Identification of quorum-sensing signal molecules and the LuxRI homologs in fish pathogen Edwardsiella tarda

New dienelactone hydrolase from microalgae bacterial community-Antibiofilm activity against fish pathogens and potential applications for aquaculture

Biofilms are resistant to many traditional antibiotics, which has led to search for new antimicrobials from different and unique sources. To harness the potential of aquatic microbial resources, we analyzed the meta-omics datasets of microalgae-bacteria communities and mined them for potential antimicrobial and quorum quenching enzymes. One of the most interesting candidates (Dlh3), a dienelactone hydrolase, is a α/β-protein with predicted eight α-helices and eight β-sheets. When it was applied to one of the major fish pathogens, Edwardsiella anguillarum, the biofilm development was reproducibly inhibited by up to 54.5%. The transcriptome dataset in presence of Dlh3 showed an upregulation in functions related to self-defense like active genes for export mechanisms and transport systems. The most interesting point regarding the biotechnological potential for aquaculture applications of Dlh3 are clear evidence of biofilm inhibition and that health and division of a relevant fish cell model (CHSE-214) was not impaired by the enzyme.

Quorum sensing signals: Aquaculture risk factor

Bacteria produce several virulence factors and cause massive mortality in fish and crustaceans. Abundant quorum sensing (QS) signals and high cell density are essentially required for the production of such virulence factors. Although several strategies have been developed to control aquatic pathogens through antibiotics and QS inhibition, the impact of pre‐existing QS signals in the aquatic environment has been overlooked. QS signals cause detrimental effects on mammalian cells and induce cell death by interfering with multiple cellular pathways. Moreover, QS signals not only function as a messenger, but also annihilate the functions of the host immune system which implies that QS signals should be designated as a major virulence factor. Despite QS signals' role has been well documented in mammalian cells, their impact on aquatic organisms is still at the budding stage. However, many aquatic organisms produce enzymes that degrade and detoxify such QS signals. In addition, physical and chemical factors also determine the stability of the QS signals in the aqueous environment. The balance between QS signals and existing QS signals degrading factors essentially determines the disease progression in aquatic organisms. In this review, we highlight the impact of QS signals on aquatic organisms and further discussed potential alternative strategies to control disease progression.

Piscirickettsia salmonis Produces a N-Acetyl-L-Homoserine Lactone as a Bacterial Quorum Sensing System-Related Molecule

Piscirickettsia salmonis is the etiological agent of piscirickettsiosis, the most prevalent disease in salmonid species in Chilean salmonids farms. Many bacteria produce N-acyl-homoserine lactones (AHLs) as a quorum-sensing signal molecule to regulate gene expression in a cell density-dependent manner, and thus modulate physiological characteristics and several bacterial mechanisms. In this study, a fluorescent biosensor system method and gas chromatography-tandem mass spectrometry (GC/MS) were combined to detect AHLs produced by P. salmonis. These analyses revealed an emitted fluorescence signal when the biosensor P. putida EL106 (RPL4cep) was co-cultured with both, P. salmonis LF-89 type strain and an EM-90-like strain Ps007, respectively. Furthermore, the production of an AHL-type molecule was confirmed by GC/MS by both P. salmonis strains, which identified the presence of a N-acetyl-L-homoserine Lactone in the supernatant extract. However, It is suggested that an alternate pathway could synthesizes AHLs, which should be address in future experiments in order to elucidate this important bacterial process. To the best of our knowledge, the present report is the first to describe the type of AHLs produced by P. salmonis.

Bacillus spp. Inhibit Edwardsiella tarda Quorum-Sensing and Fish Infection

The disruption of pathogen communication or quorum-sensing (QS) via quorum-quenching (QQ) molecules has been proposed as a promising strategy to fight bacterial infections. Bacillus spp. have recognizable biotechnology applications, namely as probiotic health-promoting agents or as a source of natural antimicrobial molecules, including QQ molecules. This study characterized the QQ potential of 200 Bacillus spp., isolated from the gut of different aquaculture fish species, to suppress fish pathogens QS. Approximately 12% of the tested Bacillus spp. fish isolates (FI). were able to interfere with synthetic QS molecules. Ten isolates were further selected as producers of extracellular QQ-molecules and their QQ capacity was evaluated against the QS of important aquaculture bacterial pathogens, namely Aeromonas spp., Vibrio spp., Photobacterium damselae, Edwardsiela tarda, and Shigella sonnei. The results revealed that A. veronii and E. tarda produce QS molecules that are detectable by the Chr. violaceum biosensor, and which were degraded when exposed to the extracellular extracts of three FI isolates. Moreover, the same isolates, identified as B. subtilis, B. vezelensis, and B. pumilus, significantly reduced the pathogenicity of E. tarda in zebrafish larvae, increasing its survival by 50%. Taken together, these results identified three Bacillus spp. capable of extracellularly quenching aquaculture pathogen communication, and thus become a promising source of bioactive molecules for use in the biocontrol of aquaculture bacterial diseases.

Target genes directly regulated by Eha are required for Edwardsiella tarda survival within macrophages

Eha is a virulence regulator in Edwardsiella tarda (E. tarda). The present study examined how Eha regulated its target genes to affect the bacterial survival within the cells. We constructed the reporter a pGEX-4T-ehaflag plasmid expressing Eha tagged at its C terminus with the flag epitope, and introduced the plasmid into an eha mutant ET13 strain, and obtained a Cehaflag strain. The expression and activity of an EhaFlag fusion protein restored the survival of the Cehaflag as the wild type in macrophages by Western blotting and intracellular survival experiments. We used a monoclonal anti-Flag antibody to precipitate EhaFlag-DNA complexes using chromatic immunoprecipitation (ChIP). We then designed primers based on the differentially-expressed genes identified from RNA-sequencing, and identified ten Eha-interacting genes by qPCR. We amplified the promoter regions of the ten genes and the eha gene from ET13 strain by PCR, constructed pBD-P_targetlacZ and pBD-P_ehalacZ plasmids. The eha gene directly and positively regulated these target genes, and be negatively auto-regulated by Eha in E. tarda, as determined by comparing their β-Galactosidase activities. These target genes were distributed in the categories involved in the bacterial growth, movement and resistance to H₂O₂ or acid. We further constructed a ETATCC_RS15225 mutant (△dcuA1), a ETATCC_ RS14855 mutant (△flgK) anda ETATCC_RS07650 mutant (ΔtnaA), and a partial complementary strains of △eha-tnaA and △eha-flgK and the complementary strains of CΔflgK and CΔtnaA. The ETATCC_RS15225 gene probably encoded a transporter protein DcuA1 at outer membrane with SDS-PAGE and RT-PCR. The ETATCC _RS14855 gene probably encoded FlgK protein and affected the bacterial motility. The ETATCC_RS07650 gene encoded Tryptophanase, which affected the bacterial survival within macrophages. With the assistance of these above strains, our results showed that the eha gene was able to regulate the ETATCC_RS15225 gene to express its outer membrane protein DcuA1, the ETATCC _RS14855 gene to control the flagellar motility and the ETATCC_RS07650 to affect the bacterial survival within macrophages. With the combination of other functions of above three genes, our results suggested that Eha directly regulates the target genes to affect E. tarda to survive within the cells.

Influence of N-acylhomoserine lactonase silver nanoparticles on the quorum sensing system of Helicobacter pylori: A potential strategy to combat biofilm formation

The treatment of Helicobacter pylori usually fails due to their ability to form biofilms and resistance to antibiotics. This might potentially lead to gastric carcinoma and mucosa-associated lymphoid tissue lymphoma. In the present study, we elucidate the potential role of N-acylhomoserine lactonase stabilized silver nanoparticles (AiiA-AgNPs) in treating biofilms produced by H. pylori. AiiA-AgNPs inhibited quorum sensing (QS) by degradation of QS molecules, thereby reducing biofilm formation, urease production, and altering cell surface hydrophobicity of H. pylori. AiiA-AgNPs showed no cytotoxic effects on RAW 264.7 macrophages at the effective concentration (1-5 µM) of antibiofilm activity. In addition, AiiA-AgNP in high concentration (80-100 µM) exhibited cytotoxicity against HCT-15 carcinoma cells, depicting its therapeutic role in treating cancer.

Saline Environments as a Source of Potential Quorum Sensing Disruptors to Control Bacterial Infections: A Review

Saline environments, such as marine and hypersaline habitats, are widely distributed around the world. They include sea waters, saline lakes, solar salterns, or hypersaline soils. The bacteria that live in these habitats produce and develop unique bioactive molecules and physiological pathways to cope with the stress conditions generated by these environments. They have been described to produce compounds with properties that differ from those found in non-saline habitats. In the last decades, the ability to disrupt quorum-sensing (QS) intercellular communication systems has been identified in many marine organisms, including bacteria. The two main mechanisms of QS interference, i.e., quorum sensing inhibition (QSI) and quorum quenching (QQ), appear to be a more frequent phenomenon in marine aquatic environments than in soils. However, data concerning bacteria from hypersaline habitats is scarce. Salt-tolerant QSI compounds and QQ enzymes may be of interest to interfere with QS-regulated bacterial functions, including virulence, in sectors such as aquaculture or agriculture where salinity is a serious environmental issue. This review provides a global overview of the main works related to QS interruption in saline environments as well as the derived biotechnological applications.

Effective quorum quenching with a conformation-stable recombinant lactonase possessing a hydrophilic polymeric shell fabricated via electrospinning

Quorum sensing (QS) in Gram-negative bacteria is frequently regulated by the diffusible signal N-acylhomoserine lactone (AHL) along with the production of virulence factors in pathogens. To inhibit QS, we fabricated heat-resistant, long-term-stable AHL-lactonase AiiM by electrospinning (ES) aqueous polyvinyl alcohol (PVA) solution containing genetically engineered AiiM with a maltose-binding protein (MBP) tag. MBP-AiiM was immobilized via its inclusion within a dense PVA shell formed during the drying process of ES, followed by cross-linking between hydroxyl groups on PVA. Secondary structure analysis via circular dichroism suggested no conformational change in the MBP-AiiM during ES. Even after pre-heating of MBP-AiiM/PVA fiber mats at 70 °C for 24 h, QS-dependent prodigiosin production in the model pathogen Serratia marcescens AS-1 was effectively inhibited to 0.13% that of the control. Additionally, relative prodigiosin production was reduced to ~20% that of the control after 5-month storage in buffer solution. These results suggest that a shear-thinning process using an entangled PVA aggregate during elongational changes to fibrous domains and a drying process during ES contributes not to enzymatic inactivation caused by conformational changes, but rather to the fabrication of a dense PVA shell around the MBP-AiiM molecules to protect them from disruptors including heating. The developed quorum-quenching enzyme has high potential to inhibit AHL-mediated QS frequently appearing in various Gram-negative bacteria.

Effects of Sulfide Flavors on AHL-Mediated Quorum Sensing and Biofilm Formation of Hafnia alvei

In this study, 10 different sulfide flavor compounds commonly used as food additives were screened for antiquorum-sensing activity. Among these, diallyl disulfide (DADS) and methyl 2-methyl-3-furyl disulfide (MMFDS) were found to exert the strongest inhibition against violacein production in Chromobacterium violaceum 026, the tested biosensor strain. DADS and MMFDS also inhibited the growth of Hafnia alvei H4, yielding MIC values of 48 and 41.6 mM, respectively. In addition, DADS and MMFDS also inhibited the ability of H. alvei H4 to produce acyl-homoserine lactone as demonstrated by the reduced level of C6-HSL in the supernatant of DADS-treated culture. At concentrations corresponding to 1/4 MIC, DADS, and MMFDS inhibited the swarming ability of H. alvei H4 by 73.50% and 76.43%, respectively, while having virtually no effect on cell growth. The same concentrations of DADS and MMFDS also completely inhibited the formation of biofilm. These antiquorum sensing effects of DADS and MMFDS involved changes in the expression of the quorum-sensing genes luxI and luxR. Quantitative RT-PCR analysis showed that the mRNA levels of both genes were significantly reduced by DADS and MMDFS at concentrations below their MICs. However, further test using a mutant strain of H. alvei lacking luxR (ΔluxR) revealed significant reduction in luxI mRNA level upon treatment of the strain with DADS or MMDFS, but no change in luxR mRNA level occurred when a luxI-lacking mutant (ΔluxI) was treated with these compounds. The result therefore suggested that the antiquorum-sensing effect of DADS and MMFDS against H. alvei H4 might operate mainly through the inhibition of luxI expression in the cells.

PRACTICAL APPLICATION

The sulfide flavors compounds used in this paper are commonly used in food processing in China and are listed in the national standard of Chinese food additives GB2760-2014. The application of sulfide flavors in food processing can enhance aroma and prevent food spoilage.

AHL-lactonase expression in three marine emerging pathogenic Vibrio spp. reduces virulence and mortality in brine shrimp (Artemia salina) and Manila clam (Venerupis philippinarum)

Bacterial infectious diseases produced by Vibrio are the main cause of economic losses in aquaculture. During recent years it has been shown that the expression of virulence genes in some Vibrio species is controlled by a population-density dependent gene-expression mechanism known as quorum sensing (QS), which is mediated by the diffusion of signal molecules such as N-acylhomoserine lactones (AHLs). QS disruption, especially the enzymatic degradation of signalling molecules, known as quorum quenching (QQ), is one of the novel therapeutic strategies for the treatment of bacterial infections. In this study, we present the detection of AHLs in 34 marine Vibrionaceae strains. Three aquaculture-related pathogenic Vibrio strains, V. mediterranei VibC-Oc-097, V. owensii VibC-Oc-106 and V. coralliilyticus VibC-Oc-193 were selected for further studies based on their virulence and high production of AHLs. This is the first report where the signal molecules have been characterized in these emerging marine pathogens and correlated to the expression of virulence factors. Moreover, the results of AHL inactivation in the three selected strains have been confirmed in vivo against brine shrimps (Artemia salina) and Manila clams (Venerupis philippinarum). This research contributes to the development of future therapies based on AHL disruption, the most promising alternatives for fighting infectious diseases in aquaculture.

Identification and validation of sRNAs in Edwardsiella tarda S08

Bacterial small non-coding RNAs (sRNAs) are known as novel regulators involved in virulence, stress responsibility, and so on. Recently, a lot of new researches have highlighted the critical roles of sRNAs in fine-tune gene regulation in both prokaryotes and eukaryotes. Edwardsiella tarda (E. tarda) is a gram-negative, intracellular pathogen that causes edwardsiellosis in fish. Thus far, no sRNA has been reported in E. tarda. The present study represents the first attempt to identify sRNAs in E. tarda S08. Ten sRNAs were validated by RNA sequencing and quantitative PCR (qPCR). ET_sRNA_1 and ET_sRNA_2 were homolous to tmRNA and GcvB, respectively. However, the other candidate sRNAs have not been reported till now. The cellular abundance of 10 validated sRNA was detected by qPCR at different growth phases to monitor their biosynthesis. Nine candidate sRNAs were expressed in the late-stage of exponential growth and stationary stages of growth (36~60 h). And the expression of the nine sRNAs was growth phase-dependent. But ET_sRNA_10 was almost expressed all the time and reached the highest peak at 48 h. Their targets were predicted by TargetRNA2 and each sRNA target contains some genes that directly or indirectly relate to virulence. These results preliminary showed that sRNAs probably play a regulatory role of virulence in E. tarda.

Identification of Unanticipated and Novel N-Acyl L-Homoserine Lactones (AHLs) Using a Sensitive Non-Targeted LC-MS/MS Method

N-acyl L-homoserine lactones (AHLs) constitute a predominant class of quorum-sensing signaling molecules used by Gram-negative bacteria. Here, we report a sensitive and non-targeted HPLC-MS/MS method based on parallel reaction monitoring (PRM) to identify and quantitate known, unanticipated, and novel AHLs in microbial samples. Using a hybrid quadrupole-high resolution mass analyzer, this method integrates MS scans and all-ion fragmentation MS/MS scans to allow simultaneous detection of AHL parent-ion masses and generation of full mass spectra at high resolution and high mass accuracy in a single chromatographic run. We applied this method to screen for AHL production in a variety of Gram-negative bacteria (i.e. B. cepacia, E. tarda, E. carotovora, E. herbicola, P. stewartii, P. aeruginosa, P. aureofaciens, and R. sphaeroides) and discovered that nearly all of them produce a larger set of AHLs than previously reported. Furthermore, we identified production of an uncommon AHL (i.e. 3-oxo-C7-HL) in E. carotovora and P. stewartii, whose production has only been previously observed within the genera Serratia and Yersinia. Finally, we used our method to quantitate AHL degradation in B. cepacia, E. carotovora, E. herbicola, P. stewartii, P. aeruginosa, P. aureofaciens, the non-AHL producer E. coli, and the Gram-positive bacterium B. subtilis. We found that AHL degradation ability varies widely across these microbes, of which B. subtilis and E. carotovora are the best degraders, and observed that there is a general trend for AHLs containing long acyl chains (≥10 carbons) to be degraded at faster rates than AHLs with short acyl chains (≤6 carbons).

The biological effect of metal ions on the granulation of aerobic granular activated sludge

As a special biofilm structure, microbial attachment is believed to play an important role in the granulation of aerobic granular activated sludge (AGAS). This experiment was to investigate the biological effect of Ca(2+), Mg(2+), Cu(2+), Fe(2+), Zn(2+), and K(+) which are the most common ions present in biological wastewater treatment systems, on the microbial attachment of AGAS and flocculent activated sludge (FAS), from which AGAS is always derived, in order to provide a new strategy for the rapid cultivation and stability control of AGAS. The result showed that attachment biomass of AGAS was about 300% higher than that of FAS without the addition of metal ions. Different metal ions had different effects on the process of microbial attachment. FAS and AGAS reacted differently to the metal ions as well, and in fact, AGAS was more sensitive to the metal ions. Specifically, Ca(2+), Mg(2+), and K(+) could increase the microbial attachment ability of both AGAS and FAS under appropriate concentrations, Cu(2+), Fe(2+), and Zn(2+) were also beneficial to the microbial attachment of FAS at low concentrations, but Cu(2+), Fe(2+), and Zn(2+) greatly inhibited the attachment process of AGAS even at extremely low concentrations. In addition, the acylated homoserine lactone (AHL)-based quorum sensing system, the content of extracellular polymeric substances and the relative hydrophobicity of the sludges were greatly influenced by metal ions. As all these parameters had close relationships with the microbial attachment process, the microbial attachment may be affected by changes of these parameters.

Insights into the virulence-related genes of Edwardsiella tarda isolated from turbot in Europe: genetic homogeneity and evidence for vibrioferrin production

Edwardsiella tarda has long been known as a pathogen that causes severe economic losses in aquaculture industry. Insights gained on E. tarda pathogenesis may prove useful in the development of new methods for the treatment of infections as well as preventive measures against future outbreaks. In this report, we have established the correlation between the presence of virulence genes, related with three aspects typically involved in bacterial pathogenesis (chondroitinase activity, quorum sensing and siderophore-mediated ferric uptake systems), in the genome of E. tarda strains isolated from turbot in Europe and their phenotypic traits. A total of 8 genes were tested by PCR for their presence in 73 E. tarda isolates. High homogeneity was observed in the presence/absence pattern of all the strains. Positive results in the amplification of virulence-related genes were correlated with the detection of chondroitinase activity in agar plates, in vivo AHL production during fish infection and determination of type of siderophore produced by E. tarda. To the best of our knowledge, this is the first study carried out with European strains on potential virulence factors. Furthermore, we demonstrated for the first time that E. tarda produces the siderophore vibrioferrin.

Isolation of AHL-degrading bacteria from micro-algal cultures and their impact on algal growth and on virulence of Vibrio campbellii to prawn larvae

Inactivation of quorum sensing (QS) signal molecules, such as acylhomoserine lactones (AHLs) of pathogenic bacteria, has been proposed as a novel method to combat bacterial diseases in aquaculture. Despite the importance of micro-algae for aquaculture, AHL degradation by bacteria associated with micro-algal cultures has thus far not been investigated. In this study, we isolated Pseudomonas sp. NFMI-T and Bacillus sp. NFMI-C from open cultures of the micro-algae Tetraselmis suecica and Chaetoceros muelleri, respectively. An AHL degradation assay showed that either monocultures or co-cultures of the isolates were able to degrade the AHL N-hexanoyl-L-homoserine lactone. In contrast, only Bacillus sp. NFMI-C was able to inactivate N-hydroxybutanoyl-L-homoserine lactone, the AHL produced by Vibrio campbellii. The isolated bacteria were able to persist for up to 3 weeks in conventionalized micro-algal cultures, indicating that they were able to establish and maintain themselves within open algal cultures. Using gnotobiotic algal cultures, we found that the isolates did not affect growth of the micro-algae from which they were isolated, whereas a mixture of both isolates increased the growth of Tetraselmis and decreased the growth of Chaetoceros. Finally, addition of Bacillus sp. NFMI-C to the rearing water of giant river prawn (Macrobrachium rosenbergii) larvae significantly improved survival of the larvae when challenged with pathogenic V. campbellii, whereas it had no effect on larval growth.

Mechanisms of quorum sensing and strategies for quorum sensing disruption in aquaculture pathogens

In many countries, infectious diseases are a considerable threat to aquaculture. The pathogenicity of micro-organisms that infect aquaculture systems is closely related to the release of virulence factors and the formation of biofilms, both of which are regulated by quorum sensing (QS). Thus, QS disruption is a potential strategy for preventing disease in aquaculture systems. QS inhibitors (QSIs) not only inhibit the expression of virulence-associated genes but also attenuate the virulence of aquaculture pathogens. In this review, we discuss QS systems in important aquaculture pathogens and focus on the relationship between QS mechanisms and bacterial virulence in aquaculture. We further elucidate QS disruption strategies for targeting aquaculture pathogens. Four main types of QSIs that target aquaculture pathogens are discussed based on their mechanisms of action.

In Vivo Programmed Gene Expression Based on Artificial Quorum Networks

The quorum sensing (QS) system, as a well-functioning population-dependent gene switch, has been widely applied in many gene circuits in synthetic biology. In our work, an efficient cell density-controlled expression system (QS) was established via engineering of the Vibrio fischeri luxI-luxR quorum sensing system. In order to achieve in vivo programmed gene expression, a synthetic binary regulation circuit (araQS) was constructed by assembling multiple genetic components, including the quorum quenching protein AiiA and the arabinose promoter ParaBAD, into the QS system. In vitro expression assays verified that the araQS system was initiated only in the absence of arabinose in the medium at a high cell density. In vivo expression assays confirmed that the araQS system presented an in vivo-triggered and cell density-dependent expression pattern. Furthermore, the araQS system was demonstrated to function well in different bacteria, indicating a wide range of bacterial hosts for use. To explore its potential applications in vivo, the araQS system was used to control the production of a heterologous protective antigen in an attenuated Edwardsiella tarda strain, which successfully evoked efficient immune protection in a fish model. This work suggested that the araQS system could program bacterial expression in vivo and might have potential uses, including, but not limited to, bacterial vector vaccines.

Comparative proteomic study of Edwardsiella tarda strains with different degrees of virulence

Edwardsiella tarda is an enteric opportunistic pathogen that causes a great loss in aquaculture. This species has been described as a phenotypical homogeneous group; in contrast, serological studies and molecular typing revealed a wide heterogeneity. In this work, a proteomic study of differential expression of a virulent isolate from turbot cultured in the Norwest of Spain in comparison with an avirulent collection strain was performed in order to recognize proteins involved in virulence. One hundred and three proteins that presented different abundance were successfully identified and classified into 11 functional categories according to their biological processes: amino acid, carbohydrate and lipid metabolism, tricarboxylic cycle, stress response and protein fate, protein synthesis, biogenesis of cellular components, cell rescue defence and virulence, cell membrane and transport, signal transduction and purine and pyrimidine metabolism. Twenty three protein spots detected only in turbot isolate were identified. It was shown that the same proteins appeared in different spots in the two isolates. Mass spectra obtained by MALDITOF/TOF of some of these proteins and DNA sequencing explained the changes as a result of different amino acid sequences. Several proteins related with the virulence of E. tarda (FliC, ArnA or FeSODI) were only detected in the turbot European isolate. This article is part of a Special Issue entitled: HUPO 2014.

Generation and evaluation of virulence attenuated mutants of Edwardsiella tarda as vaccine candidates to combat edwardsiellosis in flounder (Paralichthys olivaceus)

Edwardsiella tarda is an intracellular pathogen that causes edwardsiellosis in fish. The development of a live attenuated vaccine may be an effective approach for preventing this disease in fish. In this study, we introduced deletions of esrB, esaC, evpH, rpoS, and purA into the E. tarda LSE40ΔaroA strain, thereby generating five double-gene mutants (ΔaroAΔesrB, ΔaroAΔesaC, ΔaroAΔrpoS, ΔaroAΔevpH, and ΔaroAΔpurA) and two triple-gene mutants (ΔaroAΔesrBΔevpH and ΔaroAΔesaCΔevpH). When blue gourami (Trichogaster trichopterus) was used as a fish model for the primary screening and evaluation of the vaccine candidates, all mutants were attenuated significantly by more than 2 to 3 logs in terms of the 50% lethal dose (LD(50)). Five double-gene mutants yielded relative percentage survival (RPS) rates of 26.1-82.6% after challenge with wild-type E. tarda. The ΔaroAΔesrB mutant that conferred the highest RPS (82.6%) in blue gourami was also evaluated in flounder (Paralichthys olivaceus). After vaccination via intramuscular (i.m.) injection or immersion, this mutant could persist in the flounder for 14-35 days and it induced higher serum antibody titers than the control fish (P < 0.01). Flounder vaccinated via i.m. injection at doses of 10(3)-10(7) CFU/fish had RPS rates of 14.3-66.7% after i.m. challenge with 10(4) CFU/fish using wild-type E. tarda. Flounder vaccinated via immersion at a dose of 10(7) CFU/ml exhibited 100% RPS against immersion challenge with 10(7) CFU/ml using wild-type E. tarda. These results indicate that the ΔaroAΔesrB mutant could be used as an effective live vaccine to combat edwardsiellosis in flounder.

Deciphering the role of coumarin as a novel quorum sensing inhibitor suppressing virulence phenotypes in bacterial pathogens

The rapid unchecked rise in antibiotic resistance over the last few decades has led to an increased focus on the need for alternative therapeutic strategies for the treatment and clinical management of microbial infections. In particular, small molecules that can suppress microbial virulence systems independent of any impact on growth are receiving increased attention. Quorum sensing (QS) is a cell-to-cell signalling communication system that controls the virulence behaviour of a broad spectrum of bacterial pathogens. QS systems have been proposed as an effective target, particularly as they control biofilm formation in pathogens, a key driver of antibiotic ineffectiveness. In this study, we identified coumarin, a natural plant phenolic compound, as a novel QS inhibitor, with potent anti-virulence activity in a broad spectrum of pathogens. Using a range of biosensor systems, coumarin was active against short, medium and long chain N-acyl-homoserine lactones, independent of any effect on growth. To determine if this suppression was linked to anti-virulence activity, key virulence systems were studied in the nosocomial pathogen Pseudomonas aeruginosa. Consistent with suppression of QS, coumarin inhibited biofilm, the production of phenazines and swarming motility in this organism potentially linked to reduced expression of the rhlI and pqsA quorum sensing genes. Furthermore, coumarin significantly inhibited biofilm formation and protease activity in other bacterial pathogens and inhibited bioluminescence in Aliivibrio fischeri. In light of these findings, coumarin would appear to have potential as a novel quorum sensing inhibitor with a broad spectrum of action.

N-hexanoyl-L-homoserine lactone-degrading Pseudomonas aeruginosa PsDAHP1 protects zebrafish against Vibrio parahaemolyticus infection

Four strains of N-hexanoyl-L-homoserine lactone (AHL)-degrading Pseudomonas spp., named PsDAHP1, PsDAHP2, PsDAHP3, and PsDAHP4 were isolated and identified from the intestine of Fenneropenaeus indicus. PsDAHP1 showed the highest AHL-degrading activity among the four isolates. PsDAHP1 inhibited biofilm-forming exopolysaccharide and altered cell surface hydrophobicity of virulent green fluorescent protein (GFP)-tagged Vibrio parahaemolyticus DAHV2 (GFP-VpDAHV2). Oral administration of PsDAHP1 significantly reduced zebrafish mortality caused by GFP-VpDAHV2 challenge, and inhibited colonisation of GFP-VpDAHV2 in the gills and intestine of zebrafish as evidence by confocal laser scanning microscope and selective plating. Furthermore, zebrafish receiving PsDAHP1-containing feed had increased phagocytic cells of its leucocytes, increased serum activities of superoxide dismutase and lysozyme. The results suggest that Pseudomonas aeruginosa PsDAHP1 could protect zebrafish from V. parahaemolyticus infection by inhibiting biofilm formation and enhancing defence mechanisms of the fish.

Quorum-quenching activity of the AHL-lactonase from Bacillus licheniformis DAHB1 inhibits Vibrio biofilm formation in vitro and reduces shrimp intestinal colonisation and mortality

Vibrio parahaemolyticus is a significant cause of gastroenteritis resulting from the consumption of undercooked sea foods and often cause significant infections in shrimp aquaculture. Vibrio virulence is associated with biofilm formation and is regulated by N-acylated homoserine lactone (AHL)-mediated quorum sensing. In an attempt to reduce vibrio colonisation of shrimps and mortality, we screened native intestinal bacilli from Indian white shrimps (Fenneropenaeus indicus) for an isolate which showed biofilm-inhibitory activity (quorum quenching) against the pathogen V. parahaemolyticus DAHP1. The AHL-lactonase (AiiA) expressed by one of these, Bacillus licheniformis DAHB1, was characterised as having a broad-spectrum AHL substrate specificity and intrinsic resistance to the acid conditions of the shrimp intestine. Purified recombinant AiiA inhibited vibrio biofilm development in a cover slip assay and significantly attenuated infection and mortality in shrimps reared in a recirculation aquaculture system. Investigation of intestinal samples also showed that AiiA treatment also reduced vibrio viable counts and biofilm development as determined by confocal laser scanning microscopy (CLSM) imaging. These findings suggest that the B. licheniformis DAHB1 quorum-quenching AiiA might be developed for use as a prophylactic treatment to inhibit or reduce vibrio colonisation and mortality of shrimps in aquaculture.

Global and phylogenetic distribution of quorum sensing signals, acyl homoserine lactones, in the family of Vibrionaceae

Bacterial quorum sensing (QS) and the corresponding signals, acyl homoserine lactones (AHLs), were first described for a luminescent Vibrio species. Since then, detailed knowledge has been gained on the functional level of QS; however, the abundance of AHLs in the family of Vibrionaceae in the environment has remained unclear. Three hundred and one Vibrionaceae strains were collected on a global research cruise and the prevalence and profile of AHL signals in this global collection were determined. AHLs were detected in 32 of the 301 strains using Agrobacterium tumefaciens and Chromobacterium violaceum reporter strains. Ethyl acetate extracts of the cultures were analysed by ultra-high performance liquid chromatography-high resolution mass spectrometry (MS) with automated tandem MS confirmation for AHLs. N-(3-hydroxy-hexanoyl) (OH-C6) and N-(3-hydroxy-decanoyl) (OH-C10) homoserine lactones were the most common AHLs found in 17 and 12 strains, respectively. Several strains produced a diversity of different AHLs, including N-heptanoyl (C7) HL. AHL-producing Vibrionaceae were found in polar, temperate and tropical waters. The AHL profiles correlated with strain phylogeny based on gene sequence homology, however not with geographical location. In conclusion, a wide range of AHL signals are produced by a number of clades in the Vibrionaceae family and these results will allow future investigations of inter- and intra-species interactions within this cosmopolitan family of marine bacteria.

In vitro cytotoxic effects of gold nanoparticles coated with functional acyl homoserine lactone lactonase protein from Bacillus licheniformis and their antibiofilm activity against Proteus species

N-acylated homoserine lactonases are known to inhibit the signaling molecules of the biofilm-forming pathogens. In this study, gold nanoparticles were coated with N-acylated homoserine lactonase proteins (AiiA AuNPs) purified from Bacillus licheniformis. The AiiA AuNPs were characterized by UV-visible spectra, Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), and X-ray diffraction (XRD). The synthesized AiiA AuNPs were found to be spherical in shape and 10 to 30 nm in size. Treatment with AiiA protein-coated AuNPs showed maximum reduction in exopolysaccharide production, metabolic activities, and cell surface hydrophobicity and potent antibiofilm activity against multidrug-resistant Proteus species compared to treatment with AiiA protein alone. AiiA AuNPs exhibited potent antibiofilm activity at 2 to 8 μM concentrations without being harmful to the macrophages. We conclude that at a specific dose, AuNPs coated with AiiA can kill bacteria without harming the host cells, thus representing a potential template for the design of novel antibiofilm and antibacterial protein drugs to decrease bacterial colonization and to overcome the problem of drug resistance. In summary, our data suggest that the combined effect of the lactonase and the gold nanoparticles of the AiiA AuNPs has promising antibiofilm activity against biofilm-forming and multidrug-resistant Proteus species.

In vitro quenching of fish pathogen Edwardsiella tarda AHL production using marine bacterium Tenacibaculum sp. strain 20J cell extracts

Quorum quenching (QQ) has become an interesting alternative for solving the problem of bacterial antibiotic resistance, especially in the aquaculture industry, since many species of fish-pathogenic bacteria control their virulence factors through quorum sensing (QS) systems mediated by N-acylhomoserine lactones (AHLs). In a screening for bacterial strains with QQ activity in different marine environments, Tenacibaculum sp. strain 20J was identified and selected for its high degradation activity against a wide range of AHLs. In this study, the QQ activity of live cells and crude cell extracts (CCEs) of strain 20J was characterized and the possibilities of the use of CCEs of this strain to quench the production of AHLs in cultures of the fish pathogen Edwardsiella tarda ACC35.1 was explored. E. tarda ACC35.1 produces N-hexanoyl-L-homoserine lactone (C6-HSL) and N-oxohexanoyl-L-homoserine lactone (OC6-HSL). This differs from profiles registered for other E. tarda strains and indicates an important intra-specific variability in AHL production in this species. The CCEs of strain 20J presented a wide-spectrum QQ activity and, unlike Bacillus thuringiensis serovar Berliner ATCC10792 CCEs, were effective in eliminating the AHLs produced in E. tarda ACC35.1 cultures. The fast and wide-spectrum AHL-degradation activity shown by this member of the Cytophaga-Flexibacter-Bacteroidetes group consolidates this strain as a promising candidate for the control of AHL-based QS pathogens, especially in the marine fish farming industry.

The effect of quorum sensing and extracellular proteins on the microbial attachment of aerobic granular activated sludge

In this study, vanillin, a quorum sensing (QS) blocker, and proteinase K were employed to investigate the effect of QS and extracellular proteins on the microbial attachment of aerobic granular activated sludge (AGAS). Results clearly showed that both vanillin and proteinase K could reduce attachment potential of AGAS, and the combined use of them was more effective in reducing attachment biomass of AGAS. The contents of N-acylhomoserine lactones (AHLs) and extracellular proteins were reduced in the presence of vanillin and proteinase K. Besides, it was found that extracellular proteins could promote microbial attachment of AGAS, and it was also revealed that AHLs-mediated QS might be involved in microbial attachment of AGAS through the regulation of extracellular proteins. This study suggested that both QS and extracellular proteins might play important roles in the development of "AGAS biofilm" from the perspective of the biofilm.

Are there acyl-homoserine lactones within mammalian intestines?

Many Proteobacteria are capable of quorum sensing using N-acyl-homoserine lactone (acyl-HSL) signaling molecules that are synthesized by LuxI or LuxM homologs and detected by transcription factors of the LuxR family. Most quorum-sensing species have at least one LuxR and one LuxI homolog. However, members of the Escherichia, Salmonella, Klebsiella, and Enterobacter genera possess only a single LuxR homolog, SdiA, and no acyl-HSL synthase. The most obvious hypothesis is that these organisms are eavesdropping on acyl-HSL production within the complex microbial communities of the mammalian intestinal tract. However, there is currently no evidence of acyl-HSLs being produced within normal intestinal communities. A few intestinal pathogens, including Yersinia enterocolitica, do produce acyl-HSLs, and Salmonella can detect them during infection. Therefore, a more refined hypothesis is that SdiA orthologs are used for eavesdropping on other quorum-sensing pathogens in the host. However, the lack of acyl-HSL signaling among the normal intestinal residents is a surprising finding given the complexity of intestinal communities. In this review, we examine the evidence for and against the possibility of acyl-HSL signaling molecules in the mammalian intestine and discuss the possibility that related signaling molecules might be present and awaiting discovery.

The presence and role of bacterial quorum sensing in activated sludge

Activated sludge used for wastewater treatment globally is composed of a high‐density microbial community of great biotechnological significance. In this study the presence and purpose of quorum sensing via N‐acylated‐l‐homoserine lactones (AHLs) in activated sludge was explored. The presence of N‐heptanoyl‐l‐homoserine lactone in organic extracts of sludge was demonstrated along with activation of a LuxR‐based AHL monitor strain deployed in sludge, indicating AHL‐mediated gene expression is active in sludge flocculates but not in the bulk aqueous phase. Bacterial isolates from activated sludge were screened for AHL production and expression of phenotypes commonly but not exclusively regulated by AHL‐mediated gene transcription. N‐acylated‐l‐homoserine lactone and exoenzyme production were frequently observed among the isolates. N‐acylated‐l‐homoserine lactone addition to sludge upregulated chitinase activity and an AHL‐ and chitinase‐producing isolate closely related to Aeromonas hydrophila was shown to respond to AHL addition with upregulation of chitinase activity. N‐acylated‐l‐homoserine lactones produced by this strain were identified and genes ahyI/R and chiA, encoding AHL production and response and chitinase activity respectively, were sequenced. These experiments provide insight into the relationship between AHL‐mediated gene expression and exoenzyme activity in activated sludge and may ultimately create opportunities to improve sludge performance.

Mutation of tryptophanase gene tnaA in Edwardsiella tarda reduces lipopolysaccharide production, antibiotic resistance and virulence

A fish pathogen Edwardsiella tarda LTB-4 produced various indole alkaloids, including indole, 2-(1H-indol-3-yl)ethanol, 4-di(1H-indol-3-yl)methylphenol, tri(1H-indol-3-yl)methane and 2-[2,2-bis(1H-indol-3-yl)]ethylphenylamine. Indole was the most abundant among these indole alkaloids. E. tarda LTB-4 produced indoles during its whole growth phase and maintained a high level (around 35.5 µM) during the stationary phase. The relevant tryptophanase (TnaA) gene tnaA was cloned from LTB-4 and conditionally expressed in Escherichia coli; the recombinant TnaA catalysed L-tryptophan to indole. A tnaA in-frame deletion mutant ΔtnaA was constructed through double cross-over allelic exchange by means of the suicide vector pRE118; deletion of tnaA caused some phenotypic changes including decreased swarming and twitching motility, lipopolysaccharide production and multiple antibiotic resistances. Also, subtherapeutic doses of chloromycetin, carbenicillin and tetracyline could cause the decrease of bacterial growth, but greatly induce the production of indole by E. tarda. Most importantly, attenuated virulence of the ΔtnaA mutant to zebra fish by increasing the LD50 for about 55-fold indicated that TnaA involved in the virulence of E. tarda.

Quorum quenching bacteria protect Macrobrachium rosenbergii larvae from Vibrio harveyi infection

AIMS

In this study, we investigated the effect of N-acyl homoserine lactone-degrading bacterial enrichment cultures (ECs) on larviculture of the giant freshwater prawn Macrobrachium rosenbergii.

METHODS AND RESULTS

The larval performance in terms of larval growth, larval survival, larval quality, duration of the larval rearing process and microflora levels in the rearing water as well as inside the prawn gut was investigated. The application of the EC bacteria was performed in two ways: by adding them directly into the larval rearing water and via enriched Artemia nauplii used for larval feeding. The results of the study demonstrated that both ECs that were tested had a similar positive effect on larval survival and larval quality, whereas they did not affect larval growth or the duration of the larval rearing process.

CONCLUSIONS

Under normal hatchery conditions, the optimal EC densities were found to be 10(6) CFU ml(-1) for adding into the rearing water and 5 × 10(8) CFU ml(-1) for enrichment of Artemia nauplii used for feeding of the larvae. In the hatchery, the ECs can be grown on waste streams of Artemia hatching.

SIGNIFICANCE AND IMPACT OF THE STUDY

Application of this kind of ECs could lead to a more sustainable aquaculture production, by replacing the use of antibiotics to control diseases.

Disruption of bacterial cell-to-cell communication by marine organisms and its relevance to aquaculture

Bacterial disease is one of the most critical problems in commercial aquaculture. Although various methods and treatments have been developed to curb the problem, yet they still have significant drawbacks. A novel and environmental-friendly approach in solving this problem is through the disruption of bacterial communication or quorum sensing (QS). In this communication scheme, bacteria regulate their own gene expression by producing, releasing, and sensing chemical signals from the environment. There seems to be a link between QS and diseases through the regulation of certain phenotypes and the induction of virulence factors responsible for pathogen-host association. Several findings have reported that numerous aquatic organisms such as micro-algae, macro-algae, invertebrates, or even other bacteria have the potential to disrupt QS. The mechanism of action varies from degradation of signals through enzymatic or chemical inactivation to antagonistic as well as agonistic activities. This review focuses on the existing marine organisms that are able to interfere with QS with potential application for aquaculture as bacterial control.

QseBC controls flagellar motility, fimbrial hemagglutination and intracellular virulence in fish pathogen Edwardsiella tarda

The inter-kingdom communication with the mammalian hosts mediated by autoinducer-3 (AI-3)/epinephrine (Epi)/norepinephrine (NE), and transduced by two-component systems QseBC has recently been described. As a fish pathogen and opportunistic pathogen for human beings, Edwardsiella tarda develops surface structures such as flagellar and fimbriae to cause different hemagglutination phenotypes and serotypes and initiate pathogen-host recognition and invasion process. E. tarda survives within macrophages in fish using type III secretion system (TTSS). Here, the genes of E. tarda two-component system, qseB and qseC, were found to be co-transcribed. Phylogenetic analysis indicated that evolution of QseC strongly correlated to different host niches. Compared with the wild type and their complemented strains, ΔqseB and ΔqseC mutants exhibited significant impaired flagellar motilities. Mammalian Epi was able to stimuli the flagellar motility in E. tarda via QseBC. Hemagglutination caused by fimbriae was induced in ΔqseB but repressed in ΔqseC. Disruption of qseB or qseC down-regulated the intracellular expressions of TTSS elements EseB and EsaC, and impaired their intracellular survival capabilities as well as in vivo competitive abilities. Furthermore, in vitro tests indicated that expression of EseB was induced by Epi via QseBC. Our results revealed that the QseBC system modified the virulence-related surface structures (flagellum, fimbriae and secretion system) and that hormone might stimulate the virulence of the pathogen in fish.

Quorum sensing and bacterial biofilms

This review describes the chemistry of the bacterial biofilms including the chemistry of their constituents and signalling compounds that mediate or inhibit the formation of biofilms. Systems are described with special emphasis, in which quorum sensing molecules (autoinducers) trigger the formation of biofilms. In the first instance, N-acyl-L-homoserine lactones (AHLs) are the focus of this review, whereas the inter-species signal known as furanosyl borate diester and peptide autoinducers used by Gram-positive bacteria are not discussed in detail. Since the first discovery of an AHL autoinducer from Vibrio fischeri a large and further increasing number of different AHL structures from Gram-negative bacteria have been identified. This review gives a summary of all known AHL autoinducers and producing bacterial species. A few systems are discussed, where biofilm formation is suppressed by enzymatic degradation of AHL molecules or interference of secondary metabolites from other species with the quorum sensing systems of communicating bacteria. Finally, the multi-channel quorum sensing system, the intracellular downstream processing of the signal, and the resulting response of whole populations including biofilm formation are discussed for the Vibrio genus that has been extensively investigated.

Motility and the expression of the flagellin protein FliC are negatively regulated by quorum sensing in Edwardsiella tarda

Edwardsiella tarda is a gram-negative bacterial pathogen of fish and animals. A number of gram-negative bacteria have quorum-sensing systems and produce N-acylhomoserine lactone (AHL) that they use as a quorum-sensing signaling molecule. We have already reported that E. tarda NUF251 produces AHLs and has the AHL-synthase gene, edwI. Inactivation of NUF251 edwI induces expression of an approximately 45 kDa extracellular protein, identified as a flagellin encoded by FliC. Mutation of edwI also changes the motility pattern of NUF251 from a radial expansion pattern to concentric rings. The addition of exogenous AHL was capable of restoring normal motility to NUF251 edwI mutants. These results demonstrate that quorum sensing negatively regulates motility and expression of the FliC protein.

Genome sequence of the versatile fish pathogen Edwardsiella tarda provides insights into its adaptation to broad host ranges and intracellular niches

BACKGROUND

Edwardsiella tarda is the etiologic agent of edwardsiellosis, a devastating fish disease prevailing in worldwide aquaculture industries. Here we describe the complete genome of E. tarda, EIB202, a highly virulent and multi-drug resistant isolate in China.

METHODOLOGY/PRINCIPAL FINDINGS

E. tarda EIB202 possesses a single chromosome of 3,760,463 base pairs containing 3,486 predicted protein coding sequences, 8 ribosomal rRNA operons, and 95 tRNA genes, and a 43,703 bp conjugative plasmid harboring multi-drug resistant determinants and encoding type IV A secretion system components. We identified a full spectrum of genetic properties related to its genome plasticity such as repeated sequences, insertion sequences, phage-like proteins, integrases, recombinases and genomic islands. In addition, analysis also indicated that a substantial proportion of the E. tarda genome might be devoted to the growth and survival under diverse conditions including intracellular niches, with a large number of aerobic or anaerobic respiration-associated proteins, signal transduction proteins as well as proteins involved in various stress adaptations. A pool of genes for secretion systems, pili formation, nonfimbrial adhesions, invasions and hemagglutinins, chondroitinases, hemolysins, iron scavenging systems as well as the incomplete flagellar biogenesis might feature its surface structures and pathogenesis in a fish body.

CONCLUSION/SIGNIFICANCE

Genomic analysis of the bacterium offered insights into the phylogeny, metabolism, drug-resistance, stress adaptation, and virulence characteristics of this versatile pathogen, which constitutes an important first step in understanding the pathogenesis of E. tarda to facilitate construction of a practical effective vaccine used for combating fish edwardsiellosis.

Quorum sensing in veterinary pathogens: mechanisms, clinical importance and future perspectives

Under certain circumstances the individuals of a bacterial population may find advantages in acting together and making "collective decisions". This phenomenon is better known as quorum sensing. When the concentration of signal molecules produced by the surrounding bacteria exceeds a certain threshold, the bacterial population acts as a single organism, collectively expressing virulence genes, biofilm forming genes, etc. Several mechanisms of quorum sensing are discussed, each with its distinct signal molecules and respective receptors. Some of these mechanisms are restricted to sensing intraspecies signalling, but interspecies and even interkingdom signalling have also been described. Several veterinary pathogens such as Staphylococcus aureus, Staphylococcus pseudintermedius, Pseudomonas aeruginosa and Salmonella Typhimurium use quorum sensing as a means to optimize virulence gene expression and host colonization. Therefore, targeting of the QS mechanisms may provide a novel strategy for combating bacterial infections, also in veterinary medicine.

Absolute configuration and antimicrobial activity of acylhomoserine lactones

(S)-N-Heptanoylhomoserine lactone is an uncommon acyl odd-chain natural product employed by many Gram-negative bacteria as a signaling substance in chemical communication mechanisms known as quorum sensing. The absolute configuration determination of the metabolite produced by the phytopathogen Pantoea ananatis Serrano is reported herein. As with all other substances of this class, the lactone moiety possesses S configuration, corroborating the hypothesis that it shares the same biosynthetic pathway as the (S)-N-hexanoylhomoserine lactone and also that some LuxI homologues can accept both hexanoyl- and heptanoyl-ACP as precursors. Evaluation of the antimicrobial activity of enantiomeric acylhomoserine lactones against three Gram-positive bacteria (Bacillus cereus, B. subtilis, and Staphylococcus aureus) revealed important features between absolute configuration and antimicrobial activity. The N-heptanoylhomoserine lactone was considerably less active than the 3-oxo derivatives. Surprisingly, non-natural (R)-N-(3-oxo-octanoyl)homoserine lactone was as active as the S enantiomer against B. cereus, while the synthetic racemic product was less active than either enantiomer.

A review of the functionality of probiotics in the larviculture food chain

During the past two decades, the use of probiotics as an alternative to the use of antibiotics has shown to be promising in aquaculture, particularly in fish and shellfish larviculture. This article reviews the studies on probiotics in larviculture, focusing on the current knowledge of their in vivo mechanisms of action. The article highlights that the in vivo mechanisms of action largely remain to be unravelled. Several methodologies are suggested for further in vivo research, including studies on gut microbiota composition, the use of gnotobiotic animals as test models, and the application of molecular techniques to study host-microbe and microbe-microbe interactions.

The plant pathogen Pantoea ananatis produces N-acylhomoserine lactone and causes center rot disease of onion by quorum sensing

A number of gram-negative bacteria have a quorum-sensing system and produce N-acyl-l-homoserine lactone (AHL) that they use them as a quorum-sensing signal molecule. Pantoea ananatis is reported as a common colonist of wheat heads at ripening and causes center rot of onion. In this study, we demonstrated that P. ananatis SK-1 produced two AHLs, N-hexanoyl-l-homoserine lactone (C6-HSL) and N-(3-oxohexanoyl)-l-homoserine lactone (3-oxo-C6-HSL). We cloned the AHL-synthase gene (eanI) and AHL-receptor gene (eanR) and revealed that the deduced amino acid sequence of EanI/EanR showed high identity to those of EsaI/EsaR from P. stewartii. EanR repressed the ean box sequence and the addition of AHLs resulted in derepression of ean box. Inactivation of the chromosomal eanI gene in SK-1 caused disruption of exopolysaccharide (EPS) biosynthesis, biofilm formation, and infection of onion leaves, which were recovered by adding exogenous 3-oxo-C6-HSL. These results demonstrated that the quorum-sensing system involved the biosynthesis of EPS, biofilm formation, and infection of onion leaves in P. ananatis SK-1.