumaroxidans with either M. hungatei or M. formicicum for a year in a fed-batch growth mode to stimulate aggregation. Millimeter-scale aggregates were observed in both co-cultures within the first 5 months of cultivation. In addition, we detected quorum sensing molecules, specifically N-acyl homoserine lactones, in co-culture supernatants preceding the formation of macro-aggregates (with diameter of more than 20 μm). Comparative transcriptomics revealed higher expression of genes related to signal transduction, polysaccharide secretion and metal transporters in the late-aggregation state co-cultures, compared to the initial ones. This is the first study to report in detail both biochemical and physiological changes associated with the aggregate formation in syntrophic methanogenic co-cultures. KEYPOINTS: • Syntrophic co-cultures formed mm-scale aggregates within 5 months of fed-batch cultivation. • N-acyl homoserine lactones were detected during the formation of aggregates. • Aggregated co-cultures exhibited upregulated expression of adhesins- and polysaccharide-associated genes.

Benefits of fungal-to-bacterial quorum quenching as anti-biofouling strategy in membrane bioreactors for wastewater treatment and water reuse

This study addresses membrane biofouling in membrane bioreactors (MBRs) by exploring fungal-to-bacterial quorum quenching (QQ) strategies. While most research has been focused on bacterial-to-bacterial QQ tactics, this study identified fungal strain Vanrija sp. MS1, which is capable of degrading N-acyl-homoserine lactones (signaling molecules of Gram-negative bacteria). To determine the benefits of fungal over bacterial strains, after immobilization on fluidizing spherical beads in an MBR, MS1 significantly reduced the fouling rate by 1.8-fold compared to control MBR, decreased extracellular polymeric substance levels in the biofilm during MBR operation, and favorably changed microbial community and bacterial network, resulting in biofouling mitigation. It is noteworthy that, unlike Rhodococcus sp. BH4, MS1 enhanced QQ activity when switching from neutral to acidic conditions. These results suggest that MS1 has the potential for the effective treatment of acidic industrial wastewater sources such as semiconductor and secondary battery wastewater using MBRs.

Impact of nutrient deficiency on biological sewage treatment - Perspectives towards urine source segregation

Human urine contains 9 g/L of nitrogen (N) and 0.7 g/L of phosphorus (P). The recovery of N and P from urine helps close the nutrient loop and increase resource circularity in the sewage treatment sector. Urine contributes an average of 80 % N and 50 % P in sewage, whereby urine source segregation could reduce the burden of nutrient removal in sewage treatment plants (STPs) but result in N and P deficiency and unintended negative consequences. This review examines the potential impacts of N and P deficiency on the removal of organic carbon and nutrients, sludge characteristics and greenhouse gas emissions in activated sludge processes. The details of how these impacts affect the operation of STPs were also included. This review helps foresee operational challenges that established STPs may face when dealing with nutrient-deficient sewage in a future where source separation of urine is the norm. The findings indicate that the requirement of nitrification-denitrification and biological P removal processes could shrink at urine segregation above 80 % and 100 %, respectively. Organic carbon, N and biological P removal processes can be severely affected under full urine segregation. The decrease in solid retention time due to urine segregation increases treatment capacity up to 48 %. Sludge flocculation and settleability would deteriorate due to changes in extracellular polymeric substances and induce various forms of bulking. Beneficially, N deficiency reduces nitrous oxide emissions. These findings emphasise the importance of considering and preparing for impacts caused by urine source segregation-induced nutrient deficiency in sewage treatment processes.

Protective effects of antibiotic resistant bacteria on susceptibles in biofilm: Influential factors, mechanism, and modeling

In environmental biofilms, antibiotic-resistant bacteria facilitate the persistence of susceptible counterparts under antibiotic stresses, contributing to increased community-level resistance. However, there is a lack of quantitative understanding of this protective effect and its influential factors, hindering accurate risk assessment of biofilm resistance in diverse environment. This study isolated an opportunistic Escherichia coli pathogen from soil, and engineered it with plasmids conferring antibiotic resistance. Protective effects of the ampicillin resistant strain (AmpR) on their susceptible counterparts (AmpS) were observed in ampicillin-stress colony biofilms. The concentration of ampicillin delineated protective effects into 3 zones: continuous protection (<1 MIC of AmpS), initial AmpS/R dependent (1-8 MIC of AmpS), and ineffective (>8 MIC of AmpS). Intriguingly, Zone 2 exhibited a surprising "less is more" phenomenon tuned by the initial AmpS/R ratio, where biofilm with an initially lower AmpR (1:50 vs 50:1) harbored 30-90 % more AmpR after 24 h growth under antibiotic stress. Compared to AmpS, AmpR displayed superiority in adhesion, antibiotic degradation, motility, and quorum sensing, allowing them to preferentially colonize biofilm edge and areas with higher ampicillin. An agent-based model incorporating protective effects successfully simulated tempo-spatial dynamics of AmpR and AmpS influenced by antibiotic stress and initial AmpS/R. This study provides a holistic view on the pervasive but poorly understood protective effects in biofilm, enabling development of better risk assessment and precisely targeted control strategies of biofilm resistance in diverse environment.

Enhanced AHL-mediated quorum sensing accelerates the start-up of biofilm reactors by elevating the fitness of fast-growing bacteria in sludge and biofilm communities

Quorum sensing (QS)-based manipulations emerge as a promising solution for biofilm reactors to overcome challenges from inefficient biofilm formation and lengthy start-ups. However, the ecological mechanisms underlying how QS regulates microbial behaviors and community assembly remain elusive. Herein, by introducing different levels of N-acyl-homoserine lactones, we manipulated the strength of QS during the start-up of moving bed biofilm reactors and compared the dynamics of bacterial communities. We found that enhanced QS elevated the fitness of fast-growing bacteria with high ribosomal RNA operon (rrn) copy numbers in their genomes in both the sludge and biofilm communities. This led to notably increased extracellular substance production, as evidenced by strong positive correlations between community-level rrn copy numbers and extracellular proteins and polysaccharides (Pearson's r = 0.529-0.830, P < 0.001). Network analyses demonstrated that enhanced QS significantly promoted the ecological interactions among taxa, particularly cooperative interactions. Bacterial taxa with higher network degrees were more strongly correlated with extracellular substances, suggesting their crucial roles as public goods in regulating bacterial interactions and shaping network structures. However, the assembly of more cooperative communities in QS-enhanced reactors came at the cost of decreased network stability and modularity. Null model and dissimilarity-overlap curve analysis revealed that enhanced QS strengthened stochastic processes in community assembly and rendered the universal population dynamics more convergent. Additionally, these shaping effects were consistent for both the sludge and biofilm communities, underpinning the planktonic-to-biofilm transition. This work highlights that QS manipulations efficiently drive community assembly and confer specialized functional traits to communities by recruiting taxa with specific life strategies and regulating interspecific interactions. These ecological insights deepen our understanding of the rules governing microbial societies and provide guidance for managing engineering ecosystems.

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.

Bacillus altitudinis AD13-4 Enhances Saline-Alkali Stress Tolerance of Alfalfa and Affects Composition of Rhizosphere Soil Microbial Community

Saline and alkaline stresses limit plant growth and reduce crop yield. Soil salinization and alkalization seriously threaten the sustainable development of agriculture and the virtuous cycle of ecology. Biofertilizers made from plant growth-promoting rhizobacteria (PGPR) not only enhance plant growth and stress tolerance, but also are environmentally friendly and cost-effective. There have been many studies on the mechanisms underlying PGPRs enhancing plant salt resistance. However, there is limited knowledge about the interaction between PGPR and plants under alkaline-sodic stress. To clarify the mechanisms underlying PGPR's improvement of plants' tolerance to alkaline-sodic stress, we screened PGPR from the rhizosphere microorganisms of local plants growing in alkaline-sodic land and selected an efficient strain, Bacillus altitudinis AD13-4, as the research object. Our results indicate that the strain AD13-4 can produce various growth-promoting substances to regulate plant endogenous hormone levels, cell division and differentiation, photosynthesis, antioxidant capacity, etc. Transcriptome analysis revealed that the strain AD13-4 significantly affected metabolism and secondary metabolism, signal transduction, photosynthesis, redox processes, and plant-pathogen interactions. Under alkaline-sodic conditions, inoculation of the strain AD13-4 significantly improved plant biomass and the contents of metabolites (e.g., soluble proteins and sugars) as well as secondary metabolites (e.g., phenols, flavonoids, and terpenoids). The 16S rRNA gene sequencing results indicated that the strain AD13-4 significantly affected the abundance and composition of the rhizospheric microbiota and improved soil activities and physiochemical properties. Our study provides theoretical support for the optimization of saline-alkali-tolerant PGPR and valuable information for elucidating the mechanism of plant alkaline-sodic tolerance.

First report on pollutant accumulation and associated microbial communities in the freshwater sponge Spongilla lacustris (Linnaeus, 1759) from the sub-Arctic Pasvik River (Norway)

This explorative study was aimed at first characterizing the sponge Spongilla lacustris (Linnaeus, 1759) from the sub-Arctic Pasvik River (Northern Fennoscandia), in terms of associated microbial communities and pollutant accumulation. Persistent organic pollutants were determined in sponge mesohyl tissues, along with the estimation of the microbial enzymatic activity rates, prokaryotic abundance and morphometric traits, and the analysis of the taxonomic bacterial diversity by next-generation sequencing techniques. The main bacterial groups associated with S. lacustris were Alphaproteobacteria and Gammaproteobacteria, followed by Chloroflexi and Acidobacteria. The structure of the S. lacustris-associated bacterial communities was in sharp contrast to those of the bacterioplankton, being statistically close to those found in sediments. Dieldrin was measured at higher concentrations in the sponge tissues (3.1 ± 0.4 ng/g) compared to sediment of the same site (0.04 ± 0.03 ng/g). Some taxonomic groups were possibly related to the occurrence of certain contaminants, as was the case of Patescibacteria and dieldrin. Obtained results substantially contribute to the still scarce knowledge of bacterial community diversity, activities, and ecology in freshwater sponges. PRACTITIONER POINTS: Microbial community associated with Spongilla lacustris is probably shaped by the occurrence of certain contaminants, mainly dieldrin and heavy metals. A higher accumulation of dieldrin in the sponge mesohyl tissues than in sediment was determined. S. lacustris is suggested as sponge species to be used as a sentinel of pesticide pollution in the Pasvik River. S. lacustris, living in tight contact with soft substrates, harbored communities more similar to sediment than water communities.

Metagenomic analysis revealed highly diverse carbon fixation microorganisms in a petroleum-hydrocarbon-contaminated aquifer

As one of the ultimate products of hydrocarbon biodegradation, inorganic carbon always be used to evaluate hydrocarbon biodegradation rates in petroleum-hydrocarbon-contaminated (PHC) aquifers. The evaluation method was challenged because of the existence of carbon fixation microorganisms, which may uptake inorganic carbons and consequently cause the biodegradation rates to be underestimated. We wonder if there are carbon fixation microorganisms in PHC aquifers. Although an extremely limited number of carbon fixation microorganisms in PHC sites have been studied in previous studies, the vast majority of microorganisms that participate in carbon fixation have not been systematically identified. To systematically reveal carbon fixation microorganisms and their survival environmental conditions, high-throughput metagenomic sequencing technologies, which are characterized by culture-independent, unbiased, and comprehensive methods for the detection and taxonomic characterization of microorganisms, were introduced to analyze the groundwater samples collected from a PHC aquifer. Results showed that 1041 genera were annotated as carbon fixation microorganisms, which accounted for 49% of the total number of genera in the PHC aquifer. Carbon fixation genes involved in Calvin-Benson-Bassham (CBB), 3-hydroxy propionate (3HP), reductive tricarboxylic acid (rTCA), and Wood-Ljungdahl (WL) cycles accounted for 2%, 41%, 34%, and 23% of the total carbon fixation genes, respectively, and 3HP, rTCA, and WL can be deemed as the dominant carbon fixation pathways. Most of the identified carbon fixation microorganisms are potential hydrocarbon biodegraders, and the most abundant carbon fixation microorganisms, such as Microbacterium, Novosphingobium, and Reyranella, were just the most abundant microorganisms in the aquifer system. It's deduced that most of the microorganisms in the aquifer were facultative autotrophic, and undertaking the dual responsibilities of degrading hydrocarbons to inorganic carbon and uptaking inorganic carbon to biomass.

Inhibition of quorum sensing serves as an effective strategy to mitigate the risks of human bacterial pathogens in soil

The coexistence of antibiotic resistance genes (ARGs), mobile genetic elements (MGEs), and virulence factor genes (VFGs) in human bacterial pathogens (HBPs) increases their risks to ecological security and human health and no effective strategy is available. Herein, we demonstrated two typical quorum sensing (QS) interfering agents, 4-nitropyridine-N-oxide (4-NPO, a QS inhibitor) and Acylase Ⅰ (a quorum quenching (QQ) enzyme), effectively decreased the abundance of HBPs by 48.30% and 72.54%, respectively, which was accompanied by the reduction of VFGs, ARGs, and MGEs. The decrease in QS signals mediated by QS interfering agents disturbed bacterial communication and inhibited biofilm formation. More importantly, QS interfering agents reduced the intra-species and inter-species conjugation frequencies among bacteria, considerably inhibiting the dissemination of ARGs and VFGs via horizontal gene transfer. Furthermore, the QS interfering agents did not significantly affect the metabolic function of other nonpathogenic microorganisms in the soil. Collectively, our study provides an effective and eco-friendly strategy to mitigate the risks of HBPs in soil.

Study of the biofilm mechanism of C4-HSL and C6-HSL in the degradation of quinoline

Quinoline is a hard-to-degrade organic compound widely found in coal chemical wastewater, that seriously affects the ecological environment and human health. A number of biochemical methods are already available for quinoline degradation, but the use of microbial community sensing for quinoline degradation has not been studied in depth. Therefore, this paper focuses on the enhanced mechanism of quorum-sensing signaling molecules in the biofilm formation process during quinoline degradation by functional strains of bacteria. In this paper, the effects of the signal molecules C4-HSL and C6-HSL on the adhesion ability, colony diameter, biofilm formation ability and biofilm morphology of functional strains of quinoline degrading bacteria (Ochrobactrum sp., LC-1) were investigated, and the results showed that both signal molecules promoted the biofilm formation process during the degradation of quinoline by exhibiting an efficient biofortification effect. Both signal molecules could enhance the colony diameter of strain LC-1, where C4-HSL could enhance the biomass of strain LC-1 and stimulate the secretion of extracellular polysaccharides; and C6-HSL could induce the enhancement of adhesion performance and the secretion of extracellular proteins from strain LC-1; both molecules together enhanced the biofilm formation process of strain LC-1. This study has practical application in the degradation of quinoline in coal chemical wastewater.

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.

N-acyl homoserine lactone mediating initial adhesion of microalgal biofilm formation

While pioneering methods have demonstrated that bacterial N-acyl homoserine lactone (AHL) signaling molecules can influence the growth and self-aggregation of suspended microalgae, whether AHLs can affect the initial adhesion to a carrier has remained an open question. Here we revealed that the microalgae exhibited different adhesion potential under AHL mediation, where the performance was affiliated to both AHL types and concentrations. The result can be well explained by the interaction energy theory, where the energy barrier between the carriers and the cells varied due to AHL mediation. Depth analyses revealed that AHL acted through modifying the properties of the surface electron donor of the cells, which were dependent upon three major components, i.e., extracellular protein (PN) secretion, the PN secondary structure, and the PN amino acid composition. These findings expand the known diversity of AHLs mediation on microalgal initial adhesion and metabolisms, which may interface with other major cycles and become helpful to theoretically guide the application of AHLs in microalgal culture and harvesting.

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.

A peptide of PilZ domain-containing protein controls wastewater-treatment-membrane biofouling by inducing bacterial attachment

Membrane-based wastewater reclamation is used to mitigate water scarcity; however, irreversible biofouling is an elusive problem that hinders the efficiency of a forward-osmosis (FO) membrane-based process, and the protein responsible for fouling is unknown. Herein, we identified fouling proteins by analyzing the microbiome and proteome of wastewater extracellular polymeric substances responsible for strong irreversible FO-membrane fouling. The IGLSSLPR peptide of a PilZ domain-containing protein was found to recruit bacterial attachment when immobilized on the membrane surface while suppressing it when dissolved, in a similar manner to the Arg-Gly-Asp (RGD) peptide in mammalian cell cultures. Bacteria adhere to IGLSSLPR and poly-l-lysine-coated membranes with similar energies and exhibit water fluxes that decline similarly, which is ascribable to interaction as strong as electrostatic interactions in the peptide-coated membranes. We conclude that IGLSSLPR is the key domain responsible for membrane fouling and can be used to develop antifouling technology against bacteria, which is similar to the current usage of RGD peptide in mammalian cell cultures.

Hierarchical stringent response behaviors of activated sludge system to stressed conditions

The stringent response of activated sludge systems to either stressed or harmful environments is important for the stable operation of activated sludge, which is examined by taking copper ion (Cu²⁺) as a stress model in this study. When weak stress was employed (Cu²⁺ ≤ 2.5 mg/L), the N-acyl-homoserine lactones (AHLs) of C6-, C8-, and C10-HSL increased by 30 %, 13 %, and 127 %, respectively, while the redox sensor green (RSG) intensity decreased by 28 %. Encountering the increased stress (2.5 mg/L < Cu²⁺ ≤ 5 mg/L), bacteria concentration in the supernatant increased by 87 %. However, the respiration rates of autotrophic and heterotrophic bacteria (SOUR_a and SOUR_h) and adenosine triphosphate decreased by 52 %, 18 %, and 27 %, respectively, and the flocs disintegrated with a diameter decreasing from 57 to 51 μm. When the stress became more serious (Cu²⁺ > 5 mg/L), the respiration rates continued to decline, but the quasi-endogenous respiration ratio (R_q/t) increased from 31 % to 47 %. Negligible changes occurred in the endogenous respiration rate (SOUR_e), adenosine diphosphate, and adenosine monophosphate. Based on these results, a hierarchical stringent response model of the activated sludge system to stressed conditions was proposed, and these responses were evaluated by respirogram. The initial response to weak stress was related to the most sensitive signals of quorum sensing and RSG intensity, well described by the quasi-endogenous respiration rate. The adaptive response to increased stress was the proactive migrations of low- and high-nucleic-acid bacteria to the supernatant, causing the looseness and even disintegration of sludge flocs, well described by SOUR_a, SOUR_h, and R_q/t. The lethal response to lethal stress was related to endogenous metabolic processes, well described by SOUR_e. This work provides new insights into understanding the stringent response of activated sludge systems to some stressed conditions. It helps to regulate the stability of activated sludge systems with respirogram technology.

Insights to Fe(II) on the fate of humic acid and humic acid Fe complex with biogeobattery effect in simultaneous partial nitritation, anammox and denitrification (SNAD) system

The role of Fe(II) on the humic acid (HA) transformation and the effects of humic acid Fe (HA-Fe) on simultaneous partial nitrification, anammox and denitrification (SNAD) system were investigated. After adding Fe(II), the HA content decreased and the HA inhibition on the SNAD system was released. Results showed that Fe(II) and HA formed the lower water-soluble HA-Fe, promoting the HA removal. HA-Fe with stronger electron transfer capacity constituted the interface with microorganisms to forming the biogeobattery effect. This accelerated the microbial electron transfer, as well as improved the key enzymes and ATP, indicating that HA-Fe stimulated the microbial activity of the SNAD system. Microbial community and quorum sensing analysis further demonstrated that HA-Fe enhanced the mutual symbiosis between electroactive and nitrogen removal bacteria, to ensure the stability of the SNAD system. The study provided references for efficient HA removal and revealed the biogeobattery effect of HA-Fe in the SNAD system.

Sensitive or tolerant functional microorganisms under cadmium stress: suggesting potential specific interaction network characteristics in the rhizosphere system of karst potato

The heavy metal cadmium (Cd) pollution in Chinese karst soils threatens food security, and microorganisms play an important role in regulating the migration and transformation of Cd in the soil-plant system. Nevertheless, the interaction characteristics between key microbial communities and environmental factors in response to Cd stress in specific crop environmental systems need to be explored. In this study, the soil (ferralsols)-microbe-crop (potato) system was taken as the object to explore the potato rhizosphere microbiome, using toxicology and molecular biology approaches, to explore the potato rhizosphere soil properties, microbial stress characteristics, and important microbial taxa under Cd stress. We hypothesized that different members of fungal and bacterial microbiome would regulate the resilience of potato rhizosphere and plants to Cd stress in the soil environment. Meanwhile, individual taxa will have different roles in the contaminated rhizosphere ecosystem. We found that soil pH was the main environmental factor affecting fungal community structure; urea-decomposing and nitrate-reducing functional bacteria as well as endosymbiotic and saprophytic functional fungi gradually decreased. In particular, Basidiomycota may play a key role in preventing the migration of Cd from the soil to plants (potato). These findings provide important candidates for screening the cascade of Cd inhibition (detoxification/regulation) from soil to microorganisms to plants. Our work provides an important foundation and research insights for the application of microbial remediation technology in the karst cadmium-contaminated farmland.

Effects of biochar addition towards the treatment of blackwater in anaerobic dynamic membrane bioreactor (AnDMBR): Comparison among room temperature, mesophilic and thermophilic conditions

Effects of biochar addition on the performance of anaerobic dynamic membrane bioreactor (AnDMBR) under different temperatures for blackwater treatment was investigated. When the organic load ratios (OLR) was 1.0 g COD/L·d, the specific methane yield for the three biochar-amended reactors were 125.7, 148.0 and 182.3 mLCH₄/g COD_removed, respectively. Compare to those digesters without biochar participation, the methane production in the thermophilic reactor with biochar increased by 12% while the other two digesters increased by 6-10%. An analysis of membrane filtration resistance showed a reduction in total resistance (R_t) of 6.2 × 10¹¹-7.3 × 10¹¹ m^-1 when biochar was added to the three reactors. The thermophilic reactors with biochar increased the relative abundance of Methanothermobacter and promoted gene expression of metabolic pathways related to hydrolysis, acid production and methane production. Overall, biochar showed great potential as an inexpensive conductive material to increase methane production with reduced membrane fouling in AnDMBR systems.

Diversity of Bacteria with Quorum Sensing and Quenching Activities from Hydrothermal Vents in the Okinawa Trough

Quorum sensing (QS) is a chemical communication system by which bacteria coordinate gene expression and social behaviors. Quorum quenching (QQ) refers to processes of inhibiting the QS pathway. Deep-sea hydrothermal vents are extreme marine environments, where abundant and diverse microbial communities live. However, the nature of chemical communication in bacteria inhabiting the hydrothermal vent is poorly understood. In this study, the QS and QQ activities with N-acyl homoserine lactones (AHLs) as the autoinducer were detected in bacteria isolated from hydrothermal vents in the Okinawa Trough. A total of 18 and 108 isolates possessed AHL-producing and AHL-degrading abilities, respectively. Bacteria mainly affiliated with Rhodobacterales, Hyphomicrobiales, Enterobacterales and Sphingomonadales showed QS activities; QQ was mainly associated with Bacillales, Rhodospirillales and Sphingomonadales. The results showed that the bacterial QS and QQ processes are prevalent in hydrothermal environments in the Okinawa Trough. Furthermore, QS significantly affected the activities of extracellular enzymes represented by β-glucosidase, aminopeptidase and phosphatase in the four isolates with higher QS activities. Our results increase the current knowledge of the diversity of QS and QQ bacteria in extreme marine environments and shed light on the interspecific relationships to better investigate their dynamics and ecological roles in biogeochemical cycling.

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

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

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

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

The Role of Quorum Sensing Molecules in Bacterial-Plant Interactions

Quorum sensing (QS) is a system of communication of bacterial cells by means of chemical signals called autoinducers, which modulate the behavior of entire populations of Gram-negative and Gram-positive bacteria. Three classes of signaling molecules have been recognized, Al-1, Al-2, Al-3, whose functions are slightly different. However, the phenomenon of quorum sensing is not only concerned with the interactions between bacteria, but the whole spectrum of interspecies interactions. A growing number of research results confirm the important role of QS molecules in the growth stimulation and defense responses in plants. Although many of the details concerning the signaling metabolites of the rhizosphere microflora and plant host are still unknown, Al-1 compounds should be considered as important components of bacterial-plant interactions, leading to the stimulation of plant growth and the biological control of phytopathogens. The use of class 1 autoinducers in plants to induce beneficial activity may be a practical solution to improve plant productivity under field conditions. In addition, researchers are also interested in tools that offer the possibility of regulating the activity of autoinducers by means of degrading enzymes or specific inhibitors (QSI). Current knowledge of QS and QSI provides an excellent foundation for the application of research to biopreparations in agriculture, containing a consortia of AHL-producing bacteria and QS inhibitors and limiting the growth of phytopathogenic organisms.

Biomineralization and AHLs-guided quorum sensing enhanced phosphorus recovery in the alternating aerobic/anaerobic biofilm system under metal ion stress

The alternating aerobic/anaerobic biofilm system had been applied for phosphorus (P) enrichment and recovery because of the advantage of low energy consumption and high efficiency. The metal ions and N-acyl-L-homoserine lactones (AHLs) in system were studied to better clarify the mechanism of P uptake/release under metal ion stress. The results indicated that the increase of metal ions stimulated the release of AHLs, and AHLs-guided quorum sensing (QS) enhanced P uptake. Moreover, biomineralization could stimulate the increase of P content in biofilm (P_biofilm). Meanwhile, some ortho-p was converted to short-chain poly-p in extracellular polymer substance (EPS), and others were transferred into cell through EPS to synthesize poly-p. With the P_biofilm increased, more P could be absorbed/released due to the shift in the metabolic model of polyphosphate accumulating organisms (PAOs). The release of AHLs between microorganisms was also inhibited when PAOs reached the state of P saturation (75.6 ± 2.5 mg/g SS), which meant that the effect of signaling function would tend to stabilize, and the 169.2 ± 2.6 mg/L P concentration in the enriched solution was obtained due to the P release was inhibited. Moreover, P was rapidly transferred to the new enriched solution after the P was recovered, and PAOs restored its capability of P uptake/release. In addition, ³¹P-NMR analysis demonstrated that EPS played a major role in PAOs compared to cell, and inorganic phosphorus (IP) played an essential role in the uptake/release of P compared to organic phosphorus (OP). Furthermore, the microbiological analysis showed that Candidatus Accumulibacter was positively correlated with AHLs (P < 0.05). This study provided essential support for clarifying the P metabolism mechanism of PAOs.

Wastewater treatment with nanomaterials for the future: A state-of-the-art review

Aquatic and terrestrial ecosystems are both threatened by toxic wastewater. The unique properties of nanomaterials are currently being studied thoroughly for treating sewage. Nanomaterials also have the advantage of being capable of removing organic matter, fungi, and viruses from wastewater. Advanced oxidation processes are used in nanomaterials to treat wastewater. Additionally, nanomaterials have a large effective area of contact due to their tiny dimensions. The adsorption and reactivity of nanomaterials are strong. Wastewater treatment would benefit from the development of nanomaterial technology. Second, the paper provides a comprehensive analysis of the unique characteristics of nanomaterials in wastewater treatment, their proper use, and their prospects. In addition to focusing on their economic feasibility, since limited forms of nanomaterials have been manufactured, it is also necessary to consider their feasibility in terms of their technical results. According to this study, the significant adsorption area, excellent chemical reaction, and electrical conductivity of nanoparticles (NPs) contribute to the successful treatment of wastewater.

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.

Molecular Mechanisms and Applications of N-Acyl Homoserine Lactone-Mediated Quorum Sensing in Bacteria

Microbial biodiversity includes biotic and abiotic components that support all life forms by adapting to environmental conditions. Climate change, pollution, human activity, and natural calamities affect microbial biodiversity. Microbes have diverse growth conditions, physiology, and metabolism. Bacteria use signaling systems such as quorum sensing (QS) to regulate cellular interactions via small chemical signaling molecules which also help with adaptation under undesirable survival conditions. Proteobacteria use acyl-homoserine lactone (AHL) molecules as autoinducers to sense population density and modulate gene expression. The LuxI-type enzymes synthesize AHL molecules, while the LuxR-type proteins (AHL transcriptional regulators) bind to AHLs to regulate QS-dependent gene expression. Diverse AHLs have been identified, and the diversity extends to AHL synthases and AHL receptors. This review comprehensively explains the molecular diversity of AHL signaling components of Pseudomonas aeruginosa, Chromobacterium violaceum, Agrobacterium tumefaciens, and Escherichia coli. The regulatory mechanism of AHL signaling is also highlighted in this review, which adds to the current understanding of AHL signaling in Gram-negative bacteria. We summarize molecular diversity among well-studied QS systems and recent advances in the role of QS proteins in bacterial cellular signaling pathways. This review describes AHL-dependent QS details in bacteria that can be employed to understand their features, improve environmental adaptation, and develop broad biomolecule-based biotechnological applications.

N-Acyl-Homoserine Lactone (AHL)-Mediated Microalgal-Bacterial Communication Driving Chlorella-Activated Sludge Bacterial Biofloc Formation

N-acyl-homoserine lactones (AHLs) as autoinducers of Gram-negative bacteria for quorum sensing regulation have shown positive effects on the production of aromatic proteins in extracellular polymeric substances (EPSs) during bioflocculation. To investigate the role of AHLs in aromatic protein production, a Chlorella-bacteria system with great bioflocculation was established via fed-batch cultivation. Tryptophan and aromatic proteins as the main compounds in the EPS of bioflocs showed an increasing trend during fed-batch cultivation. The Chlorella cells only secreted tryptophan rather than aromatic proteins during axenic cultivation. N-dodecanoyl-l-homoserine lactone (C12-HSL) was correlated with the flocculation activity and extracellular protein content of bioflocs during fed-batch cultivation. The addition of exogenous C12-HSL enhanced the flocculation activity of the Chlorella-bacteria system and aromatic protein production in the EPS. Chlorella cells sensed exogenous C12-HSL and significantly upregulated the aromatic protein synthesis pathway during axenic cultivation. In addition, vanillin as a quorum-sensing inhibitor suppressed the positive effect of C12-HSL on flocculation activity and aromatic protein production and synthesis. This result indicated that vanillin intercepts the response of Chlorella cells to C12-HSL. Overall, C12-HSL is supposed to be an important signal molecule to achieve communication between Chlorella and Gram-negative bacteria and subsequently induce Chlorella cells to produce aromatic proteins for biofloc formation.

Regulation of exogenous acyl homoserine lactones on sludge settling performance: Monitoring via ultrasonic time-domain reflectometry

Despite extensive studies, a comprehensive solution for sludge bulking has not yet been found. This study improves the sludge settling performance via quorum sensing (QS) by adding exogenous acyl homoserine lactones (AHLs). First, a novel approach based on ultrasonic time-domain reflectometry, which can automatically and in-situ assess a sludge volume index (SVI), was developed using the displacement in the ultrasonic spectra as a feasible indicator (R² = 0.98, p < 0.01). Next, the effects of typical AHLs, i.e., 3OC6-HSL, C12-HSL, and 3OC14-HSL, on sludge settling properties were investigated. Results indicated that the three AHLs significantly promoted the sludge settleability by 1.90, 2.03, and 1.62 times, respectively. The regulation mechanisms were investigated from the perspective of sludge physicochemical properties and biological community interactions. The draining degree of water to extracellular polymeric substances (EPS) significantly increased (p < 0.05) with all three AHLs. Meanwhile, the hydrophobic tryptophan content increased with the addition of 3OC6-HSL and C12-HSL. Hence, EPS hydrophobicity was promoted, which is conducive to microbial aggregation. In addition, molecular ecological networks of activated sludge (AS) indicated that bacterial community structures were more complex and species interactions were more intense when adding 3OC6-HSL and C12-HSL. Meanwhile, additional keystones were identified, with the proportion of QS species increasing by 63.6% and 22.2%, respectively. Exogenous 3OC6-HSL eventually decreased the gross relative abundance of filamentous bacteria by 2.37%. Overall, appropriate AHLs could enhance community stability and microbial cooperation by strengthening the communication hub role of QS species, thereby suppressing the overgrowth of filamentous bacteria and improving the sludge settleability. This study provides an effective strategy to determine the appropriate AHL to rapidly eliminate filamentous bulking problems.

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.

AHLS-pred: a novel sequence-based predictor of acyl-homoserine-lactone synthases using machine learning algorithms

Acyl-homoserine-lactones (AHLs), as the major quorum sensing (QS) signalling molecules in Gram-negative bacteria, have shown great application potential in regulating biological nutrient removal process. The identification of AHLs synthases plays an essential role in in-depth research on QS mechanisms and applications of biological wastewater treatment processes. This work proposed the first prediction model for AHLs synthases based on machine learning algorithms, namely, AHLS-pred. The training dataset AHLS1400 and the independent testing dataset AHLS132 for AHLSs prediction were first established. Three sequence-based feature extraction methods are utilized to generate feature descriptors, namely, amino acid composition, dipeptide composition and G-gap dipeptide composition respectively. Subsequently, the optimal features were obtained based on the sorted feature descriptors (in F-score order) and the sequential forward search strategy. By comparing five different machine learning algorithms, the final prediction model is trained with support vector machine classifier on AHLS1400 in fivefold cross-validation with the best performance (ACC = 99.43%, MCC = 0.989, AUC = 0.997). The results show that AHLS-pred achieves an ACC of 94.70%, MCC of 0.894 and AUC of 0.995 on the independent testing dataset AHLS132. It demonstrates that AHLS-pred is a promising and powerful prediction method for accelerating the process of AHLSs computational identification.

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

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

Benthic Microbial Communities in a Seasonally Ice-Covered Sub-Arctic River (Pasvik River, Norway) Are Shaped by Site-Specific Environmental Conditions

The Pasvik River experiences chemical, physical, and biological stressors due to the direct discharges of domestic sewage from settlements located within the catchment and runoff from smelter and mine wastes. Sediments, as a natural repository of organic matter and associated contaminants, are of global concern for the possible release of pollutants in the water column, with detrimental effects on aquatic organisms. The present study was aimed at characterizing the riverine benthic microbial community and evaluating its ecological role in relation to the contamination level. Sediments were sampled along the river during two contrasting environmental periods (i.e., beginning and ongoing phases of ice melting). Microbial enzymatic activities, cell abundance, and morphological traits were evaluated, along with the phylogenetic community composition. Amplified 16S rRNA genes from bacteria were sequenced using a next-generation approach. Sediments were also analyzed for a variety of chemical features, namely particulate material characteristics and concentration of polychlorobiphenyls, polycyclic aromatic hydrocarbons, and pesticides. Riverine and brackish sites did not affect the microbial community in terms of main phylogenetic diversity (at phylum level), morphometry, enzymatic activities, and abundance. Instead, bacterial diversity in the river sediments appeared to be influenced by the micro-niche conditions, with differences in the relative abundance of selected taxa. In particular, our results highlighted the occurrence of bacterial taxa directly involved in the C, Fe, and N cycles, as well as in the degradation of organic pollutants and toxic compounds.

N-acyl-homoserine lactones in extracellular polymeric substances from sludge for enhanced chloramphenicol-degrading anode biofilm formation in microbial fuel cells

Exploring an efficient acclimation strategy to obtain robust bioanodes is of practical significance for antibiotic wastewater treatment by bioelectrochemical systems (BESs). This study investigated the effects of two acclimation conditions on chloramphenicol (CAP)-degrading anode biofilm formation in microbial fuel cells (MFCs). The one was continuously added the extracellular polymeric substances (EPS) extracted from anaerobic sludge and increasing concentrations of CAP after the first start-up phase, while the other was added the EPS-1 (N-acyl-homoserine lactones, namely AHLs were extracted from the EPS) at the same conditions. The results demonstrated that AHLs in the sludge EPS played a crucial role for enhanced CAP-degrading anode biofilm formation in MFCs. The AHL-regulation could not only maintain stable voltage outputs but also significantly accelerate CAP removal in the EPS MFC. The maximum voltage of 653.83 mV and CAP removal rate of 1.21 ± 0.05 mg/L·h were attained from the EPS MFC at 30 mg/L of CAP, which were 0.84 and 1.57 times higher than those from the EPS-1 MFC, respectively. These improvements were largely caused by the thick and 3D structured biofilm, strong and homogeneous cell viability throughout the biofilm, and high protein/polysaccharide ratio along with more conductive contents in the biofilm EPS. Additionally, AHLs facilitated the formation of a biofilm with rich biodiversity and balanced bacterial proportions, leading to more beneficial mutualism among different functional bacteria. More bi-functional bacteria (for electricity generation and antibiotic resistance/degradation) were specifically enriched by AHLs as well. These findings provide quorum sensing theoretical knowledge and practical instruction for rapid antibiotic-degrading electrode biofilm acclimation in BESs.

Rapid recovery of mixed culture polyhydroxyalkanoate production system from EPS bulking using azithromycin

The long-term stable operation of the mixed culture polyhydroxyalkanoate (PHA) enrichment stage is the guarantee for the continuous synthesis of PHA, however extracellular polymeric substances (EPS) sludge bulking occurred from time to time may cause the operation fail. In order to solve this problem, as a quencher of signal molecules and antibiotic, azithromycin (AZM) was used in the two systems with different modes to recover the sedimentation capacity of the sludge. The results showed that AZM addition resulted in the reduction of polysaccharide /protein (PS/PN) ratio in EPS and significant improvement of the sedimentation capacity of the sludge. Quorum quenching of AZM or aiiA gene maintained the sedimentation ability of the sludge in a relay mode. By adding AZM, the growth of Thauera and Flavobacterium, which caused sludge bulking, was inhibited. Paracoccus, a strong PHA producer, has been enriched to ensure that the maximum PHA synthesis of the system.

Phosphorus deficiency leads to the loosening of activated sludge: The role of exopolysaccharides in aggregation

Whether phosphorus deficiency in influent will affect the aggregation and sedimentation of activated sludge needs to be further clarified. This paper systematically studied the structure, aggregation and settlement of activated sludge, and the composition, properties and chemical structure of extracellular polymers and microbial community structure of sludge under different influent phosphorus contents to determine the causes of sludge aggregation and structural deterioration. The results show that phosphorus deficiency in influent leads to a decrease in the aggregation capacity and a loose structure of activated sludge, and the reduction of hydrophobic interactions is the main factor of sludge aggregation and structural deterioration. The content, functional groups and protein secondary structure of extracellular protein were almost unchanged. An increase in the content and hydrophilicity of extracellular polysaccharide (PS) results in a decrease in sludge hydrophobicity. Under phosphorus deficiency, the changes in microbial species related to PS secretion were the reasons for the increase in PS content and hydrophilicity. The negative effects of PS content and hydrophilicity on sludge aggregation and structure are important findings of this work and are expected to be useful for better understanding the restoration of activated sludge performance in the treatment of phosphorus-deficient wastewater.

Similarities and Differences in Quorum Sensing-Controlled Bioluminescence between Photobacterium phosphoreum T3 and Vibrio qinghaiensis sp.-Q67

Quorum sensing is a density-dependent mechanism using chemical signal molecules termed autoinducers to regulate diverse biological processes in bacteria, including bioluminescence. However, the correlation between growth and light emission of two typical luminescent bacteria, Photobacterium phosphoreum T3 and Vibrio qinghaiensis sp.-Q67, is still unclear. This study investigates the variations of bioluminescence and the light-emission-involved gene expression of the above two strains, respectively, showing that bioluminescence is population density-dependent. Furthermore, the effect of crude extracts (175, 350, 700 and 1750 mg/L) from the bacterial culture that contains the potential autoinducers on the bioluminescence is explored. At the exponential and the early stationary growth phase, T3 did not exhibit an obvious light intensity and cell density change after adding crude extracts at 175 and 350 mg/L, while the light intensity decreased at 700 and 1750 mg/L, showing a luminescence inhibition. For Q67, the light intensity increased dramatically with crude extract concentration. These results suggest that the bioluminescence process of both T3 and Q67 is controlled by quorum sensing. Furthermore, the different response modes of these two strains to autoinducers imply that the two strains could be applied to different compounds for toxicity assesses.

Colonization of biofilm in wastewater treatment: A review

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

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

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

Acylase enzymes disrupting quorum sensing alter the transcriptome and phenotype of Pseudomonas aeruginosa, and the composition of bacterial biofilms from wastewater treatment plants

Biofilms represent an essential way of life and colonization of new environments for microorganisms. This feature is regulated by quorum sensing (QS), a microbial communication system based on autoinducer molecules, such as N-acyl-homoserine lactones (AHLs) in Gram negative bacteria. In artificial ecosystems, like Wastewater Treatment Plants (WWTPs), biofilm attachment in filtration membranes produces biofouling. In this environment, the microbial communities are mostly composed of Gram-negative phyla. Thus, we used two AHLs-degrading enzymes, obtained from Actinoplanes utahensis (namely AuAAC and AuAHLA) to determine the effects of degradation of QS signals in the biofilm formation, among other virulence factors, of a Pseudomonas aeruginosa strain isolated from a WWTP, assessing molecular mechanisms through transcriptomics. Besides, we studied the possible effects on community composition in biofilms from activated sludge samples. Although the studied enzymes only degraded the AHLs involved in one of the four QS systems of P. aeruginosa, these activities produced the deregulation of the complete QS network. In fact, AuAAC -the enzyme with higher catalytic efficiency- deregulated all the four QS systems. However, both enzymes reduced the biofilm formation and pyocyanin and protease production. The transcriptomic response of P. aeruginosa affected QS related genes, moreover, transcriptomic response to AuAAC affected mainly to QS related genes. Regarding community composition of biofilms, as expected, the abundance of Gram-negative phyla was significantly decreased after enzymatic treatment. These results support the potential use of such AHLs-degrading enzymes as a method to reduce biofilm formation in WWTP membranes and ameliorate bacterial virulence.

Beneficial biofilms: A mini-review of strategies to enhance biofilm formation for biotechnological applications

The capacity of bacteria to form biofilms is an important trait for their survival and persistence. Biofilms occur naturally in soil and aquatic environments, are associated with animals ranging from insects to humans and are also found in built environments. They are typically encountered as a challenge in healthcare, food industry, and water supply ecosystems. In contrast, they are known to play a key role in the industrial production of commercially valuable products, environmental remediation processes, and in microbe-catalysed electrochemical systems for energy and resource recovery from wastewater. While there are many recent articles on biofilm control and removal, review articles on promoting biofilm growth for biotechnological applications are unavailable. Biofilm formation is a tightly regulated response to perturbations in the external environment. The multi-stage process, mediated by an assortment of proteins and signaling systems, involves the attachment of bacterial cells to a surface followed by their aggregation in a matrix of extracellular polymeric substances. Biofilms can be promoted by altering the external environment in a controlled manner, supplying molecules that trigger the aggregation of cells and engineering genes associated with biofilm development. This mini-review synthesizes findings from studies that have described such strategies and highlights areas needing research attention.

Facilitating biofilm formation of Pseudomonas aeruginosa via exogenous N-Acy-L-homoserine lactones stimulation: Regulation on the bacterial motility, adhesive ability and metabolic activity

The N-Acy-L-homoserine lactones (AHLs) mediated quorum sensing (QS) system exhibited important ecological significance in bacterial biofilm formation. However, the previous studies mainly focused on indigenous AHLs while the role of exogenous AHLs has remained unclear. This study evaluated the roles of exogenous AHLs on the biofilm formation of Pseudomonas aeruginosa. Both the C6-HSL and C8-HSL promoted the biofilm formation of P. aeruginosa with an enhancement of 2.47 and 1.88 times, respectively. Further analysis showed that exogenous AHLs contributed greatly to the adhesive ability instead of growth rate. Also, the bacterial motility and metabolic activities were significantly improved by AHLs. Moreover, the microbial functional genes (i.e. lasI, lasR, rhlI and rhlR) involved in regulating the biofilm formation were highly expressed in AHLs reactors. These findings expanded the knowledge of AHLs functions in mediating biofilm formation, and provided insightful guidance on the biofilm regulation in the wastewater treatment via biofilm technology.

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.