Development of an extraction method and LC–MS analysis for N-acylated-l-homoserine lactones (AHLs) in wastewater treatment biofilms

Development of an analytical method to quantify N-acyl-homoserine lactones in bacterial cultures, river water, and treated wastewater

N-Acyl-homoserine lactones (AHL) play a major role in the communication of Gram-negative bacteria. They influence processes such as biofilm formation, swarming motility, and bioluminescence in the aquatic environment. A comprehensive analytical method was developed to elucidate the "chemical communication" in pure bacterial cultures as well as in the aquatic environment and engineered environments with biofilms. Due to the high diversity of AHLs and their low concentrations in water, a sensitive and selective LC-ESI-MS/MS method combined with solid-phase extraction was developed for 34 AHLs, optimized and validated to quantify AHLs in bacterial conditioned medium, river water, and treated wastewater. Furthermore, the developed method was optimized in terms of enrichment volume, internal standards, limits of detection, and limits of quantification in several matrices. An unanticipated variety of AHLs was detected in the culture media of Pseudomonas aeruginosa (in total 8 AHLs), Phaeobacter gallaeciensis (in total 6 AHLs), and Methylobacterium mesophilicum (in total 15 AHLs), which to our knowledge have not been described for these bacterial cultures so far. Furthermore, AHLs were detected in river water (in total 5 AHLs) and treated wastewater (in total 3 AHLs). Several detected AHLs were quantified (in total 24) using a standard addition method up to 7.3±1.0 µg/L 3-Oxo-C12-AHL (culture media of P. aeruginosa).

Metabolic insights from mass spectrometry imaging of biofilms: A perspective from model microorganisms

Biofilms are dense aggregates of bacterial colonies embedded inside a self-produced polymeric matrix. Biofilms have received increasing attention in medical, industrial, and environmental settings due to their enhanced survival. Their characterization using microscopy techniques has revealed the presence of structural and cellular heterogeneity in many bacterial systems. However, these techniques provide limited chemical detail and lack information about the molecules important for bacterial communication and virulence. Mass spectrometry imaging (MSI) bridges the gap by generating spatial chemical information with unmatched chemical detail, making it an irreplaceable analytical platform in the multi-modal imaging of biofilms. In the last two decades, over 30 species of biofilm-forming bacteria have been studied using MSI in different environments. The literature conveys both analytical advancements and an improved understanding of the effects of environmental variables such as host surface characteristics, antibiotics, and other species of microorganisms on biofilms. This review summarizes the insights from frequently studied model microorganisms. We share a detailed list of organism-wide metabolites, commonly observed mass spectral adducts, culture conditions, strains of bacteria, substrate, broad problem definition, and details of the MS instrumentation, such as ionization sources and matrix, to facilitate future studies. We also compared the spatial characteristics of the secretome under different study designs to highlight changes because of various environmental influences. In addition, we highlight the current limitations of MSI in relation to biofilm characterization to enable cross-comparison between experiments. Overall, MSI has emerged to become an important approach for the spatial/chemical characterization of bacterial biofilms and its use will continue to grow as MSI becomes more accessible.

A FadR-Type Regulator Activates the Biodegradation of Polycyclic Aromatic Hydrocarbons by Mediating Quorum Sensing in Croceicoccus naphthovorans Strain PQ-2

Bacteria employ multiple transcriptional regulators to orchestrate cellular responses to adapt to constantly varying environments. The bacterial biodegradation of polycyclic aromatic hydrocarbons (PAHs) has been extensively described, and yet, the PAH-related transcriptional regulators remain elusive. In this report, we identified an FadR-type transcriptional regulator that controls phenanthrene biodegradation in Croceicoccus naphthovorans strain PQ-2. The expression of fadR in C. naphthovorans PQ-2 was induced by phenanthrene, and its deletion significantly impaired both the biodegradation of phenanthrene and the synthesis of acyl-homoserine lactones (AHLs). In the fadR deletion strain, the biodegradation of phenanthrene could be recovered by supplying either AHLs or fatty acids. Notably, FadR simultaneously activated the fatty acid biosynthesis pathway and repressed the fatty acid degradation pathway. As intracellular AHLs are synthesized with fatty acids as substrates, boosting the fatty acid supply could enhance AHL synthesis. Collectively, these findings demonstrate that FadR in C. naphthovorans PQ-2 positively regulates PAH biodegradation by controlling the formation of AHLs, which is mediated by the metabolism of fatty acids. IMPORTANCE Master transcriptional regulation of carbon catabolites is extremely important for the survival of bacteria that face changes in carbon sources. Polycyclic aromatic hydrocarbons (PAHs) can be utilized as carbon sources by some bacteria. FadR is a well-known transcriptional regulator involved in fatty acid metabolism; however, the connection between FadR regulation and PAH utilization in bacteria remains unknown. This study revealed that a FadR-type regulator in Croceicoccus naphthovorans PQ-2 stimulated PAH biodegradation by controlling the biosynthesis of the acyl-homoserine lactone quorum-sensing signals that belong to fatty acid-derived compounds. These results provide a unique perspective for understanding bacterial adaptation to PAH-containing environments.

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.

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

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

Comprehensive chiral GC-MS/MS and LC-MS/MS methods for identification and determination of N-acyl homoserine lactones

N-acyl homoserine lactones (N-HLs) are signaling molecules synthesized by gram-negative bacteria to communicate in a process called quorum sensing. Most reported methods for the analysis of N-HLs, which are chiral molecules, do not distinguish between enantiomers. Typical examples include biosensors, liquid chromatography with UV detection, gas chromatography coupled with a mass spectrometer (GC-MS) and liquid chromatography coupled with mass spectrometer (LC-MS). Recently, the production of both D,L-N-HLs have been reported in Vibrio fischeri and Burkholderia cepacia. Concentrations of the D-N-HLs were found at the limit of quantification for the employed method. Therefore, for further studies of the role of the D-N-HLs in bacterial physiology, more sensitive, reliable, and selective analytical methods are necessary. In this work, such comprehensive chiral analytical methods for the identification and determination of 18 N-HLs using solid phase extraction followed by GC-MS/MS and LC-MS/MS analyses were developed. Extraction recoveries for the more hydrophilic C4 N-HLs were <10% of all other N-HLs, thus offering a possible explanation as to their lack of detection in previous studies. The chiral separations of all 18 N-HLs derivatives were accomplished by the complementary GC-MS/MS and LC-MS/MS methods. The limit of detection for LC-MS/MS method was as low as 1 ppb. The limit of detection for the GC-MS/MS method was found to be one to three orders of magnitude higher than the LC-MS/MS method. Due to the high extraction recovery and a preconcentration factor of 100, concentrations as low as 10 ppt can be detected by LC-MS/MS in biological samples. The LC-MS/MS approach provided greater enantioselectivity for the larger, more hydrophobic N-HLs while GC-MS/MS provided better enantioselectivity for the smaller N-HLs.

Two strategies of stubborn biofouling strains surviving from NaClO membrane cleaning: EPS shielding and/or quorum sensing

The declined performance of repeated chemically-enhanced-backwashing (CEB) seriously hampered the sustainable operation of membrane bioreactor (MBR) in long-term, and could be partially attributed to the strengthened anti-cleaning properties of residual stubborn microbes. Although plenty of research has been done towards either the model strains or the whole post-CEB microbial community, little was known about the resisting behavior of practical stubborn strains when confronting oxidative stresses induced by NaClO. Hence, this study isolated 21 strains from samples in a large-scale MBR plant with routine CEB treatment. To unravel how they survive and affect membrane fouling, their anti-oxidation ability, fouling potential and quorum sensing (QS) effect before and after NaClO stimuli were evaluated. The composition and molecular weight distribution of extracellular polymeric substance (EPS) were also investigated to understand their roles during the anti-CEB process. It was found that typical stubborn strains tended to secrete more EPS as protective shields, where polysaccharides (especially the ones >1 kDa) made major contribution. However, sometimes EPS could not well resist the stimuli, with consequent low survival rate and high intracellular ROS level. Under such circumstances, stubborn strains would rather choose to be sensitive with surged QS level and quick population regrowth to maintain vitality under the oxidative stresses. Both strategies aggravated biofouling and eventually enhanced the anti-cleaning properties of biofilm.

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.

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.

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.

Impact of quorum sensing signaling molecules in gram-negative bacteria on host cells: current understanding and future perspectives

Quorum sensing is a molecular signaling-based communication mechanism in prokaryotes. In the basic mode, signaling molecules released by certain bacteria are sensed by intracellular receptors or membrane-bound receptors of other members in the community, leading to the collective isogenic signaling molecule synthesis and synchronized activities. This regulation is important for the symbiosis of the bacterium with the host, as well as virulence and biofilm formation. Notably, quorum sensing signaling molecules are not only able to control microbial community behavior but can likewise regulate the physiological status of host cells. Here, we provide a comprehensive review of the importance of quorum sensing signaling molecules in gram-negative bacteria in regulating host cell function and gut health, and suggest possible opportunities for application in combating human and animal diseases by blocking the pathways through which quorum sensing signaling molecules exert their functions.

C12-HSL is an across-boundary signal molecule that could alleviate fungi Galactomyces's filamentation: A new mechanism on activated sludge bulking

Fungal bulking is caused by fungi excessive growth and morphological changes, resulting from the evolution toward fungi dominant activated sludge. Communication across fungi and bacteria boundary that mediated by bacterial signal molecules (SMs) probably is the central induce caused fungal bulking occurrence. In this work, it intended to identify the bacterial SM that affected fungal bulking, and verified its roles in regulate the spore germination and hyphal growth. We found C12-HSL concentration decreased significantly from 12.36 to 3.38 ng/g-VSS (P < 0.05) when fungal sludge bulking happened, and filamentous Galactomyces's relatively abundant was correlatively enriched. To test the effects of this SM, trace commercial C12-HSL was added to pure cultured Galactomyces, in which spore germination rates decreased by 20 % and hyphal extension inhibited by 15 %. Ras1-cAMP-PKA and mitogen-activated protein kinase (MAPK) pathways of Galactomyces were responsible for signal C12-HSL transduction, which inhibited peroxisome biosynthesis, suppressed the biological activity of the actin cytoskeleton, and disrupted intercellular organelle transport. All these results showed C12-HSL was the functional SM that could suppress the development of fungal filamentous. This study provided a new insight into the sludge bulking mechanism from view of cross-kingdom communication.

Straightforward N-Acyl Homoserine Lactone Discovery and Annotation by LC-MS/MS-based Molecular Networking

N-Acyl-l-homoserine lactones (AHLs) are a large family of signaling molecules in "quorum sensing" communication. This mechanism is present in a number of bacterial physiological phenomena, including pathogenic phenomena. In this study, we described a simple and accessible way to detect, annotate, and quantify these compounds from bacterial culture media. Analytical standards and ethyl acetate bacterial extracts containing AHLs were analyzed by an ultra-high-performance liquid chromatography system coupled to a mass spectrometer using a nontargeted FullMS data-dependent MS² method. The results were processed in MZmine2 and then analyzed by a Feature-Based Molecular Networking (FBMN) workflow in the Global Natural Products Social Networking (GNPS) platform for the discovery and annotation of known and unknown AHLs. Our group analyzed 31 AHL standards and included the MS² spectra in the spectral library of the GNPS platform. We also provide the 31 standard AHL spectrum list for inclusion in molecular networking analyses. FBMN analysis annotated 30 out of 31 standards correctly. Then, as an example, a set of five bacterial extracts was prepared for AHL annotation. Following the method described in this Article, 5 known and 11 unknown AHLs were properly annotated using the FBMN-based molecular network approach. This study offers the possibility for the automatic annotation of known AHLs and the search for nonreferenced AHLs in bacterial extracts in a somewhat straightforward approach even without acquiring analytical standards. The method also provides relative quantification information.

Bacterial Lighthouses—Real-Time Detection of Yersinia enterocolitica by Quorum Sensing

Foodborne zoonotic pathogens have a severe impact on food safety. The demand for animal-based food products (meat, milk, and eggs) is increasing, and therefore faster methods are necessary to detect infected animals or contaminated food before products enter the market. However, conventional detection is based on time-consuming microbial cultivation methods. Here, the establishment of a quorum sensing-based method for detection of foodborne pathogens as Yersinia enterocolitica in a co-cultivation approach using a bacterial biosensor carrying a special sensor plasmid is described. We combined selective enrichment with the simultaneous detection of pathogens by recording autoinducer-1-induced bioluminescent response of the biosensor. This new approach enables real-time detection with a calculated sensitivity of one initial cell in a sample after 15.3 h of co-cultivation, while higher levels of initial contamination can be detected within less than half of the time. Our new method is substantially faster than conventional microbial cultivation and should be transferrable to other zoonotic foodborne pathogens. As we could demonstrate, quorum sensing is a promising platform for the development of sensitive assays in the area of food quality, safety, and hygiene.

Biological mechanism of alleviating membrane biofouling by porous spherical carriers in a submerged membrane bioreactor

In this study, porous spherical carriers were fixed around the hollow fiber membrane module to mitigate membrane biofouling. Two MBRs (R1 without carriers, R2 with carriers) were operated for 31 days under identical operating conditions to investigate the effects of the carriers on the reactor performances, the production of extracellular polymeric substances (EPS), the level of N-acyl-homoserine lactones (AHLs), and the microbial communities. The results showed that the presence of carriers in MBR was conducive to nitrogen removal and decreased the total membrane filtration resistance by about 1.7 times. Slower transmembrane pressure (TMP) rise-up, thinner bio-cakes, lower EPS production, and fewer tryptophan and aromatic proteins substances on the membrane surface were observed in R2. The polysaccharides secretion of EPS in bio-cakes was mainly regulated by C4-HSL and 3OC6-HSL in the presence of carriers. The microbial community analysis revealed that carriers addition reduced the relative abundance of EPS and AHL producing bacteria in the membrane bio-cakes and enriched the accumulation of functional bacteria conducive to nutrient removal in the mixed liquor. This study provided an in-depth understanding for the application of porous spherical carriers to alleviate membrane biofouling.

N-acyl-homoserine-lactones signaling as a critical control point for phosphorus entrapment by multi-species microbial aggregates

Quorum sensing (QS) has been extensively studied in pure stains of microorganisms, but the ecological roles of QS in multi-species microbial aggregates are poorly understood due to the aggregates' heterogeneity and complexity, in particular the phosphorus (P) entrapment, a key aspect of element cycling. Using periphytic biofilm as a microbial-aggregate model, we addressed how QS signaling via N-acyl-homoserine-lactones (AHLs) regulated P entrapment. The most-abundant AHLs detected were C8-HSL, 3OC8-HSL, and C12-HSL, are the primary regulator of P entrapment in the periphytic biofilm. QS signaling-AHL is a beneficial molecule for bacterial growth in periphytic biofilm and the addition of these three AHLs optimized polyphosphate accumulating organisms (PAOs) community. Growth promotion was accompanied by up-regulation of pyrimidine, purine and energy metabolism. Both intra- and extra-cellular P entrapment were enhanced in the addition of AHLs. AHLs increased extracellular polymeric substances (EPS) production to drive extracellular P entrapment, via up-regulating amino acids biosynthesis and amino sugar/nucleotide sugar metabolism. Also, AHLs improved intracellular P entrapment potential by regulating genes involved in inorganic-P accumulation (ppk, ppx) and P uptake and transport (pit, pstSCAB). This proof-of-concept evidence about how QS signaling regulates P entrapment by microbial aggregates paves the way for managing QS to enhance P removal by microbial aggregates in aquatic environments.

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

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

N-acyl homoserine lactone molecules assisted quorum sensing: effects consequences and monitoring of bacteria talking in real life

Bacteria utilize small signal molecules to monitor population densities. Bacteria arrange gene regulation in a method called Quorum Sensing (QS). The most widespread signalling molecules are N-Acyl Homoserine Lactones (AHLs/HSLs) for Gram-negative bacteria communities. QS plays significant role in the organizing of the bacterial gene that adapts to harsh environmental conditions for bacteria. It is involved in the arrangement of duties, such as biofilm formation occurrence, virulence activity of bacteria, production of antibiotics, plasmid conjugal transfer incident, pigmentation phenomenon and production of exopolysaccharide (EPS). QS obviously impacts on human health, agriculture and environment. AHL-related QS researches have been extensively studied and understood in depth for cell to cell intercommunication channel in Gram-negative bacteria. It is understood that AHL-based QS research has been extensively studied for cell-to-cell communication in Gram-negative bacteria; hence, a comprehensive study of AHLs, which are bacterial signal molecules, is required. The purpose of this review is to examine the effects of QS-mediated AHLs in many areas by looking at them from a different perspectives, such as clinic samples, food industry, aquatic life and wastewater treatment system.

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

Quorum sensing (QS) plays a crucial role during initial biofilm formation, however the QS threshold and the response of biofilm formation towards N-acyl-homoserine lactones (AHLs) remains largely unknown due to the limitation of nondestructive online methods for monitoring bacterial adherence and the complexity of QS system, which limits the application of QS signal reagents in biofilm reactors. In this study, bacterial QS threshold and its response of biofilm formation to AHLs in purely cultured Sphingomonas rubra biofilm as well as in three different wastewater biofilms #1-3 were investigated via real time cell analysis (RTCA). The main perspective was to study the biomass adherence in response to 12 different forms of AHLs at different concentrations. Results showed that bacterial adhesion was significantly improved by exogenous AHLs with the maximum increase of 2.26-, 2.36-, 2.52-, and 2.80- times biomass production in the four respective biofilms. Although the preferred form of AHL differed for various biofilms, the long-chain AHLs (12-14 carbons) resulted in an overall improvement of bacterial adhesion due to their stronger hydrophobicity and hydrolysis resistance. In addition, bacterial QS threshold of AHLs was observed to have a wide range of concentration from 10 ng/L to 10 μg/L. Meanwhile, QS response time to AHLs also showed a significant difference in different biofilms. Biofilm #2 inoculated with bulking sludge had lower QS threshold of 10 ng/L and faster response to most AHLs that is less than 6 h. Thus, considering the improvement of biofilm adhesion by AHLs, 10 ng/L of C12-HSL, 10 ng/L of C12-HSL, and 10 ng/L of C6-HSL were preferentially selected for wastewater biofilms #1-3 respectively. Unexpectedly, adding high-concentration of AHLs detected in sludges did not significantly improved the bacterial adhesion. Infact the addition of these AHLs at low concentrations or even undetected concentrations substantially improved bacterial adhesion, which could be explained by bacterial communities composition. According to the Pearson correlation analysis, 62% of the top 50 most abundant genera in bacterial communities were significantly negatively related to the response time of multiple AHLs, representing their fast QS response. The QS bacteria, Dechloromonas and Nitrospira have fast QS response for C4-HSL and C8-HSL while, Comamonadaceae has fast QS response for 3OC8-HSL, 3OC10-HSL, 3OC12-HSL, and 3OC14-HSL. In contrast, the rest 38% of the top most abundant genera, such as Ferruginibacter, Hyphomicrobium, and Terrimonas quickly responded to only one AHL, showing significant negative relationship with the response time of C6-HSL. Overall, this study provides an effective and convenient means to select appropriate AHL reagents to promote bacterial adhesion in biofilm systems. Moreover, it also suggests that exogenous AHLs may be useful in improving the settling property of bulking sludge.

Quorum sensing regulates 'swim-or-stick' lifestyle in the phycosphere

Interactions between phytoplankton and bacteria play major roles in global biogeochemical cycles and oceanic nutrient fluxes. These interactions occur in the microenvironment surrounding phytoplankton cells, known as the phycosphere. Bacteria in the phycosphere use either chemotaxis or attachment to benefit from algal excretions. Both processes are regulated by quorum sensing (QS), a cell–cell signalling mechanism that uses small infochemicals to coordinate bacterial gene expression. However, the role of QS in regulating bacterial attachment in the phycosphere is not clear. Here, we isolated a Sulfitobacter pseudonitzschiae F5 and a Phaeobacter sp. F10 belonging to the marine Roseobacter group and an Alteromonas macleodii F12 belonging to Alteromonadaceae, from the microbial community of the ubiquitous diatom Asterionellopsis glacialis. We show that only the Roseobacter group isolates (diatom symbionts) can attach to diatom transparent exopolymeric particles. Despite all three bacteria possessing genes involved in motility, chemotaxis, and attachment, only S. pseudonitzschiae F5 and Phaeobacter sp. F10 possessed complete QS systems and could synthesize QS signals. Using UHPLC–MS/MS, we identified three QS molecules produced by both bacteria of which only 3‐oxo‐C_16:1‐HSL strongly inhibited bacterial motility and stimulated attachment in the phycosphere. These findings suggest that QS signals enable colonization of the phycosphere by algal symbionts.

Syntrophic metabolism of phenol in the anodic degradation within a Phenol-Cr(VI) coupled microbial electrolysis cell

Bioelectrochemical system (BESs) has been applied to treat refractory wastewaters such as phenolic wastewater since microbial anodic oxidation driven by electroactive bacteria is believed to enhance decomposition of organic matters. Considering that most of electroactive bacteria are sensitive to phenol and cannot utilize it directly, it was assumed that fermentative bacteria and electroactive bacteria in mixed-culture BESs cooperated to degrade phenol. To clarify this assumption, a microbial electrolysis cell (MEC) for phenol degradation with Cr(VI)-reduction bio-cathode was developed in this study. Results showed that phenol served as anodic electron donor was more efficient than acetate for cathodic reduction of Cr(VI) since the slow release of acetate from phenol degradation with fermentative bacteria might make full use of acetate as electron donor for anodic oxidation. The production of quorum sensing (QS) signal molecules were higher in phenolic anolyte, confirming the syntrophic metabolism among phenol-degrading bacteria and electroactive bacteria. Cyclic voltammetry (CV) test and Fourier transform infrared spectroscopy (FT-IR) indicated that phenolic anolyte and anodic sludge had higher electron transfer ability due to enhanced bio-electrochemisty processes in syntrophic metabolism.

Analysis of N-Acy-L-homoserine lactones (AHLs) in wastewater treatment systems using SPE-LLE with LC-MS/MS

The occurrence and distribution of N-acyl homoserine lactones (AHLs) in membrane bioreactors (MBRs) treating wastewater has garnered much attention as they have been shown to play critical role in biofouling. There is a need to develop a single method capable of analysing AHLs in various wastewater with comparable and reliable performance. A novel and robust method was proposed for trace analysis of 11 AHLs in wastewater treatment systems treating domestic and industrial wastewater. This method utilised solid phase extraction (SPE) to extract AHLs from wastewater followed by liquid-liquid extraction (LLE) to extract AHLs from the SPE eluant, and used N-heptanoyl-dl-homoserine lactone (C7-HSL) as an internal standard. There was no need to prepare matrix-matched calibration curve for accurate quantification of AHLs in the liquid chromatography tandem mass chromatography (LC-MS/MS) analysis. The developed method was validated with six different types of domestic and industrial wastewater with regard to AHLs recoveries and matrix effects. For treated domestic and industrial wastewater, the relative recoveries ranged from 75% to 130% and the matrix effects ranged from 89% to 122%. This method exhibited remarkable improvement compared with single SPE. The results also indicated that inclusion of LLE after SPE could effectively alleviate matrix effects, which may be because of the removal of relatively hydrophilic interferences by using dichloromethane to extract AHLs from the SPE eluant composing of methanol and water. The limits of detection of the AHLs were all below 5 ng/L for the tested wastewater samples. The developed method of SPE-LLE with LC-MS/MS was applied to analyse AHLs in four lab-scale and one pilot-scale wastewater treatment systems. Wide spectrum of AHLs with alkanoyl chains ranging from C4 to C14 were detected with concentrations ranging from 2.7 to 299.2 ng/L. This method is capable of identifying and quantifying trace levels of AHLs in various wastewater treatment systems and can help us better understand the mechanisms of AHL-mediated quroum sensing in various wastewater treatment systems.

In-situ monitoring AHL-mediated quorum-sensing regulation of the initial phase of wastewater biofilm formation

Initial attachment plays an important role in biofilm formation in wastewater treatment processes. However, the initial attachment process mediated by N-acyl-homoserine lactones (AHLs) is difficult to be fully understood due to the lack of non-invasive and on-line investigation techniques. In this study, the AHL-regulated wastewater biofilm attachment was quantified using ultrasonic time-domain reflectometry (UTDR) as an in-situ and non-invasive monitoring technique. Results demonstrated that the reversible adhesion time in municipal and industrial wastewaters was significantly decreased in the presence of exogenous AHLs. Biofilm thickness in municipal and industrial wastewaters increased significantly with the addition of exogenous AHLs. Also, the addition of acylase delayed the initial biofilm formation (lengthened reversible adhesion time and decreased biofilm thickness and density). Compared with biofilm behavior in the presence of low concentrations of AHLs (4.92 ± 0.17 μg/L), both reversible adhesion time and biofilm thickness were not significantly increased (p > 0.05) with an increase in AHL concentration (9.75 ± 0.41 μg/L). Furthermore, the addition of exogenous AHLs resulted in significant changes in the attached bacterial community structures, in which both QS and quorum-quenching (QQ) bacteria were stimulated. The current work presents an effective approach to in-situ monitoring of the regulation of AHL-mediated QS in the initial attachment of biofilms, especially in the reversible adhesion process, which may provide a potential strategy to facilitate biofilm establishment in wastewater treatment processes.

Undesirable effects of exogenous N-acyl homoserine lactones on moving bed biofilm reactor treating medium-strength synthetic wastewater

Exogenous AHLs are gradually reported to facilitate biofilm growth, however, whether they play a universal role in promoting biofilm formation and pollutants removal remains to be investigated. The pollutant removal, biofilm properties, microbial community and the distribution of AHLs were investigated in three lab-scale MBBRs by continuous dosing 100 nM N-Hexanoyl-L-homoserine lactone (C6-HSL) and N-Octanoyl-L-homoserine lactone (C8-HSL) in synthetic wastewater under normal nutrition (40 mg/L NH₄⁺-N with C/N = 20). Results showed that adding AHLs didn't affect organics removal and exogenous C6-HSL even significantly suppressed NH₄⁺-N removal by 0.44-20.29% after 16 days (p < 0.05). The introduction of AHLs both facilitated biofilm growth and extracellular polymeric substances secretion while suppressed ATP production especially during the stable operation period, with 48.96% by C6-HSL (p < 0.05) and 27.25% by C8-HSL, respectively. Exogenous AHLs inhibited the proliferation of Chryseobacterium, resulting in improvement in biofilm growth and it probably mediated ATP synthesis through regulating the release of 3OHC12-HSL in aqueous phase. Organics removal and biofilm growth were mainly attributed to the combined actions of multitudinous AHLs in biofilm phase rather than that in aqueous phase. The counterintuitive conclusions obtained in this study highlighted the importance of legitimately applying exogenous AHLs to accelerate biofilm formation and the start-up of MBBR in wastewater treatment.

Insights into the effects of carbon source on sequencing batch reactors: Performance, quorum sensing and microbial community

Effects of carbon source on the performance, quorum sensing (QS) and microbial communities in the sequencing batch reactors were investigated in this work. Among the chosen carbon source, sodium acetate (R1), glucose (R2), starch (R3) and Tween 80 (R4), sodium acetate was the best carbon source for nutrient removal, while starch was favorable for inducing the sludge bulking, and Tween 80 was beneficial to the production of extracellular polymeric substances (EPS) and proliferation of Microthrix parvicella. Additionally, the R² value of linear correlation between sludge settleability and particle size in four reactors followed an order of R1 > R2 > R3 > R4. Moreover, Person correlation analysis showed that various significant correlations were observed in reactors fed with different carbon sources and the QS mainly mediated the production and component of EPS. High-throughput sequencing analysis revealed that the carbon source affected microbial communities and the Canonical correspondence analysis results indicated that QS related to microbial communities. It was inferred that the interactions between microbial communities and QS affected system performance.

Insight into mature biofilm quorum sensing in full-scale wastewater treatment plants

Quorum sensing (QS) has been thoroughly investigated during initial biofilm formation stages, while the role of QS in mature biofilms has received little research attention. This study assessed QS in 22 biofilm samples from full-scale wastewater treatment plants in China. Results showed that the concentration of acyl-homoserine lactones (AHLs) in various biofilm bound forms, ranged from 15.63 to 609.76 ng/g. The highest concentration of AHLs was found in the tightly bound biofilm fraction, while the lowest concentrations were observed in the surface biofilm fraction. Environmental variables, C/N ratio and temperature, were found to be significant factors influencing biofilm AHL distribution (p < 0.01). Higher C/N ratios (ranging from 3 to 12) and low temperatures contributed to the higher concentration of AHLs in biofilms. Dominant AHLs (C10-HSL and C12-HSL) were significantly associated with biofilm activity (R² = 0.98/0.97, p < 0.05), with the tightly bound biofilm fraction (TB-biofilm) presenting the highest activity (ATP concentration). Biofilm aging and re-formation processes were more active in the surface biofilm layer (S-biofilm), while the stable structure of the TB-biofilm layer which is attached to the surface of bio-carriers ensures high biofilm activity. This study furthers our understanding of the roles of AHLs in the regulation of mature biofilm activities.

Quorum sensing signaling distribution during the development of full-scale municipal wastewater treatment biofilms

Acylated homoserine lactone (AHL)-mediated quorum sensing (QS) is ecologically important in multi-species systems in laboratory-scale studies; however, little is known about QS in the biofilm formation process in full-scale wastewater treatment plants, which is driven by multiple environmental variables. Here, a model integrated fixed-film-activated sludge system was employed in full-scale municipal wastewater treatment plant to investigate the AHL distribution during the biofilm development process in response to variable environmental factors. The whole biofilm development process can be divided into three phases: initial biofilm attachment process (week 1 to 3), biofilm development and mature phase (week 4 to 6), and biofilm detachment and reformation process (week 7 to 17). N-decanoyl-DL-homoserine lactone (C10-HSL) and N-dodecanoyl-DL-homoserine lactone (C12-HSL) presented high concentrations during the biofilm formation process, which was closely related with the biofilm initial attachment process. The AHL concentration in biofilms was higher than in activated sludge. During the initial attachment process, tryptophan and protein-like substances related to biological substance were strongly positively correlated with all detected AHL concentrations (p < 0.05). Three environmental variables (total nitrogen, pH, and Na⁺) were closely related to AHL distribution in municipal wastewater biofilms. High wastewater pH was found to contribute to a low AHL concentration. AHLs in the biofilm were significantly (p < 0.01) influenced by the concentration of Na⁺, and higher concentrations of Na⁺ (10.84-18.58 mg/L) in wastewater treatment plants potentially contribute to the biofilm formation processes. In addition, bacteria with nitrogen removal ability showed QS functionality. The results of this study indicate that AHL-based regulation of tryptophan and protein-like substances related to biological substance production was significantly influenced by the surrounding chemical environment, which has been underestimated in previous studies. AHL-mediated QS potentially suggests a novel solution for the advanced AHL-based regulation of the biofilm development processes.

Identification of Quorum-Sensing Molecules of N-Acyl-Homoserine Lactone in Gluconacetobacter Strains by Liquid Chromatography-Tandem Mass Spectrometry

Many Gram-negative bacteria can regulate gene expression in a cell density-dependent manner via quorum-sensing systems using N-acyl-homoserine lactones (AHLs), which are typical quorum-sensing signaling molecules, and thus modulate physiological characteristics. N-acyl-homoserine lactones are small chemical molecules produced at low concentrations by bacteria and are, therefore, difficult to detect. Here, a biosensor system method and liquid chromatography-tandem mass spectrometry were combined to detect and assay AHL production. As demonstrated by liquid chromatography-tandem mass spectrometry, Gluconacetobacter xylinus CGMCC No. 2955, a Gram-negative acetic acid-producing bacterium and a typical bacterial cellulose (BC) biosynthesis strain, produces six different AHLs, including N-acetyl-homoserine lactone, N-butanoyl-homoserine lactone, N-hexanoyl-homoserine lactone, N-3-oxo-decanoyl-homoserine lactone, N-dodecanoyl-homoserine lactone, and N-tetradecanoyl-homoserine lactone. Gluconacetobacter sp. strain SX-1, another Gram-negative acetic acid-producing bacterium, which can synthesize BC, produces seven different AHLs including N-acetyl-homoserine lactone, N-butanoyl-homoserine lactone, N-hexanoyl-homoserine lactone, N-3-oxo-octanoyl-homoserine lactone, N-decanoyl-homoserine lactone, N-dodecanoyl-homoserine lactone, and N-tetradecanoyl-homoserine lactone. These results lay the foundation for investigating the relationship between BC biosynthesis and quorum-sensing systems.

Identification and Quantification of Acylated Homoserine Lactones in Shewanella baltica, the Specific Spoilage Organism of Pseudosciaena crocea, by Ultrahigh-Performance Liquid Chromatography Coupled to Triple Quadrupole Mass Spectrometry

Bacteria communicate with one another using chemical signal molecules called autoinducers, and the most prevalent autoinducers used by Gram-negative bacteria are N-acylated homoserine lactones (AHLs). However, AHLs are hardly detected in Shewanella baltica, the specific spoilage organism of Pseudosciaena crocea. In this study, we applied ultrahigh-performance liquid chromatography coupled to triple quadrupole tandem mass spectrometry to determine AHLs. This method enabled the quantification of up to 11 AHLs within 5.6 min with excellent sensitivity (ng/mL level) and linearity (r² > 0.99), which further detected 9 AHLs produced by S. baltica. Furthermore, by using a biofilm formation assay and statistical analysis, the biofilm-inducing activity of AHL in S. baltica was first revealed. Our results elucidated the physiological role of AHL in S. baltica and provided a satisfactory method to detect AHLs and a statistical model to predict food spoilage properties.