Role of N-acyl homoserine lactone (AHL)-based quorum sensing (QS) in aerobic sludge granulation

N-acyl homoserine lactone (AHL)-based quorum sensing (QS) has been recognized to play an important role in the formation of biofilm. However, aerobic granular sludge is considered as a special biofilm, and its biological implication and role of AHL-based QS still remain unclear. This study investigated the role of AHL-based QS in aerobic granulation. Results showed that AHLs were necessary to the typical aerobic granulation, and AHL-associated coordination of bacteria in sludge aggregation was sludge density dependent only when it reached a threshold of 1.010 g/mL; AHL-based QS was activated to regulate aerobic granulation. Furthermore, a quorum quenching method was firstly adopted to investigate the role of AHLs in aerobic granules. Results showed inhibition of AHL by acylase that reduced the AHL content in aerobic granules and further weakened its attachment potential, which proved that AHLs play an important role in the formation of aerobic granules. Additionally, the assay of quorum quenching not only proved that AHL-based QS could regulate EPS production but also provided additional evidence for the role of AHLs in aerobic granulation by regulating EPS content and its component proportion.     

Multiplicity of Quorum Quenching Enzymes: A Potential Mechanism to Limit Quorum Sensing Bacterial Population

Bacteria express certain of their characteristics especially, pathogenicity factors at high cell densities. The process is termed as quorum sensing (QS). QS operates via signal molecules such as acylhomoserine lactones (AHLs). Other bacteria inhibit QS through the inactivation of AHL signals by producing enzymes like AHL-lactonases and -acylases. Comparative genomic analysis has revealed the multiplicity of genes for AHL lactonases (up to 12 copies per genome) among Bacillus spp. and that of AHL-acylases (up to 5 copies per genome) among Pseudomonas spp. This genetic evolution can be envisaged to enable host to withstand the attacks from bacterial population, which regulates its functioning through QS.     

Exogenous N‐acyl‐homoserine lactones enhance the expression of flagella of Pseudomonas syringae and activate defence responses in plants

In order to cope with pathogens, plants have evolved sophisticated mechanisms to sense pathogenic attacks and to induce defence responses. The N‐acyl‐homoserine lactone (AHL)‐mediated quorum sensing in bacteria regulates diverse physiological processes, including those involved in pathogenicity. In this work, we study the interactions between AHL‐producing transgenic tobacco plants and Pseudomonas syringae pv. tabaci 11528 (P. syringae 11528). Both a reduced incidence of disease and decrease in the growth of P. syringae 11528 were observed in AHL‐producing plants compared with wild‐type plants. The present data indicate that plant‐produced AHLs enhance disease resistance against this pathogen. Subsequent RNA‐sequencing analysis showed that the exogenous addition of AHLs up‐regulated the expression of P. syringae 11528 genes for flagella production. Expression levels of plant defence genes in AHL‐producing and wild‐type plants were determined by quantitative real‐time polymerase chain reaction. These data showed that plant‐produced AHLs activated a wide spectrum of defence responses in plants following inoculation, including the oxidative burst, hypersensitive response, cell wall strengthening, and the production of certain metabolites. These results demonstrate that exogenous AHLs alter the gene expression patterns of pathogens, and plant‐produced AHLs either directly or indirectly enhance plant local immunity during the early stage of plant infection.     

Design, synthesis, and biological evaluation of abiotic, non-lactone modulators of LuxR-type quorum sensing

Quorum sensing (QS) is a cell-cell signaling mechanism that allows bacteria to monitor their population size and alter their behavior at high cell densities. Gram-negative bacteria use N-acylated L-homoserine lactones (AHLs) as their primary signals for QS. These signals are susceptible to lactone hydrolysis in biologically relevant media, and the ring-opened products are inactive QS signals. We have previously identified a range of non-native AHLs capable of strongly agonizing and antagonizing QS in Gram-negative bacteria. However, these abiotic AHLs are also prone to hydrolysis and inactivation and thereby have a relatively short time window for use (～12-48 h). Non-native QS modulators with reduced or no hydrolytic instability could have enhanced potencies and would be valuable as tools to study the mechanisms of QS in a range of environments (for example, on eukaryotic hosts). This study reports the design and synthesis of two libraries of new, non-hydrolyzable AHL mimics. The libraries were screened for QS modulatory activity using LasR, LuxR, and TraR bacterial reporter strains, and several new, abiotic agonists and antagonists of these receptors were identified.     

Bacterial quorum sensing in symbiotic and pathogenic relationships with hosts*

Gram-negative bacteria communicate with each other by producing and sensing diffusible signaling molecules. This mechanism is called quorum sensing (QS) and regulates many bacterial activities from gene expression to symbiotic/pathogenic interactions with hosts. Therefore, the elucidation and control of bacterial QS systems have been attracted increasing attention over the past two decades. The most common QS signals in Gram-negative bacteria are N-acyl homoserine lactones (AHLs). There are also bacteria that employ different QS systems, for example, the plant pathogen Ralstonia solanacearum utilizes 3-hydroxy fatty acid methyl esters as its QS signals. The QS system found in the endosymbiotic bacterium associated with the fungus Mortierella alpina, the development of an affinity pull-down method for AHL synthases, and the elucidation of a unique QS circuit in R. solanacearum are discussed herein.     

MomL,a Novel Marine-Derived N-Acyl Homoserine Lactonase from Muricauda olearia

Gram-negative bacteria use N-acyl homoserine lactones (AHLs) as quorum sensing (QS) signaling molecules for interspecies communication, and AHL-dependent QS is related with virulence factor production in many bacterial pathogens. Quorum quenching, the enzymatic degradation of the signaling molecule, would attenuate virulence rather than kill the pathogens, and thereby reduce the potential for evolution of drug resistance. In a previous study, we showed that Muricauda olearia Th120, belonging to the class Flavobacteriia, has strong AHL degradative activity. In this study, an AHL lactonase (designated MomL), which could degrade both short- and long-chain AHLs with or without a substitution of oxo-group at the C-3 position, was identified from Th120. Liquid chromatography-mass spectrometry analysis demonstrated that MomL functions as an AHL lactonase catalyzing AHL degradation through lactone hydrolysis. MomL is an AHL lactonase belonging to the metallo-β-lactamase superfamily that harbors an N-terminal signal peptide. The overall catalytic efficiency of MomL for C₆-HSL is ∼2.9 × 10⁵ s⁻¹ M⁻¹. Metal analysis and site-directed mutagenesis showed that, compared to AiiA, MomL has a different metal-binding capability and requires the histidine and aspartic acid residues for activity, while it shares the “HXHXDH” motif with other AHL lactonases belonging to the metallo-β-lactamase superfamily. This suggests that MomL is a representative of a novel type of secretory AHL lactonase. Furthermore, MomL significantly attenuated the virulence of Pseudomonas aeruginosa in a Caenorhabditis elegans infection model, which suggests that MomL has the potential to be used as a therapeutic agent.     

N‐ACYL HOMOSERINE LACTONe LACTONASE,AiiA, INACTIVATION OF QUORUM‐SENSING AGONISTS PRODUCED BY CHLAMYDOMONAS REINHARDTII (CHLOROPHYTA) AND CHARACTERIZATION OF aiiA TRANSGENIC ALGAE1

Eukaryotes such as plants and the unicellular green alga Chlamydomonas reinhardtii P. A. Dang. produce and secrete compounds that mimic N‐acyl homoserine lactone (AHL) bacterial quorum‐sensing (QS) signals and alter QS‐regulated gene expression in the associated bacteria. Here, we show that the set of C. reinhardtii signal‐mimic compounds that activate the CepR AHL receptor of Burkholderia cepacia are susceptible to inactivation by AiiA, an AHL lactonase enzyme of Bacillus. Inactivation of these algal mimics by AiiA suggests that the CepR‐stimulatory class of mimics produced by C. reinhardtii may have a conserved lactone ring structure in common with AHL QS signals. To examine the role of AHL mimic compounds in the interactions of C. reinhardtii with bacteria, the aiiA gene codon optimized for Chlamydomonas was generated for the expression of AiiA as a chimeric fusion with cyan fluorescent protein (AimC). Culture filtrates of transgenic strains expressing the fusion protein AimC had significantly reduced levels of CepR signal‐mimic activities. When parental and transgenic algae were cultured with a natural pond water bacterial community, a morphologically distinct, AHL‐producing isolate of Aeromonas veronii was observed to colonize the transgenic algal cultures and form biofilms more readily than the parental algal cultures, indicating that secretion of the CepR signal mimics by the alga can significantly affect its interactions with bacteria it encounters in natural environments. The parental alga was also able to sequester and/or destroy AHLs in its growth media to further disrupt or manipulate bacterial QS.     

Detection of quorum-sensing N-acyl homoserine lactone signal molecules by bacterial biosensors

Many Gram-negative bacteria use N-acyl homoserine lactones (AHLs) as quorum-sensing (QS) signal molecules. AHL QS has been the subject of extensive investigation in the last decade and has become a paradigm for bacterial intercellular signaling. Research in AHL QS has been considerably aided by simple methods devised to detect AHLs using bacterial biosensors that phenotypically respond when exposed to exogenous AHLs. This article reviews and discusses the currently available bacterial biosensors which can be used in detecting and studying the different AHLs.     

Contribution of quorum‐sensing system to hexadecane degradation and biofilm formation in Acinetobacter sp. strain DR1

Aims: To investigate roles of quorum‐sensing (QS) system in Acinetobacter sp. strain DR1 and rifampicin‐resistant variant (hereinafter DR1R). Methods and Results: The DR1 strain generated three putative acyl homoserine lactones (AHLs), while the DR1R produced only one signal and QS signal production was abrogated in the aqsI (LuxI homolog) mutant. The hexadecane‐degradation and biofilm‐formation capabilities of DR1, DR1R, and aqsI mutants were compared, along with their proteomic data. Proteomics analysis revealed that the AHL lactonase responsible for degrading QS signal was highly upregulated in both DR1R and aqsI mutant, also showed that several proteins, including ppGpp synthase, histidine kinase sensors, might be under the control of QS signalling. Interestingly, biofilm‐formation and hexadecane‐biodegradation abilities were reduced more profoundly in the aqsI mutant. These altered phenotypes of the aqsI mutant were restored via the addition of free wild‐type cell supernatant and exogenous C₁₂‐AHL. Conclusions: The QS system in strain DR1 contributes to hexadecane degradation and biofilm formation. Significance and Impact of the Study: This is the first report to demonstrate that a specific QS signal appears to be a critical factor for hexadecane degradation and biofilm formation in Acinetobacter sp. strain DR1.     

PpoR is a conserved unpaired LuxR solo of Pseudomonas putida which binds N-acyl homoserine lactones

Background  

Only a small number of Pseudomonas putida strains possess the typical N-acyl homoserine lactone quorum sensing system (AHL QS) that consists of a modular LuxR family protein and its cognate LuxI homolog that produces the AHL signal. Moreover, AHL QS systems in P. putida strains are diverse in the type of AHLs they produce and the phenotypes that they regulate.     

Quorum sensing disruption regulates hydrolytic enzyme and biofilm production in estuarine bacteria

Biofilms of heterotrophic bacteria cover organic matter aggregates and constitute hotspots of mineralization, primarily acting through extracellular hydrolytic enzyme production. Nevertheless, regulation of both biofilm and hydrolytic enzyme synthesis remains poorly investigated, especially in estuarine ecosystems. In this study, various bioassays, mass spectrometry and genomics approaches were combined to test the possible involvement of quorum sensing (QS) in these mechanisms. QS is a bacterial cell–cell communication system that relies notably on the emission of N-acylhomoserine lactones (AHLs). In our estuarine bacterial collection, we found that 28 strains (9%), mainly Vibrio, Pseudomonas and Acinetobacter isolates, produced at least 14 different types of AHLs encoded by various luxI genes. We then inhibited the AHL QS circuits of those 28 strains using a broad-spectrum lactonase preparation and tested whether biofilm production as well as β-glucosidase and leucine-aminopeptidase activities were impacted. Interestingly, we recorded contrasted responses, as biofilm production, dissolved and cell-bound β-glucosidase and leucine-aminopeptidase activities significantly increased in 4%–68% of strains but decreased in 0%–21% of strains. These findings highlight the key role of AHL-based QS in estuarine bacterial physiology and ultimately on biogeochemical cycles. They also point out the complexity of QS regulations within natural microbial assemblages.     

Opportunistic bacteria use quorum sensing to disturb coral symbiotic communities and mediate the occurrence of coral bleaching

Coral associated microorganisms, especially some opportunistic pathogens can utilize quorum-sensing (QS) signals to affect population structure and host health. However, direct evidence about the link between coral bleaching and dysbiotic microbiomes under QS regulation was lacking. Here, using 11 opportunistic bacteria and their QS products (AHLs, acyl-homoserine-lactones), we exposed Pocillopora damicornis to three different treatments: test groups (A and B: mixture of AHLs-producing bacteria and cocktail of AHLs signals respectively); control groups (C and D: group A and B with furanone added respectively); and a blank control (group E: only seawater) for 21 days. The results showed that remarkable bleaching phenomenon was observed in groups A and B. The operational taxonomic units-sequencing analysis shown that the bacterial network interactions and communities composition were significantly changed, becoming especially enhanced in the relative abundances of Vibrio, Edwardsiella, Enterobacter, Pseudomonas, and Aeromonas. Interestingly, the control groups (C and D) were found to have a limited influence upon host microbial composition and reduced bleaching susceptibility of P. damicornis. These results indicate bleaching's initiation and progression may be caused by opportunistic bacteria of resident microbes in a process under regulation by AHLs. These findings add a new dimension to our understanding of the complexity of bleaching mechanisms from a chemoecological perspective.     

Quorum-sensing molecules: Sampling,identification and characterization of N-acyl-homoserine lactone in Vibrio sp

Quorum sensing (QS) is a mechanism by which gram-negative bacteria regulate their gene expression by making use of cell density. QS is triggered by a small molecule known as an autoinducer. Typically, gram-negative bacteria such as Vibrio produce signaling molecules called acyl homoserine lactones (AHLs). However, their levels are very low, making them difficult to detect. We used thin layer chromatography (TLC) to examine AHLs in different Vibrio species, such as Vibrio alginolyticus, Vibrio parahemolyticus, and Vibrio cholerae, against a standard- Chromobacterium violaceum. Further, AHLs were characterised by high-performance liquid chromatography (HPLC) and gas chromatography-mass spectrometry (GC–MS). C4-HSL (N- butanoyl- L- homoserine lactone), C6-HSL (N- hexanoyl- L- homoserine lactone), 3-oxo-C8-HSL (N-(3-Oxooctanoyl)-DL-homoserine lactone), C8-HSL (N- octanoyl- L- homoserine lactone), C₁10-HSL (N- decanoyl- L- homoserine lactone), C12-HSL (N- dodecanoyl- L- homoserine lactone) and C14-HSL (N- tetradecanoyl- L- homoserine lactone) were identified from Vibrio. These results may provide a basis for blocking the AHL molecules of Vibrio, thereby reducing their pathogenicity and eliminating the need for antimicrobials.     

The presence, type and role of N-acyl homoserine lactone quorum sensing in fluorescent Pseudomonas originally isolated from rice rhizospheres are unpredictable

In Gram-negative bacteria, a typical quorum-sensing (QS) system involves the production and response to N-acyl homoserine lactones (AHLs). It still remains unclear as to how pivotal and conserved AHL QS is in root-colonizing rhizosphere Pseudomonas. We, therefore, performed a systematic study of AHL QS on a set of 50 rice rhizosphere Pseudomonas isolates. We also isolated the AHL QS genes in two representative strains and analyzed the role of AHL QS regulation of various phenotypes. Our results are discussed with the current knowledge of AHL QS of rhizosphere Pseudomonas, implicating a lack of conservation and an unpredictable role played by AHL QS in this group of bacteria.