Detecting and characterizing N-acyl-homoserine lactone signal molecules by thin-layer chromatography

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.

Use of Whole-Cell Bioassays for Screening Quorum Signaling, Quorum Interference, and Biofilm Dispersion

In most bacteria, a global level of regulation, termed quorum sensing (QS), exists involving intercellular communication via the production and response to cell density-dependent signal molecules. QS has been associated with a number of important features in bacteria including virulence regulation and biofilm formation. Consequently, there is considerable interest in understanding, detecting, and inhibiting QS. N-acylated homoserine lactones (AHLs) are used as extracellular QS signals by a variety of Gram-negative bacteria. Chromobacterium violaceum, commonly found in soil and water, produces the characteristic purple pigment violacein, regulated by AHL-mediated QS. Based on this readily observed pigmentation phenotype, C. violaceum strains can be used to detect various aspects of AHL-mediated QS activity. In another commonly used bioassay organism, Agrobacterium tumefaciens, QS can be detected by the use of a reporter gene such as lacZ. Here, we describe several commonly used approaches incorporating C. violaceum and A. tumefaciens that can be used to detect AHL and QS inhibitors. Due to the inherent low susceptibility of biofilm bacteria to antimicrobial agents, biofilm dispersion, whereby bacteria reenter the planktonic community, is another increasingly important area of research. At least one signal, distinct from traditional QS, has been identified and there are a variety of other environmental factors that also trigger dispersion. We describe a microtiter-based experimental strategy whereby potential biofilm dispersion compounds can be screened.

The role of chemical antifouling defence in the invasion success of Sargassum muticum: A comparison of native and invasive brown algae

Competition and fouling defence are important traits that may facilitate invasions by non-indigenous species. The 'novel weapons hypothesis' (NWH) predicts that the invasive success of exotic species is closely linked to the possession of chemical defence compounds that the recipient community in the new range is not adapted to. In order to assess whether chemical defence traits contribute to invasion success, anti-bacterial, anti-quorum sensing, anti-diatom, anti-larval and anti-algal properties were investigated for the following algae: a) the invasive brown alga Sargassum muticum from both, its native (Japan) and invasive (Germany) range, b) the two non- or weak invasive species Sargassum fusiforme and Sargassum horneri from Japan, and c) Fucus vesiculosus, a native brown alga from Germany. Crude and surface extracts and lipid fractions of active extracts were tested against common fouling organisms and zygotes of a dominant competing brown alga. Extracts of the native brown alga F. vesiculosus inhibited more bacterial strains (75%) than any of the Sargassum spp. (17 to 29%). However, Sargassum spp. from Japan exhibited the strongest settlement inhibition against the diatom Cylindrotheca closterium, larvae of the bryozoan Bugula neritina and zygotes of the brown alga F. vesiculosus. Overall, extracts of S. muticum from the invasive range were less active compared to those of the native range suggesting an adaptation to lower fouling pressure and competition in the new range resulting in a shift of resource allocation from costly chemical defence to reproduction and growth. Non-invasive Sargassum spp. from Japan was equally defended against fouling and competitors like S. muticum from Japan indicating a necessity to include these species in European monitoring programs. The variable antifouling activity of surface and crude extracts highlights the importance to use both for an initial screening for antifouling activity.

Molecular characterization of Acinetobacter baumannii isolated from Iraqi hospital environment

Healthcare-associated items are a common source of acquired infections, and hospital-acquired infections cause significant mortality and morbidity worldwide. Acinetobacter baumannii is the most prevalent infection-causing organism in the hospital environment. Hospital articles and objects are the main sources of infection with the ability to transmit some of the pathogenic microorganisms such as A. baumannii, which is considered a serious problem in therapeutic treatments. In the current study, we isolated A. baumannii from hospital sources and evaluated its antibiotic resistance, virulence factors and resistance gene determinants. The isolates were identified phenotypically as well as genotypically using PCR. In addition, their capability for biofilm formation and ten other virulence factors were measured. Of 112 samples, 21 showed growth of the target organism. Apart from A. baumannii, isolates of Candida albicans, Staphylococcus sp., Pseudomonas aeruginosa, Escherichia coli and Klebsiella pneumoniae were also grown. Antibiotic susceptibility test results considered all the A. baumannii to be multidrug-resistant isolates with the highest resistance being 100% to gentamycin, ciprofloxacin; the most effective antibiotics with 100% susceptibility was colistin and tigecycline. All A. baumannii isolates had MIC for ceftriaxone >32 mg/L. All A. baumannii isolates from the hospital environment showed multidrug resistance and had many virulence factors. They have long-term resistance to dry conditions and cause a serious public health issue.

Aii810, a Novel Cold-Adapted N-Acylhomoserine Lactonase Discovered in a Metagenome, Can Strongly Attenuate Pseudomonas aeruginosa Virulence Factors and Biofilm Formation

The pathogen Pseudomonas aeruginosa uses quorum sensing (QS) to control virulence and biofilm formation. Enzymatic disruption of quorum sensing is a promising anti-infection therapeutic strategy that does not rely on antibiotics. Here, a novel gene (aii810) encoding an N-acylhomoserine lactonase was isolated from the Mao-tofu metagenome for the first time. Aii810 encoded a protein of 269 amino acids and was expressed in Escherichia coli BL21 (DE3) in soluble form. It showed the highest activity at 20°C, and it maintained 76.5% of activity at 0°C and more than 50% activity at 0–40°C. The optimal pH was 8.0. It was stable in both neutral and slightly alkaline conditions and at temperatures below 40°C. The enzyme hydrolyzed several ρ-nitrophenyl esters, but its best substrate was ρ-nitrophenyl acetate. Its k_cat and Km values were 347.7 S^-1 and 205.1 μM, respectively. It efficiently degraded N-butyryl-L-homoserine lactone and N-(3-oxododecanoyl)-L-homoserine lactone, exceeding hydrolysis rates of 72.3 and 100%, respectively. Moreover, Aii810 strongly attenuated P. aeruginosa virulence and biofilm formation. This enzyme with high anti-QS activity was the most cold-adapted N-acylhomoserine lactonase reported, which makes it an attractive enzyme for use as a therapeutic agent against P. aeruginosa infection.

Mutation of the cyclic di-GMP phosphodiesterase gene in Burkholderia lata SK875 attenuates virulence and enhances biofilm formation

Burkholderia sp. is a gram-negative bacterium that commonly exists in the environment, and can cause diseases in plants, animals, and humans. Here, a transposon mutant library of a Burkholderia lata isolate from a pig with swine respiratory disease in Korea was screened for strains showing attenuated virulence in Caenorhabditis elegans. One such mutant was obtained, and the Tn5 insertion junction was mapped to rpfR, a gene encoding a cyclic di-GMP phosphodiesterase that functions as a receptor. Mutation of rpfR caused a reduction in growth on CPG agar and swimming motility as well as a rough colony morphology on Congo red agar. TLC analysis showed reduced AHL secretion, which was in agreement with the results from plate-based and bioluminescence assays. The mutant strain produced significantly more biofilm detected by crystal violet staining than the parent strain. SEM of the mutant strain clearly showed that the overproduced biofilm contained a filamentous structure. These results suggest that the cyclic di-GMP phosphodiesterase RpfR plays an important role in quorum sensing modulation of the bacterial virulence and biofilm formation.

Host modification of a bacterial quorum-sensing signal induces a phenotypic switch in bacterial symbionts

Bacterial communities colonize epithelial surfaces of most animals. Several factors, including the innate immune system, mucus composition, and diet, have been identified as determinants of host-associated bacterial communities. Here we show that the early branching metazoan Hydra is able to modify bacterial quorum-sensing signals. We identified a eukaryotic mechanism that enables Hydra to specifically modify long-chain 3-oxo-homoserine lactones into their 3-hydroxy-HSL counterparts. Expression data revealed that Hydra's main bacterial colonizer, Curvibacter sp., responds differentially to N-(3-hydroxydodecanoyl)-l-homoserine lactone (3OHC12-HSL) and N-(3-oxododecanoyl)-l-homoserine lactone (3OC12-HSL). Investigating the impacts of the different N-acyl-HSLs on host colonization elucidated that 3OHC12-HSL allows and 3OC12-HSL represses host colonization of Curvibacter sp. These results show that an animal manipulates bacterial quorum-sensing signals and that this modification leads to a phenotypic switch in the bacterial colonizers. This mechanism may enable the host to manipulate the gene expression and thereby the behavior of its bacterial colonizers.

Is Quorum Signaling by Mycotoxins a New Risk-Mitigating Strategy for Bacterial Biocontrol of Fusarium verticillioides and Other Endophytic Fungal Species?

Bacterial endophytes are used as biocontrol organisms for plant pathogens such as the maize endophyte Fusarium verticillioides and its production of fumonisin mycotoxins. However, such applications are not always predictable and efficient. In this work, we hypothesize and review work that quorum sensing inhibitors are produced either by fungi or by pathogenic bacteria for competitive purposes, altering the efficiency of the biocontrol organisms. Recently, quorum sensing inhibitors have been isolated from several fungi, including Fusarium species, three of which are mycotoxins. Thus, we further postulate that other mycotoxins are inhibitors or quenching metabolites that prevent the protective abilities and activities of endophytic biocontrol bacteria within intercellular spaces. To test the aforementioned suppositions, we review work detailing the use of bioassay bacteria for several mycotoxins for quorum activity. We specifically focus on the quorum use of endophytic bacteria as biocontrols for mycotoxic fungal endophytes, such as the Fusarium species and the fumonisin mycotoxins.

Membrane fouling induced by AHL-mediated soluble microbial product (SMP) formation by fouling-causing bacteria co-cultured with fouling-enhancing bacteria

Membrane fouling still remains a major obstacle for wider applications of membrane bioreactor (MBR), which is mainly caused by soluble microbial products (SMP). Identification of key bacteria responsible for SMP production is essential for mitigation of membrane fouling. Here, we investigated the effect of microbial interaction on membrane fouling. We measured the membrane fouling potentials of 13 bacterial strains isolated from a pilot-scale MBR treating domestic wastewater when they were cultivated as single-culture and co-culture. We found that fouling-causing bacteria (FCB) displayed much higher fouling potential when co-cultured even with non-FCB and mixed population (activated sludge). In particular, the fouling potential of strain S26, one of FCB, increased 26.8 times when cultivated with strain S22 (fouling-enhancing bacteria, FEB). The secretion of N-octanoyl-L-homoserine lactone (C8-HSL) was increased by co-cultivating S22 and S26 as compared with cultivating as single culture, which stimulated the production of fouling-causing SMP by S26 and consequently resulted in severe membrane fouling. This result suggests that AHL-mediated quorum-sensing (QS) regulatory system was involved in secretion of fouling-causing SMP.

Roles of quorum sensing molecules from Rhizobium etli RT1 in bacterial motility and biofilm formation

Strain RT1 was isolated from root nodules of Lens culinaris (a lentil) and characterized as Rhizobium etli (a Gram-negative soil-borne bacterium) by 16S rDNA sequencing and phylogenetic analysis. The signaling molecules produced by R. etli (RT1) were detected and identified by high-performance liquid chromatography coupled with mass spectrometry. The most abundant and biologically active N-acyl homoserine lactone molecules (3-oxo-C₈-HSL and 3-OH-C₁₄-HSL) were detected in the ethyl acetate extract of RT1. The biological role of 3-oxo-C₈-HSL was evaluated in RT1. Bacterial motility and biofilm formation were affected or modified on increasing concentrations of 3-oxo-C₈-HSL. Results confirmed the existence of cell communication in RT1 mediated by 3-oxo-C₈-HSL, and positive correlations were found among quorum sensing, motility and biofilm formation in RT1.

Quorum sensing molecules production by nosocomial and soil isolates Acinetobacter baumannii

Acinetobacter species remain alive in hospitals on various surfaces, both dry and moist, forming an important source of hospital infections. These bacteria are naturally resistant to many antibiotic classes. Although the role of the quorum sensing system in regulating the virulence factors of Acinetobacter species has not been fully elucidated, it has been reported that they play a role in bacterial biofilm formation. The biofilm formation helps them to survive under unfavorable growth conditions and antimicrobial treatments. It is based on the accumulation of bacterial communication signal molecules in the area. In this study, we compared the bacterial signal molecules of 50 nosocomial Acinetobacter baumannii strain and 20 A. baumannii strain isolated from soil. The signal molecules were detected by the biosensor bacteria (Chromobacterium violaceum 026, Agrobacterium tumefaciens A136, and Agrobacterium tumefaciens NTL1) and their separation was determined by thin-layer chromatography. As a result, it has been found that soil-borne isolates can produce 3-oxo-C8-AHL and C8-AHL, whereas nosocomial-derived isolates can produce long-chain signals such as C10-AHL, C12-AHL, C14-AHL and C16-AHL. According to these results, it is possible to understand that these signal molecules are found in the infection caused by A. baumannii. The inhibition of this signaling molecules in a communication could use to prevent multiple antibiotic resistance of these bacteria.

N-acylhomoserine lactone-regulation of genes mediating motility and pathogenicity in Pseudomonas syringae pathovar tabaci 11528

Pseudomonas syringae pathovar tabaci 11528 (P. syringae 11528) is a phytopathogen that causes wild-fire disease in soybean and tobacco plants. It utilizes a cell density-dependent regulation system known as quorum sensing (QS). In its QS system, the psyI is responsible for the biosynthesis of N-acylhomoserine lactones (AHLs). By comparing the transcripts from P. syringae 11528 wild-type strain with those of the ΔpsyI mutant using RNA sequencing (RNA-seq) technology, 1118 AHL-regulated genes were identified in the transition from exponential to stationary growth phase. Numerous AHL-regulated genes involved in pathogenicity were negatively controlled, including genes linked to flagella, chemotaxis, pilus, extracellular polysaccharides, secretion systems, and two-component system. Moreover, gene ontology and pathway enrichment analysis revealed that the most pronounced regulation was associated with bacterial motility. Finally, phenotypic assays showed that QS-regulated traits were involved in epiphytic growth of pathogens and disease development in plants. These findings imply that the AHL-mediated QS system in P. syringae 11528 plays significant roles in distinct stages of interactions between plants and pathogens, including early plant colonization and late plant infection.

Quorum sensing and RsaM regulons of the rice pathogen Pseudomonas fuscovaginae

Pseudomonas fuscovaginae (Pfv) is an emerging plant pathogen causing sheath brown rot in rice, as well as diseases in other gramineae food crops including maize, sorghum and wheat. Pfv possesses two conserved N-acyl homoserine lactone (AHL) quorum sensing (QS) systems called PfvI/R and PfsI/R, which are repressed by RsaL and RsaM, respectively. The two systems are not hierarchically organized and are involved in plant virulence. In this study the AHL QS PfsI/R, PfvI/R and RsaM regulons were determined by transcriptomic analysis. The PfsI/R system regulates 98 genes, whereas 26 genes are regulated by the PfvI/R AHL QS system; only two genes are regulated by both systems. RsaM, on the other hand, regulates over 400 genes: 206 are negatively regulated and 260 are positively regulated. More than half of the genes controlled by the PfsI/R system and 65 % by the PfvI/R system are also part of the RsaM regulon; this is due to RsaM being involved in the regulation of both systems. It is concluded that the two QS systems regulate a unique set of genes and that RsaM is a global regulator mediating the expression of different genes through the two QS systems as well as genes independently of QS.

Acinetobacter baumannii quorum-sensing signalling molecule induces the expression of drug-resistance genes

Quorum-sensing signalling molecules such as N-acyl homoserine lactones (AHLs) enable certain Gram-negative bacteria to respond to environmental changes through behaviours, such as biofilm formation and flagellar movement. The present study aimed to identify Acinetobacter baumannii AHLs and assess their influence on antibiotic resistance. A clinical isolate of A. baumannii strain S (AbS) was collected from the wound of a burn patient and high-performance liquid chromatography and tandem quadrupole or quadrupole time-of-flight high-resolution mass spectrometry was used to identify AbS AHLs. Antibiotic sensitivity was assessed in an AHL-deficient AbS mutant (AbS-M), and the expression of drug-resistance genes in the presence of meropenem in AbS, AbS-M and AbS-M treated with the AHL N-3-hydroxy-dodecanoyl-homoserine lactone (N-3-OH-C₁₂-HSL). AbS-M was more sensitive to meropenem and piperacillin than wild-type AbS, but resistance was restored by supplementation with N-3-OH-C₁₂-HSL. In addition, meropenem-treated AbS-M expressed lower levels of the drug-resistance genes oxacillinase 51, AmpC, AdeA and AdeB; treatment with N-3-OH-C₁₂-HSL also restored the expression of these genes. Overall, the results of the present study indicate that N-3-OH-C₁₂-HSL may be involved in regulating the expression of drug-resistance genes in A. baumannii. Therefore, this quorum-sensing signalling molecule may be an important target for treating multidrug-resistant A. baumannii infections.

Characterization of N-Acyl Homoserine Lactones in Vibrio tasmaniensis LGP32 by a Biosensor-Based UHPLC-HRMS/MS Method

Since the discovery of quorum sensing (QS) in the 1970s, many studies have demonstrated that Vibrio species coordinate activities such as biofilm formation, virulence, pathogenesis, and bioluminescence, through a large group of molecules called N-acyl homoserine lactones (AHLs). However, despite the extensive knowledge on the involved molecules and the biological processes controlled by QS in a few selected Vibrio strains, less is known about the overall diversity of AHLs produced by a broader range of environmental strains. To investigate the prevalence of QS capability of Vibrio environmental strains we analyzed 87 Vibrio spp. strains from the Banyuls Bacterial Culture Collection (WDCM911) for their ability to produce AHLs. This screening was based on three biosensors, which cover a large spectrum of AHLs, and revealed that only 9% of the screened isolates produced AHLs in the defined experimental conditions. Among these AHL-producing strains, Vibrio tasmaniensis LGP32 is a well-known pathogen of bivalves. We further analyzed the diversity of AHLs produced by this strain using a sensitive bioguided UHPLC-HRMS/MS approach (Ultra-High-Performance Liquid Chromatography followed by High-Resolution tandem Mass Spectrometry) and we identified C10-HSL, OH-C12-HSL, oxo-C12-HSL and C14:1-HSL as QS molecules. This is the first report that documents the production of AHL by Vibrio tasmaniensis LGP32.

Peptides as Quorum Sensing Molecules: Measurement Techniques and Obtained Levels In vitro and In vivo

The expression of certain bacterial genes is regulated in a cell-density dependent way, a phenomenon called quorum sensing. Both Gram-negative and Gram-positive bacteria use this type of communication, though the signal molecules (auto-inducers) used by them differ between both groups: Gram-negative bacteria use predominantly N-acyl homoserine lacton (AHL) molecules (autoinducer-1, AI-1) while Gram-positive bacteria use mainly peptides (autoinducer peptides, AIP or quorum sensing peptides). These quorum sensing molecules are not only involved in the inter-microbial communication, but can also possibly cross-talk directly or indirectly with their host. This review summarizes the currently applied analytical approaches for quorum sensing identification and quantification with additionally summarizing the experimentally found in vivo concentrations of these molecules in humans.

Arachis hypogaea L. produces mimic and inhibitory quorum sensing like molecules

A wide variety of plant-associated soil bacteria (rhizobacteria) communicate with each other by quorum sensing (QS). Plants are able to detect and produce mimics and inhibitor molecules of the QS bacterial communicative process. Arachis hypogaea L. (peanut) establishes a nitrogen-fixing symbiosis with rhizobia belonging to the genus Bradyrhizobium. These bacteria use a QS mechanism dependent on the synthesis of N-acyl homoserine lactones (AHLs). Given the relevance that plant-rhizobacteria interactions have at the ecological level, this work addresses the involvement of peanut in taking part in the QS mechanism. By using biosensor bacterial strains capable of detecting AHLs, a series of standard and original bioassays were performed in order to determine both (i) the production of QS-like molecules in vegetal materials and (ii) the expression of the QS mechanism throughout plant-bacteria interaction. Mimic QS-like molecules (mQS) linked to AHLs with long acyl chains (lac-AHL), and inhibitor QS-like molecules (iQS) linked to AHLs with short acyl chains (sac-AHL) were detected in seed and root exudates. The results revealed that synthesis of specific signaling molecules by the plant (such as mQS and iQS) probably modulates the function and composition of the bacterial community established in its rhizosphere. Novel bioassays of QS detection during peanut-Bradyrhizobium interaction showed an intense production of QS signals in the contact zone between root and bacteria. It is demonstrated that root exudates stimulate the root colonization and synthesis of lac-AHL by Bradyrhizobium strains in the plant rhizosphere, which leads to the early stages of the development of beneficial plant-bacteria interactions.

Quorum Sensing Inhibitors from the Sea Discovered Using Bacterial N-acyl-homoserine Lactone-Based Biosensors

Marine natural products with antibiotic activity have been a rich source of drug discovery; however, the emergence of antibiotic-resistant bacterial strains has turned attention towards the discovery of alternative innovative strategies to combat pathogens. In many pathogenic bacteria, the expression of virulence factors is under the regulation of quorum sensing (QS). QS inhibitors (QSIs) present a promising alternative or potential synergistic treatment since they disrupt the signaling pathway used for intra- and interspecies coordination of expression of virulence factors. This review covers the set of molecules showing QSI activity that were isolated from marine organisms, including plants (algae), animals (sponges, cnidarians, and bryozoans), and microorganisms (bacteria, fungi, and cyanobacteria). The compounds found and the methods used for their isolation are the emphasis of this review.

Detection of Adulterants and Contaminants in Liquid Foods-A Review

Milk and fruit juices have paramount importance in human diet. Increasing demand of these liquid foods has made them vulnerable to economic adulteration during processing and in supply chain. Adulterants are difficult to detect by consumers and thus necessitating the requirement of rapid, accurate and sensitive detection. The potential adulterants in milk and fruit juices and their limits set by different regulatory bodies have been briefly described in this review. Potential advantages and limitations of various techniques such as physicochemical methods, chromatography, immunoassays, molecular, electrical, spectroscopy with chemometrics, electronic nose, and biosensors have been described. Spectroscopy in combination with chemometrics has shown potential for rapid, precise, and sensitive detection of potential adulterants in these liquid foods.

Characterization of LuxI and LuxR Protein Homologs of N-Acylhomoserine Lactone-Dependent Quorum Sensing System in Pseudoalteromonas sp. 520P1

Pseudoalteromonas sp. 520P1 (hereafter referred to as strain 520P1) produces N-acylhomoserine lactones (AHLs), which serve as signaling molecules in Gram-negative bacterial quorum sensing. In a previous genomic analysis of the 5.25-Mb genome of strain 520P1, we detected the presence of at least one homolog of the AHL synthase gene (luxI) and five homologs of the transcriptional regulator protein gene (luxR). The LuxI homolog of strain 520P1 (PalI) contained the conserved amino acid motifs shared by all the LuxI family proteins of the different species examined here. The palI gene expressed in Escherichia coli produced two types of AHLs. In the thin-layer chromatography analysis, these AHLs showed identical mobility to the AHLs produced by strain 520P1. The five LuxR homologs of strain 520P1 (PalR1-PalR5) shared only 17-34% amino acid sequence identity, although higher identities were observed in the C-terminal DNA-binding domain. Among the five PalRs, only PalR5 displayed close homology with LuxR family proteins from other Pseudoalteromonas strains. Notably, the palR3 and palI genes were located close together and only 1021 bases apart in the genome. No cognate luxI homolog associated with the four other palR genes was detected. These characteristics of PalI and the PalRs suggest that AHL autoinducers generated by the PalI enzyme might regulate cellular metabolism in cooperation with five transcriptional regulator PalRs, each of which is presumed to play a distinctive role in bacterial signaling.

Screening for N-AHSL-Based-Signaling Interfering Enzymes

Quorum sensing (QS)-based signaling is a widespread pathway used by bacteria for the regulation of functions involved in their relation to the environment or their host. QS relies upon the production, accumulation and perception of small diffusable molecules by the bacterial population, hence linking high gene expression with high cell population densities. Among the different QS signal molecules, an important class of signal molecules is the N-acyl homoserine lactone (N-AHSL). In pathogens such as Erwinia or Pseudomonas, N-AHSL based QS is crucial to overcome the host defenses and ensure a successful infection. Interfering with QS-regulation allows the algae Delisea pulcra to avoid surface colonization by bacteria. Thus, interfering the QS-regulation of pathogenic bacteria is a promising antibiotic-free antibacterial therapeutic strategy. To date, two N-AHSL lactonases and one amidohydrolase families of N-ASHL degradation enzymes have been characterized and have proven to be efficient in vitro to control N-AHSL-based QS-regulated functions in pathogens. In this chapter, we provide methods to screen individual clones or bacterial strains as well as pool of clones for genomic and metagenomic libraries, that can be used to identify strains or clones carrying N-ASHL degradation enzymes.

Modeling quorum sensing trade-offs between bacterial cell density and system extension from open boundaries

Bacterial communities undergo collective behavioural switches upon producing and sensing diffusible signal molecules; a mechanism referred to as Quorum Sensing (QS). Exemplarily, biofilm organic matrices are built concertedly by bacteria in several environments. QS scope in bacterial ecology has been debated for over 20 years. Different perspectives counterpose the role of density reporter for populations to that of local environment diffusivity probe for individual cells. Here we devise a model system where tubes of different heights contain matrix-embedded producers and sensors. These tubes allow non-limiting signal diffusion from one open end, thereby showing that population spatial extension away from an open boundary can be a main critical factor in QS. Experimental data, successfully recapitulated by a comprehensive mathematical model, demonstrate how tube height can overtake the role of producer density in triggering sensor activation. The biotic degradation of the signal is found to play a major role and to be species-specific and entirely feedback-independent.

Diverse Profiles of AI-1 Type Quorum Sensing Molecules in Cultivable Bacteria from the Mangrove (Kandelia obovata) Rhizosphere Environment

Mangrove rhizosphere environment harbors diverse populations of microbes, and some evidence showed that rhizobacteria behavior was regulated by quorum sensing (QS). Investigating the diverse profiles of QS molecules in mangrove ecosystems may shed light on the bacterial roles and lead to a better understanding of the symbiotic interactions between plants and microbes. The aims of the current study focus on identifying AI-1 type QS signals, i.e., acyl homoserine lactones (AHLs), in Kandelia obovata rhizosphere environment. Approximately 1200 rhizobacteria were screened and 184 strains (15.3%) tested were positive. Subsequent 16s rRNA gene sequencing and dereplication analyses identified 24 species from the positive isolates, which were affiliated to three different phyla, including Proteobacteria, Firmicutes, and Actinobacteria. Thin-layer chromatography separation of extracts revealed diverse AHL profiles and detected at least one active compound in the supernatant of these 24 cultivable AHL-producers. The active extracts from these bacterial isolates were further evaluated by ultra performance liquid chromatography-mass spectrometry, and the carbon side chain length ranged from C4 to C14. This is the first report on the diversity of AI-1 type auto-inducers in the mangrove plant K. obovata, and it is imperative to expand our knowledge of plant-bacteria interactions with respect to the maintenance of wetland ecosystem health.

N-(3-oxo-hexanoyl)-homoserine lactone has a critical contribution to the quorum-sensing-dependent regulation in phytopathogen Pseudomonas syringae pv. tabaci 11528

The phytopathogen Pseudomonas syringae pv. tabaci 11528 (P. syringae 11528), causing wild-fire disease in soybean and tobacco plants, processes PsyI-PsyR quorum-sensing (QS) system, in which PsyI is the N-(3-oxo-hexanoyl)-homoserine lactone (3OC6-HSL) synthase. In comparison to P. syringae 11528 AHL-deficient mutant, 845 3OC6-HSL-dependent genes were identified using RNA sequencing (RNA-seq) in the AHL-deficient mutant grown with exogenous 3OC6-HSL in the transition from the exponential to the stationary phase, and many of them were associated with virulence, which were negatively regulated. The gene ontology and KEGG pathway enrichment analysis of those genes presented that the most pronounced regulation was involved in bacterial motility. Moreover, similar expression profiles of genes during growth phases were observed in both the wild type and the AHL-deficient mutant with exogenous 3OC6-HSL compared with the AHL-deficient mutant. These findings imply that 3OC6-HSL has a critical contribution to the QS-dependent regulation on gene expression, and 3OC6-HSL-dependent regulation may play a significant role in plant infection.

Morphological and proteomic analysis of biofilms from the Antarctic archaeon, Halorubrum lacusprofundi

Biofilms enhance rates of gene exchange, access to specific nutrients, and cell survivability. Haloarchaea in Deep Lake, Antarctica, are characterized by high rates of intergenera gene exchange, metabolic specialization that promotes niche adaptation, and are exposed to high levels of UV-irradiation in summer. Halorubrum lacusprofundi from Deep Lake has previously been reported to form biofilms. Here we defined growth conditions that promoted the formation of biofilms and used microscopy and enzymatic digestion of extracellular material to characterize biofilm structures. Extracellular DNA was found to be critical to biofilms, with cell surface proteins and quorum sensing also implicated in biofilm formation. Quantitative proteomics was used to define pathways and cellular processes involved in forming biofilms; these included enhanced purine synthesis and specific cell surface proteins involved in DNA metabolism; post-translational modification of cell surface proteins; specific pathways of carbon metabolism involving acetyl-CoA; and specific responses to oxidative stress. The study provides a new level of understanding about the molecular mechanisms involved in biofilm formation of this important member of the Deep Lake community.

Profile of Citrobacter freundii ST2, a Multi-acyl-homoserine Lactone Producer Associated with Marine Dinoflagellates

Marine algae provide a unique niche termed the phycosphere for microorganism inhabitation. The phycosphere environment is an important niche for mutualistic and competitive interactions between algae and bacteria. Quorum sensing (QS) serves as a gene regulatory system in the microbial biosphere that allows bacteria to sense the population density with signaling molecules, such as acyl-homoserine lactone (AHL), and adapt their physiological activities to their surroundings. Understanding the QS system is important to elucidate the interactions between algal-associated microbial communities in the phycosphere condition. In this study, we isolated an epidermal bacterium (ST2) from the marine dinoflagellate Scrippsiella trochoidea and evaluated its AHL production profile. Strain ST2 was classified as a member of the genus Citrobacter closely related to Citrobacter freundii by 16S rRNA gene sequence analysis. Thin-layer chromatography revealed that C. freundii ST2 secreted three active AHL compounds into the culture supernatant. Specific compounds, such as N-butyryl-L-homoserine lactone (C4-AHL), N-octanoyl-DL-homoserine lactone (C8-AHL), and N-decanoyl-DL-homoserine lactone (C10-AHL), were identified by high-resolution tandem mass spectrometry. Carbon metabolic profiling with Biolog EcoPlate™ indicated that C. freundii ST2 was widely used as a carbon source and preferred carbohydrates, amino acids, and carboxylic acids as carbon substrates. Our results demonstrated that C. freundii ST2 is a multi-AHL producer that participates in the phycosphere carbon cycle.

Purification and characterization of Pseudomonas aeruginosa LasR expressed in acyl-homoserine lactone free Escherichia coli cultures

Quorum sensing systems are essential for bacterial communication. We report here the purification and characterization of the Pseudomonas aeruginosa LasR quorum sensing regulator purified from lysates of E. coli cultures grown in the absence of added acyl-homoserine lactones (AHL). We show by isothermal titration calorimetry that LasR recognizes different AHLs with an affinity of approximately 1 μM. The affinity of LasR for its cognate 3-Oxo-C12-AHL was similar to that of other AHLs, indicating that this regulator has not evolved to preferentially recognize its cognate AHL. The α-helical content as determined by CD spectroscopy was found to be in agreement with the corresponding value derived from the homology model. Analytical ultracentrifugation studies show that LasR is a mixture of monomers and dimers and that AHL binding does not alter its oligomeric state. Thermal unfolding studies indicate that LasR has a significant thermal stability and that AHL binding does not significantly alter the unfolding temperature. Two LasR-DNA complexes were observed in electrophoretic mobility shift assays using the hcnABC promoter that has two lux boxes. Taken together, data indicate that the presence of AHLs is not a requisite for correct LasR protein folding. The protein is able to bind AHL ligands in a reversible manner, revising initial concepts of this regulator. The availability of AHL-free protein will permit further studies to determine more precisely its mode of action.

Diverse profiles of N-acyl-homoserine lactones in biofilm forming strains of Cronobacter sakazakii

The present study investigates the role of quorum sensing (QS) molecules expressed by C. sakazakii in biofilm formation and extracellular polysaccharide expression. The QS signaling was detected using Chromobacterium violaceum 026 and Agrobacterium tumefaciens NTL4(pZLR4) based bioassay. Long chain N-acyl-homoserine lactones (AHLs) with C6- C18 chain length were identified using High Performance Liquid Chromatography and Liquid Chromatography-High Resolution Mass Spectrometry. A higher Specific Biofilm Formation (SBF) index (p < 0.05) with the presence of genes associated with cellulose biosynthesis (bcsA, bcsC and bcsG) was observed in the strains. AHLs and their mechanisms can serve as novel targets for developing technologies to eradicate and prevent biofilm formation by C. sakazakii.

LasR Variant Cystic Fibrosis Isolates Reveal an Adaptable Quorum-Sensing Hierarchy in Pseudomonas aeruginosa

Chronic Pseudomonas aeruginosa infections cause significant morbidity in patients with cystic fibrosis (CF). Over years to decades, P. aeruginosa adapts genetically as it establishes chronic lung infections. Nonsynonymous mutations in lasR, the quorum-sensing (QS) master regulator, are common in CF. In laboratory strains of P. aeruginosa, LasR activates transcription of dozens of genes, including that for another QS regulator, RhlR. Despite the frequency with which lasR coding variants have been reported to occur in P. aeruginosa CF isolates, little is known about their consequences for QS. We sequenced lasR from 2,583 P. aeruginosa CF isolates. The lasR sequences of 580 isolates (22%) coded for polypeptides that differed from the conserved LasR polypeptides of well-studied laboratory strains. This collection included 173 unique lasR coding variants, 116 of which were either missense or nonsense mutations. We studied 31 of these variants. About one-sixth of the variant LasR proteins were functional, including 3 with nonsense mutations, and in some LasR-null isolates, genes that are LasR dependent in laboratory strains were nonetheless expressed. Furthermore, about half of the LasR-null isolates retained RhlR activity. Therefore, in some CF isolates the QS hierarchy is altered such that RhlR quorum sensing is independent of LasR regulation. Our analysis challenges the view that QS-silent P. aeruginosa is selected during the course of a chronic CF lung infection. Rather, some lasR sequence variants retain functionality, and many employ an alternate QS strategy involving RhlR.

Chronic Pseudomonas aeruginosa infections, such as those in patients with the genetic disease cystic fibrosis, are notable in that mutants with defects in the quorum-sensing transcription factor LasR frequently arise. In laboratory strains of P. aeruginosa, quorum sensing activates transcription of dozens of genes, many of which encode virulence factors, such as secreted proteases and hydrogen cyanide synthases. In well-studied laboratory strains, LasR-null mutants have a quorum-sensing-deficient phenotype. Therefore, the presence of LasR variants in chronic infections has been interpreted to indicate that quorum-sensing-regulated products are not important for those infections. We report that some P. aeruginosa LasR variant clinical isolates are not LasR-null mutants, and others have uncoupled a second quorum-sensing system, the RhlR system, from LasR regulation. In these uncoupled isolates, RhlR independently activates at least some quorum-sensing-dependent genes. Our findings suggest that quorum sensing plays a role in chronic P. aeruginosa infections, despite the emergence of LasR coding variants.

Genome sequencing-assisted identification and the first functional validation of N-acyl-homoserine-lactone synthases from the Sphingomonadaceae family

BACKGROUND

Members of the genus Novosphingobium have been isolated from a variety of environmental niches. Although genomics analyses have suggested the presence of genes associated with quorum sensing signal production e.g., the N-acyl-homoserine lactone (AHL) synthase (luxI) homologs in various Novosphingobium species, to date, no luxI homologs have been experimentally validated.

METHODS

In this study, we report the draft genome of the N-(AHL)-producing bacterium Novosphingobium subterraneum DSM 12447 and validate the functions of predicted luxI homologs from the bacterium through inducible heterologous expression in Agrobacterium tumefaciens strain NTL4. We developed a two-dimensional thin layer chromatography bioassay and used LC-ESI MS/MS analyses to separate, detect and identify the AHL signals produced by the N. subterraneum DSM 12447 strain.

RESULTS

Three predicted luxI homologs were annotated to the locus tags NJ75_2841 (NovINsub1), NJ75_2498 (NovINsub2), and NJ75_4146 (NovINsub3). Inducible heterologous expression of each luxI homologs followed by LC-ESI MS/MS and two-dimensional reverse phase thin layer chromatography bioassays followed by bioluminescent ccd camera imaging indicate that the three LuxI homologs are able to produce a variety of medium-length AHL compounds. New insights into the LuxI phylogeny was also gleemed as inferred by Bayesian inference.

DISCUSSION

This study significantly adds to our current understanding of quorum sensing in the genus Novosphingobium and provide the framework for future characterization of the phylogenetically interesting LuxI homologs from members of the genus Novosphingobium and more generally the family Sphingomonadaceae.

Transcriptome analysis revealed that a quorum sensing system regulates the transfer of the pAt megaplasmid in Agrobacterium tumefaciens

Background

Agrobacterium tumefaciens strain P4 is atypical, as the strain is not pathogenic and produces a for this species unusual quorum sensing signal, identified as N-(3-hydroxy-octanoyl)-homoserine lactone (3OH,C8-HSL).

Results

By sequence analysis and cloning, a functional luxI-like gene, named cinI, has been identified on the At plasmid of A. tumefaciens strain P4. Insertion mutagenesis in the cinI gene and transcriptome analyses permitted the identification of 32 cinI-regulated genes in this strain, most of them encoding proteins responsible for the conjugative transfer of pAtP4. Among these genes were the avhB genes that encode a type 4 secretion system (T4SS) involved in the formation of the conjugation apparatus, the tra genes that encode the DNA transfer and replication (Dtr) machinery and cinI and two luxR orthologs. These last two genes, cinR and cinX, exhibit an unusual organization, with the cinI gene surrounded by the two luxR orthologs. Conjugation experiments confirmed that the conjugative transfer of pAtP4 is regulated by 3OH,C8-HSL. Root colonization experiments indicated that the quorum sensing regulation of the conjugation of the pAtP4 does not confer a gain or a loss of fitness to the bacterial host in the tomato plant rhizosphere.

Conclusion

This work is the first identification of the occurrence of a quorum sensing regulation of the pAt conjugation phenomenon in Agrobacterium.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-3007-5) contains supplementary material, which is available to authorized users.

Acyl-homoserine lactone-based quorum sensing and quorum quenching hold promise to determine the performance of biological wastewater treatments: An overview

Quorum sensing (QS) is a communication process between cells, in which bacteria secrete and sense the specific chemicals, and regulate gene expression in response to population density. Quorum quenching (QQ) blocks QS system, and inhibits gene expression mediating bacterial behaviors. Given the extensive research of acyl-homoserine lactone (AHL) signals, existences and effects of AHL-based QS and QQ in biological wastewater treatments are being subject to high concern. This review summarizes AHL structure, synthesis mode, degradation mechanisms, analytical methods, environmental factors, AHL-based QS and QQ mechanisms. The existences and roles of AHL-based QS and QQ in biomembrane processes, activated sludge processes and membrane bioreactors are summarized and discussed, and corresponding exogenous regulation strategy by selective enhancement of AHL-based QS or QQ coexisting in biological wastewater treatments is suggested. Such strategies including the addition of AHL signals, AHL-producing bacteria as well as quorum quenching enzyme or bacteria can effectively improve wastewater treatment performance without killing or limiting bacterial survival and growth. This review will present the theoretical and practical cognition for bacterial AHL-based QS and QQ, suggest the feasibility of exogenous regulation strategies in biological wastewater treatments, and provide useful information to scientists and engineers who work in this field.

Involvement of Acylated Homoserine Lactones (AHLs) of Aeromonas sobria in Spoilage of Refrigerated Turbot (Scophthalmus maximus L.)

One quorum sensing strain was isolated from spoiled turbot. The species was determined by 16S rRNA gene analysis and classical tests, named Aeromonas sobria AS7. Quorum-sensing (QS) signals (N-acyl homoserine lactones (AHLs)) were detected by report strains and their structures were further determined by GC-MS. The activity changes of AHLs on strain growth stage as well as the influence of different culture conditions on secretion activity of AHLs were studied by the punch method. The result indicated that strain AS7 could induce report strains to produce typical phenotypic response. N-butanoyl-dl-homoserine lactone (C₄-HSL), N-hexanoyl-dl-homoserine lactone (C₆-HSL), N-octanoyl-dl-homoserine lactone (C₈-HSL), N-decanoyl-dl-homoserine lactone (C10-HSL), N-dodecanoyl-dl-homoserine lactone (C12-HSL) could be detected. The activities of AHLs were density-dependent and the max secretion level was at pH 8, sucrose culture, 1% NaCl and 32 h, respectively. The production of siderophore in strain AS7 was regulated by exogenous C₈-HSL, rather than C₆-HSL. Exogenous C₄-HSL and C₈-HSL accelerated the growth rate and population density of AS7 in turbot samples under refrigerated storage. However, according to the total viable counts and total volatile basic nitrogen (TVB-N) values of the fish samples, exogenous C₆-HSL did not cause spoilage of the turbot fillets. In conclusion, our results suggested that QS was involved in the spoilage of refrigerated turbot.

Degradation intermediates of polyhydroxy butyrate inhibits phenotypic expression of virulence factors and biofilm formation in luminescent Vibrio sp. PUGSK8

Luminescent vibrios are ubiquitous in the marine environment and are the causative agents of vibriosis and mass mortality in many aquatic animals. In aquatic environments, treatments cannot be limited to the diseased population alone, therefore treatment of the entire aquatic system is the only possible approach. Thus, the use of antibiotics to treat part of the infected animals requires a dose based on the entire biomass, which results in the treatment of uninfected animals as well as non-target normal microbial flora. A treatment method based on anti-virulence or quorum quenching has recently been proposed as an effective treatment strategy for aquatic animals. Polyhydroxy butyrates (PHB) are bacterial storage molecules, which accumulate in cells under nutritional stress. The degradation of PHB releases short-chain β-hydroxy butyric acid, which may act as anti-infective molecule. To date, there is very limited information on the potential anti-infective and anti-virulence mechanisms involving PHB. In this study, we aim to examine the effect of PHB on inhibition of the virulence cascade of Vibrio such as biofilm formation, luminescence, motility behaviour, haemolysin and quorum sensing. A luminescent Vibrio PUGSK8, tentatively identified as Vibrio campbellii PUGSK8 was tested in vitro for production of extracellular virulence factors and then established as a potential shrimp pathogen based on in vivo challenge experiments. The ability of Vibrio PUGSK8 to form biofilms and the effect of PHB on biofilm formation was tested in a 96-well microtitre-plate assay system. The motility behaviour of Vibrio PUGSK8 was evaluated using twitching, swimming and swarming plate assays. Reporter strains such as Chromobacterium violaceum CV026 and Agrobacterium tumefaciens were used to detect quorum-sensing molecules. Gas chromatography-mass spectrometry spectral analysis was performed to elucidate the fragmentation pattern and structure of N-hexanoyl homoserine lactone. PHB depolymerase activity in Vibrio PUGSK8 was quantified as the amount of the enzyme solution to hydrolyse 1 μg of PHB per min. An in vivo challenge experiment was performed using a gnotobiotic Artemia assay. Of the 27 isolates tested, the Vibrio PUGSK8 strain was selected for target-specific assays based on the high intensity of luminescence and production of virulence factors. The virulence cascade detected in Vibrio PUGSK8 include luminescence, motility behaviour, biofilm formation, quorum sensing and haemolysin production. Thus inhibition/degradation of the virulence cascade would be an effective approach to contain Vibrio infections in aquatic animals. In this report, we demonstrate that the degradation intermediate of PHB effectively inhibits biofilm formation, luminescence, motility behaviour, haemolysin production and the N-acyl-homoserine lactone (AHL)-mediated quorum-sensing pathway in PUGSK8. Interestingly, the growth of Vibrio PUGSK8 remains unaffected in the presence of PHB, with PHB degradation being detected in the media. PHB depolymerase activity in Vibrio PUGSK8 results in the release of degradation intermediates include a short-chain β-hydroxy butyric acid, which inhibits the virulence cascade in Vibrio PUGSK8. Thus, a molecule that targets quorum sensing and the virulence cascade and which is species/strain-specific could prove to be an effective alternative to antimicrobial agents to control the pathogenesis of Vibrio, and thereby help to contain Vibrio outbreaks in aquatic systems.

Non-thermal Plasma Exposure Rapidly Attenuates Bacterial AHL-Dependent Quorum Sensing and Virulence

The antimicrobial activity of atmospheric pressure non-thermal plasma has been exhaustively characterised, however elucidation of the interactions between biomolecules produced and utilised by bacteria and short plasma exposures are required for optimisation and clinical translation of cold plasma technology. This study characterizes the effects of non-thermal plasma exposure on acyl homoserine lactone (AHL)-dependent quorum sensing (QS). Plasma exposure of AHLs reduced the ability of such molecules to elicit a QS response in bacterial reporter strains in a dose-dependent manner. Short exposures (30-60 s) produce of a series of secondary compounds capable of eliciting a QS response, followed by the complete loss of AHL-dependent signalling following longer exposures. UPLC-MS analysis confirmed the time-dependent degradation of AHL molecules and their conversion into a series of by-products. FT-IR analysis of plasma-exposed AHLs highlighted the appearance of an OH group. In vivo assessment of the exposure of AHLs to plasma was examined using a standard in vivo model. Lettuce leaves injected with the rhlI/lasI mutant PAO-MW1 alongside plasma treated N-butyryl-homoserine lactone and n-(3-oxo-dodecanoyl)-homoserine lactone, exhibited marked attenuation of virulence. This study highlights the capacity of atmospheric pressure non-thermal plasma to modify and degrade AHL autoinducers thereby attenuating QS-dependent virulence in P. aeruginosa.

The algicidal activity of Aeromonas sp. strain GLY-2107 against bloom-forming Microcystis aeruginosa is regulated by N-acyl homoserine lactone-mediated quorum sensing

Cyanobacterial blooms have disrupted the efficient utilization of freshwater worldwide. A new freshwater bacterial strain with strong algicidal activity, GLY-2107, was isolated from Lake Taihu and identified as Aeromonas sp. It produced two algicidal compounds: 2107-A (3-benzyl-piperazine-2,5-dione) and 2107-B (3-methylindole). Both compounds exhibited potent algicidal activities against Microcystis aeruginosa, the dominant bloom-forming cyanobacterium in Lake Taihu. The EC₅₀ values (concentration for 50% maximal effect) of 3-benzyl-piperazine-2,5-dione and 3-methylindole were 4.72 and 1.10 μg ml^-1 respectively. Based on a thin-layer chromatography biosensor assay and ultra-performance liquid chromatography-coupled high resolution-tandem mass spectrometry (UPLC-HRMS/MS), the N-acyl homoserine lactone (AHL) profile of strain GLY-2107 was identified as two short side-chain AHLs: N-butyryl-homoserine lactone (C4-HSL) and N-hexanoyl-homoserine lactone (C6-HSL). The production of the two algicidal compounds was controlled by AHL-mediated quorum sensing (QS), and C4-HSL was the key QS signal for the algicidal activity of the strain GLY-2107. Moreover, 3-methylindole was found to be positively regulated by C4-HSL-mediated QS, whereas 3-benzyl-piperazine-2,5-dione might be negatively controlled by C4-HSL-mediated QS. This study suggests that a QS-regulated algicidal system may have potential use for the development of a novel control strategy for harmful cyanobacterial blooms.

Selection of the N-Acylhomoserine Lactone-Degrading Bacterium Alteromonas stellipolaris PQQ-42 and of Its Potential for Biocontrol in Aquaculture

The production of virulence factors by many pathogenic microorganisms depends on the intercellular communication system called quorum sensing, which involves the production and release of signal molecules known as autoinducers. Based on this, new-therapeutic strategies have emerged for the treatment of a variety of infections, such as the enzymatic degradation of signaling molecules, known as quorum quenching (QQ). In this study, we present the screening of QQ activity amongst 450 strains isolated from a bivalve hatchery in Granada (Spain), and the selection of the strain PQQ-42, which degrades a wide range of N-acylhomoserine lactones (AHLs). The selected strain, identified as Alteromonas stellipolaris, degraded the accumulation of AHLs and reduced the production of protease and chitinase and swimming motility of a Vibrio species in co-cultivation experiments in vitro. In the bio-control experiment, strain PQQ-42 significantly reduced the pathogenicity of Vibrio mediterranei VibC-Oc-097 upon the coral Oculina patagonica showing a lower degree of tissue damage (29.25 ± 14.63%) in its presence, compared to when the coral was infected with V. mediterranei VibC-Oc-097 alone (77.53 ± 13.22%). Our results suggest that this AHL-degrading bacterium may have biotechnological applications in aquaculture.

Programmed Lab Experiments for Biochemical Investigation of Quorum-Sensing Signal Molecules in Rhizospheric Soil Bacteria

Biochemistry courses in the Department of Molecular Biology at the National University of Río Cuarto, Argentina, are designed for undergraduate students in biology, microbiology, chemistry, agronomy, and veterinary medicine. Microbiology students typically have previous coursework in general, analytical, and organic chemistry. Programmed sequences of lab experiments allow these students to investigate biochemical problems whose solution is feasible within the context of their knowledge and experience. We previously designed and reported a programmed lab experiment that familiarizes microbiology students with techniques for detection and characterization of quorum-sensing (QS) and quorum-quenching (QQ) signal molecules. Here, we describe a sequence of experiments designed to expand the understanding and capabilities of biochemistry students using techniques for extraction and identification of QS and QQ signal molecules from peanut rhizospheric soil bacteria, including culturing and manipulation of bacteria under sterile conditions. The program provides students with an opportunity to perform useful assays, draw conclusions from their results, and discuss possible extensions of the study. © 2016 by The International Union of Biochemistry and Molecular Biology, 44:256-262, 2016.

The effect of mutation in the clpX gene on the synthesis of N-acyl-homoserine lactones and other properties of Burkholderia cenocepacia 370

In order to study the regulation of N-acyl-homoserine lactones synthesis (AHLs, the signal molecules of Quorum Sensing regulation) in Burkholderia cenocepacia strain 370 we obtained mutants with increased AHL production. One of the mutants, named BC-B6, was obtained by TnMod-RKm(r) plasposon mutagenesis. The plasposon insertion was located within the clpX gene encoding the ATPase subunit ClpX of the ClpXP protease. The mutation reduced bacterial virulence in mice intranasal infection. The results of proteomic analysis demonstrated that the expression of at least 19 proteins differed not less than 2-fold between the parental and mutant strains. 18 of the proteins were upregulated in the mutant, and one protein was downregulated. The proteins included those that involved in protein synthesis and modification, in energy production, in general metabolism, in transport and regulation. To check the effect of the clpX mutation on the AHL synthesis, a mutant with inactivated clpX gene (BC-clpX:Km(r)) was constructed by gene replacement method. This mutant also exhibited increased AHLs production. A swarming motility of both mutants was reduced compared to the original strain. Thus, the obtained results show that the clpX gene was involved in the regulation of AHL production and a number of cellular processes in B. cenocepacia 370.

Construction and phenotypic characterization of M68, an RruI quorum sensing knockout mutant of the photosynthetic alphaproteobacterium Rhodospirillum rubrum

Many bacterial species communicate using a complex system known as quorum sensing (QS) in which gene expression is controlled in response to cell density. In this study an N-acylhomoserine lactone (AHL) synthase (Rru_A3396) knockout mutant (M68) of Rhodospirillum rubrum S1H (WT) was constructed and characterized phenotypically under light anaerobic conditions. Results showed that R. rubrum WT produces unsubstituted, 3-OH and 3-oxo-substituted AHLs with acyl chains ranging from 4 to 14 carbons, with 3-OH-C8 being the most abundant. Growth, pigment content and swimming motility were found to be under the control of this LuxI-type QS system. In addition, cultivation in a low shear environment put forward the aggregative phenotype of M68 and linked biofilm formation to QS in R. rubrum S1H. Interestingly, QS-mutant M68 continued to produce decreased levels of 3-OH-C8-HSL, probably due to the presence of an extra HdtS-type AHL synthase.

Phaeobacter sp. strain Y4I utilizes two separate cell-to-cell communication systems to regulate production of the antimicrobial indigoidine

The marine roseobacter Phaeobacter sp. strain Y4I synthesizes the blue antimicrobial secondary metabolite indigoidine when grown in a biofilm or on agar plates. Prior studies suggested that indigoidine production may be, in part, regulated by cell-to-cell communication systems. Phaeobacter sp. strain Y4I possesses two luxR and luxI homologous N-acyl-L-homoserine lactone (AHL)-mediated cell-to-cell communication systems, designated pgaRI and phaRI. We show here that Y4I produces two dominantAHLs, the novel monounsaturated N-(3-hydroxydodecenoyl)-L-homoserine lactone (3OHC(12:1)-HSL) and the relatively common N-octanoyl-L-homoserine lactone (C8-HSL), and provide evidence that they are synthesized by PhaI and PgaI, respectively.A Tn5 insertional mutation in either genetic locus results in the abolishment (pgaR::Tn5) or reduction (phaR::Tn5) of pigment production. Motility defects and denser biofilms were also observed in these mutant backgrounds, suggesting an overlap in the functional roles of these systems. Production of the AHLs occurs at distinct points during growth on an agar surface and was determined by isotope dilution high-performance liquid chromatography–tandem mass spectrometry (ID-HPLC-MS/MS) analysis.Within 2 h of surface inoculation, only 3OHC(12:1)-HSL was detected in agar extracts. As surface-attached cells became established (at approximately 10 h), the concentration of 3OHC(12:1)-HSL decreased, and the concentration of C8-HSL increased rapidly over 14 h.After longer (>24-h) establishment periods, the concentrations of the two AHLs increased to and stabilized at approximately 15 nM and approximately 600 nM for 3OHC12:1-HSL and C8-HSL, respectively. In contrast, the total amount of indigoidine increased steadily from undetectable to 642 Mby 48 h. Gene expression profiles of the AHL and indigoidine synthases (pgaI, phaI, and igiD) were consistent with their metabolite profiles. These data provide evidence that pgaRI and phaRI play overlapping roles in the regulation of indigoidine biosynthesis, and it is postulated that this allows Phaeobacter sp. strain Y4I to coordinate production of indigoidine with different growth-phase-dependent physiologies.

Synthesis and analysis of stable isotope-labelled N-acyl homoserine lactones

An easy, reliable manner to make suitable, deuterated standards of AHL-molecules belonging to all three important classes of AHLs is presented, starting from a cheap and commercially available deuterium source.

Attenuation of quorum sensing-mediated virulence of Acinetobacter baumannii by Glycyrrhiza glabra flavonoids

AIM

To develop an alternative quorum quenching therapy against multidrug-resistant Acinetobacter baumannii.

METHODS & RESULTS

Activity-guided partially purified fraction (F1) from Glycyrrhiza glabra significantly (p < 0.05) reduced quorum sensing regulated virulence factors of A. baumannii viz. motility, biofilm formation and production of antioxidant enzymes. Mechanistically, F1 downregulated the expression of autoinducer synthase gene, abaI, and consequently reduced (92%) the production of 3-OH-C12-HSL as determined by ESI-MS. Q-TOF and Q-TRAP analyses suggested the presence of flavonoids viz. licoricone, glycyrin and glyzarin as the active ingredients.

CONCLUSION

This is the first report on quorum quenching activity of G. glabra linked to its flavonoids that downregulated the expression of abaI and attenuated quorum sensing regulated virulence of A. baumannii.

Functional analysis of the aefR mutation and identification of its binding site in Pseudomonas syringae pv. tabaci 11528

The TetR family transcriptional regulator AefR contributes to the regulation of the quorum-sensing system. However, the role of AefR in the regulatory network of the phytopathogen Pseudomonas syringae pathovars is not known. In this study, the phenotype of a P. syringae pv. tabaci 11528 aefR deletion mutant strain was examined. The aefR gene expression and AefR DNA-binding affinity were examined by quantitative real-time polymerase chain reaction and electrophoretic mobility shift assay, respectively. AefR was found to control quorum-sensing genes as well as the efflux genes mexE, mexF, and oprN via an indirect mechanism. AefR binds to its own operator site as well as to the palindromic sequence between positions -28 and -2 corresponding to the transcription start site of aefR, as determined by dye primer sequencing. These results suggest that P. syringae AefR modulates quorum sensing and efflux as well as its own expression, which can be exploited by strategies developed to manage this plant parasite.