Cryptic carbapenem antibiotic production genes are widespread in Erwinia carotovora: facile trans activation by the carR transcriptional regulator

Study of the Dynamics of Biofilm Formation and Elastase Activity of Pseudomonas aeruginosa in the Presence of Dodecanoyl-Homoserine Lactone

We studied the effect of early accumulation of N-3-oxo-dodecanoyl-homoserine lactone on the suppression of Pseudomonas aeruginosa reproduction, biofilm formation, and elastase activity. N-3-oxo-dodecanoyl-homoserine lactone in various concentrations was added to the P. aeruginosa culture, and changes in the concentration of bacteria and the formation of biofilms were studied in dynamics. N-3-oxo-dodecanoyl-homoserine lactone in a concentration of 25 μM, decelerated proliferation of bacterial cells during the first 6 h of culturing (p<0.05) and stimulated biofilm formation after 18 h of culturing. Elastase activity of P. aeruginosa increased significantly after addition of N-3-oxo-dodecanoyl-homoserine lactone in a concentration of 0.75 μM.

1.8 Å resolution crystal structure of the carbapenem intrinsic resistance protein CarF

The natural production of the β-lactam antibiotic carbapenem in bacteria involves a group of enzymes that form a synthetic pathway as well as proteins that protect the cell from self-intoxification by the products. Here, the crystal structure of CarF, one of the two proteins that confer resistance to synthesis of the antibiotic in the host organism, is reported. The CarF fold places it within a widely occurring structural family, indicating an ancient structural origin from which the resistance function has been derived.

The Systematic Investigation of the Quorum Sensing System of the Biocontrol Strain Pseudomonas chlororaphis subsp. aurantiaca PB-St2 Unveils aurI to Be a Biosynthetic Origin for 3-Oxo-Homoserine Lactones

The shoot endophytic biocontrol strain Pseudomonas chlororaphis subsp. aurantiaca PB-St2 produces a wide range of exoproducts, including enzymes and antibiotics. The production of exoproducts is commonly tightly regulated. In order to get a deeper insight into the regulatory network of PB-St2, the strain was systematically investigated regarding its quorum sensing systems, both on the genetic and metabolic level. The genome analysis of PB-St2 revealed the presence of four putative acyl homoserine lactone (AHL) biosynthesis genes: phzI, csaI, aurI, and hdtS. LC-MS/MS analyses of the crude supernatant extracts demonstrated that PB-St2 produces eight AHLs. In addition, the concentration of all AHL derivatives was quantified time-resolved in parallel over a period of 42 h during the growth of P. aurantiaca PB-St2, resulting in production curves, which showed differences regarding the maximum levels of the AHLs (14.6 nM– 1.75 μM) and the production period. Cloning and heterologous overexpression of all identified AHL synthase genes in Escherichia coli proved the functionality of the resulting synthases PhzI, CsaI, and AurI. A clear AHL production pattern was assigned to each of these three AHL synthases, while the HdtS synthase did not lead to any AHL production. Furthermore, the heterologous expression study demonstrated unequivocally and for the first time that AurI directs the synthesis of two 3-oxo-AHLs.

Crystal structure of the carbapenem intrinsic resistance protein CarG

In the Gram-negative enterobacterium Erwinia (Pectobacterium) and Serratia sp. ATCC 39006, intrinsic resistance to the carbapenem antibiotic 1-carbapen-2-em-3-carboxylic acid is mediated by the CarF and CarG proteins, by an unknown mechanism. Here, we report a high-resolution crystal structure for the Serratia sp. ATCC 39006 carbapenem resistance protein CarG. This structure of CarG is the first in the carbapenem intrinsic resistance (CIR) family of resistance proteins from carbapenem-producing bacteria. The crystal structure shows the protein to form a homodimer, in agreement with results from analytical gel filtration. The structure of CarG does not show homology with any known antibiotic resistance proteins nor does it belong to any well-characterised protein structural family. However, it is a close structural homologue of the bacterial inhibitor of invertebrate lysozyme, PliI-Ah, with some interesting structural variations, including the absence of the catalytic site responsible for lysozyme inhibition. Both proteins show a unique β-sandwich fold with short terminal α-helices. The core of the protein is formed by stacked anti-parallel sheets that are individually very similar in the two proteins but differ in their packing interface, causing the splaying of the two sheets in CarG. Furthermore, a conserved cation binding site identified in CarG is absent from the homologue.

Activation and silencing of secondary metabolites in Streptomyces albus and Streptomyces lividans after transformation with cosmids containing the thienamycin gene cluster from Streptomyces cattleya

Activation and silencing of antibiotic production was achieved in Streptomyces albus J1074 and Streptomyces lividans TK21 after introduction of genes within the thienamycin cluster from S. cattleya. Dramatic phenotypic and metabolic changes, involving activation of multiple silent secondary metabolites and silencing of others normally produced, were found in recombinant strains harbouring the thienamycin cluster in comparison to the parental strains. In S. albus, ultra-performance liquid chromatography purification and NMR structural elucidation revealed the identity of four structurally related activated compounds: the antibiotics paulomycins A, B and the paulomenols A and B. Four volatile compounds whose biosynthesis was switched off were identified by gas chromatography-mass spectrometry analyses and databases comparison as pyrazines; including tetramethylpyrazine, a compound with important clinical applications to our knowledge never reported to be produced by Streptomyces. In addition, this work revealed the potential of S. albus to produce many others secondary metabolites normally obtained from plants, including compounds of medical relevance as dihydro-β-agarofuran and of interest in perfume industry as β-patchoulene, suggesting that it might be an alternative model for their industrial production. In S. lividans, actinorhodins production was strongly activated in the recombinant strains whereas undecylprodigiosins were significantly reduced. Activation of cryptic metabolites in Streptomyces species might represent an alternative approach for pharmaceutical drug discovery.

Quorum quenching enzymes and their application in degrading signal molecules to block quorum sensing-dependent infection

With the emergence of antibiotic-resistant strains of bacteria, the available options for treating bacterial infections have become very limited, and the search for a novel general antibacterial therapy has received much greater attention. Quorum quenching can be used to control disease in a quorum sensing system by triggering the pathogenic phenotype. The interference with the quorum sensing system by the quorum quenching enzyme is a potential strategy for replacing traditional antibiotics because the quorum quenching strategy does not aim to kill the pathogen or limit cell growth but to shut down the expression of the pathogenic gene. Quorum quenching enzymes have been identified in quorum sensing and non-quorum sensing microbes, including lactonase, acylase, oxidoreductase and paraoxonase. Lactonase is widely conserved in a range of bacterial species and has variable substrate spectra. The existence of quorum quenching enzymes in the quorum sensing microbes can attenuate their quorum sensing, leading to blocking unnecessary gene expression and pathogenic phenotypes. In this review, we discuss the physiological function of quorum quenching enzymes in bacterial infection and elucidate the enzymatic protection in quorum sensing systems for host diseases and their application in resistance against microbial diseases.

Discovery of indolotryptoline antiproliferative agents by homology-guided metagenomic screening

Natural product discovery by random screening of broth extracts derived from cultured bacteria often suffers from high rates of redundant isolation, making it ever more challenging to identify novel biologically interesting natural products. Here we show that homology-based screening of soil metagenomes can be used to specifically target the discovery of new members of traditionally rare, biomedically relevant natural product families. Phylogenetic analysis of oxy-tryptophan dimerization gene homologs found within a large soil DNA library enabled the identification and recovery of a unique tryptophan dimerization biosynthetic gene cluster, which we have termed the bor cluster. When heterologously expressed in Streptomyces albus, this cluster produced an indolotryptoline antiproliferative agent with CaMKIIδ kinase inhibitory activity (borregomycin A), along with several dihydroxyindolocarbazole anticancer/antibiotics (borregomycins B-D). Similar homology-based screening of large environmental DNA libraries is likely to permit the directed discovery of new members within other previously rare families of bioactive natural products.

Does Microcystis aeruginosa have quorum sensing?

Quorum sensing (QS) is a system of cell-to-cell communication by means of intercellular signaling molecules to coordinate a set of targeted gene expression or repression in many Gram-negative bacteria; it plays important roles for bacteria in adaptation to adverse environmental conditions. In this study, we first demonstrated that Microcystis aeruginosa PCC-7820 could produce QS-related signal acylated homoserine lactones (AHLs) among the metabolite of axenic M. aeruginosa, based on bioassay and liquid chromatography-mass spectrometry (LC-MS) analysis. The concentration of the AHLs in the culture medium was cell density dependent and reached a maximum of 18 nM at 1.03 × 10(7) cells mL(-1), 30 days after inoculation. The regulation mechanism of QS in M. aeruginosa and its possible role in bloom formation are discussed.

Quorum sensing and expression of virulence in pectobacteria

Quorum sensing (QS) is a population density-dependent regulatory mechanism in which gene expression is coupled to the accumulation of a chemical signaling molecule. QS systems are widespread among the plant soft-rotting bacteria. In Pectobacterium carotovorum, at least two QS systems exist being specified by the nature of chemical signals involved. QS in Pectobacterium carotovorum uses N-acylhomoserine lactone (AHL) based, as well as autoinducer-2 (AI-2) dependent signaling systems. This review will address the importance of the QS in production of virulence factors and interaction of QS with other regulatory systems in Pectobacterium carotovorum.

Acyldepsipeptide HDAC inhibitor production induced in Burkholderia thailandensis

Natural product gene clusters are often tightly regulated, resulting in gene cluster silencing in laboratory fermentation studies. The systematic overexpression of transcription factors (TFs) associated with biosynthetic gene clusters found in the genome of Burkholderia thailandensis E264 identified a set of TFs that, when overexpressed, alter the secondary metabolome of this bacterium. The isolation and characterization of burkholdacs A and B, two new acyldepsitripeptide histone deacetylase inhibitors produced by B. thailandensis overexpressing the TF bhcM, is reported.

Mutations in rpsL that confer streptomycin resistance show pleiotropic effects on virulence and the production of a carbapenem antibiotic in Erwinia carotovora

Spontaneous streptomycin-resistant derivatives of Erwinia carotovora subsp. carotovora strain ATTn10 were isolated. Sequencing of the rpsL locus (encoding the ribosomal protein S12) showed that each mutant was missense, with a single base change, resulting in the substitution of the wild-type lysine by arginine, threonine or asparagine at codon 43. Phenotypic analyses showed that the rpsL mutants could be segregated into two groups: K43R mutants showed reduced production of the β-lactam secondary metabolite 1-carbapen-2-em-3 carboxylic acid (Car), but little effect on exoenzyme production or virulence in potato tuber tests. By contrast, the K43N and K43T mutations were pleiotropic, resulting in reduced exoenzyme production and virulence, as well as diminished Car production. The effect on Car production was due to reduced transcription of the quorum-sensing-dependent car biosynthetic genes. The effects of K43N and K43T mutations on Car production were partially alleviated by provision of an excess of the quorum-sensing signalling molecule N-(3-oxohexanoyl)-l-homoserine lactone. Finally, a proteomic analysis of the K43T mutant indicated that the abundance of a subset of intracellular proteins was affected by this rpsL mutation.

Quorum sensing-controlled Evr regulates a conserved cryptic pigment biosynthetic cluster and a novel phenomycin-like locus in the plant pathogen, Pectobacterium carotovorum

Pectobacterium carotovorum SCRI193 is a phytopathogenic Gram-negative bacterium. In this study, we have identified a novel cryptic pigment biosynthetic locus in P. carotovorum SCRI193 which we have called the Pectobacterium orange pigment (pop) cluster. The pop cluster is flanked by two tRNA genes and contains genes that encode non-ribosomal peptide synthases and polyketide synthase and produces a negatively charged polar orange pigment. Orange pigment production is activated when an adjacent transcriptional activator sharing sequence similarity with the Erwinia virulence regulator (Evr) is overexpressed. Evr was shown to positively activate its own transcription and that of the pigment biosynthetic genes and an unlinked locus encoding a phenomycin homologue. In addition, the expression of Evr and orange pigment production was shown to be regulated by N-(3-oxohexanoyl)-HSL (OHHL) quorum sensing and have a virulence phenotype in potato. Finally, by comparative genomics and Southern blotting we demonstrate that this pigment biosynthetic cluster is present in multiple P. carotovorum spp., Pectobacterium brasiliensis 1692 and a truncated version of the cluster is present in Pectobacterium atrosepticum. The conserved nature of this cluster in P. carotovorum and P. brasiliensis suggests that the pop cluster has an important function in these broad-host-range soft rotting bacteria, which is no longer required in the narrow-host-range P. atrosepticum SCRI1043.

The role of antibiotics and antibiotic resistance in nature

Investigations of antibiotic resistance from an environmental prospective shed new light on a problem that was traditionally confined to a subset of clinically relevant antibiotic-resistant bacterial pathogens. It is clear that the environmental microbiota, even in apparently antibiotic-free environments, possess an enormous number and diversity of antibiotic resistance genes, some of which are very similar to the genes circulating in pathogenic microbiota. It is difficult to explain the role of antibiotics and antibiotic resistance in natural environments from an anthropocentric point of view, which is focused on clinical aspects such as the efficiency of antibiotics in clearing infections and pathogens that are resistant to antibiotic treatment. A broader overview of the role of antibiotics and antibiotic resistance in nature from the evolutionary and ecological prospective suggests that antibiotics have evolved as another way of intra- and inter-domain communication in various ecosystems. This signalling by non-clinical concentrations of antibiotics in the environment results in adaptive phenotypic and genotypic responses of microbiota and other members of the community. Understanding the complex picture of evolution and ecology of antibiotics and antibiotic resistance may help to understand the processes leading to the emergence and dissemination of antibiotic resistance and also help to control it, at least in relation to the newer antibiotics now entering clinical practice.

A novel plant ferredoxin-like protein and the regulator Hor are quorum-sensing targets in the plant pathogen Erwinia carotovora

Quorum sensing (QS), a population-density-sensing mechanism, controls the production of the main virulence determinants, the plant cell-wall-degrading enzymes (PCWDEs) of the soft-rot phytopathogen Erwinia carotovora subsp. carotovora. In this study, we used random transposon mutagenesis with a gusA reporter construct to identify two new QS-controlled genes encoding the regulator Hor and a plant ferredoxin-like protein, FerE. The QS control of the identified genes was executed by the QS regulators ExpR1 and ExpR2 and mediated by the global repressor RsmA. Hor was shown to contribute to bacterial virulence at least partly through its control of PCWDE production. Our results showed that FerE contributes to oxidative stress tolerance and in planta fitness of the bacteria and suggest that QS could be central to control of oxidative stress tolerance. The presence of the FerE protein appears to be rather unique in heterotrophic bacteria and suggests an acquisition of the corresponding gene from plant host by horizontal gene transfer.

Quorum sensing, virulence and secondary metabolite production in plant soft-rotting bacteria

Quorum sensing describes the ability of bacteria to sense their population density and respond by modulating gene expression. In the plant soft-rotting bacteria, such as Erwinia, an arsenal of plant cell wall-degrading enzymes is produced in a cell density-dependent manner, which causes maceration of plant tissue. However, quorum sensing is central not only to controlling the production of such destructive enzymes, but also to the control of a number of other virulence determinants and secondary metabolites. Erwinia synthesizes both N-acylhomoserine lactone (AHL) and autoinducer-2 types of quorum sensing signal, which both play a role in regulating gene expression in the phytopathogen. We review the models for AHL-based regulation of carbapenem antibiotic production in Erwinia. We also discuss the importance of quorum sensing in the production and secretion of virulence determinants by Erwinia, and its interplay with other regulatory systems.

Strategies to unravel the function of orphan biosynthesis pathways: recent examples and future prospects

The recent increase and availability of whole genome sequences have revised our view of the metabolic capabilities of microorganisms. From these data, a large number of orphan biosynthesis pathways have been identified by bio-informatics. Orphan biosynthetic pathways are gene clusters for which the encoded natural product is unknown. It is worthy to note that the number of orphan pathways coding for putative natural products outnumbers by far the number of currently known metabolites for a given organism. Whilst Streptomyces coelicolor was known to produce only 4 secondary metabolites, the genome analysis revealed 18 additional orphan biosynthetic pathways. It is intriguing to note that this is not a particular case because analysis of other microbial genomes originating from myxobacteria, cyanobacteria and filamentous fungi showed the presence of a comparable or even larger number of orphan pathways. The discovery of these numerous pathways represents a treasure trove, which is likely to grow exponentially in the future, uncovering many novel and possibly bio-active compounds. The few natural products that have been correlated with their orphan pathway are merely the tip of the iceberg, whilst plenty of metabolites await discovery. The recent strategies and methods to access these promising hidden natural products are discussed in this review.

Quorum sensing in Erwinia species

The term quorum sensing (QS) refers to the ability of bacteria to regulate gene expression according to the accumulation of signalling molecules that are made by every cell in the population. The erwiniae group of bacteria are often phytopathogens and the expression of a number of their important virulence determinants and secondary metabolites is under QS control. The erwiniae utilise two types of QS signalling molecules: N-acyl homoserine lactones and AI-2-type signalling molecules. Here, we review the regulatory networks involving QS in the soft rot erwiniae.

Identification of the central quorum sensing regulator of virulence in the enteric phytopathogen, Erwinia carotovora: the VirR repressor

In the Gram-negative phytopathogen, Erwinia carotovora ssp. atroseptica (Eca) virulence depends on the production of a N-(3-oxohexanoyl)-L-homoserine lactone (OHHL) quorum sensing (QS) signal. This work identifies the elusive 'missing link' between QS and virulence in Erwinia. We have identified and characterized a novel regulator of virulence, VirR, in Eca and show that a virR mutation completely restores virulence factor production to an Eca mutant unable to synthesize OHHL. This effect of the virR mutation translates to a restoration of virulence to wild-type levels and thus provides evidence that VirR acts to prevent the production of virulence factors at low cell density. We also show that, in Eca, transcription of virulence genes is controlled by OHHL and that this control is effected through the action of VirR. We also demonstrate that the VirR regulatory pathway is present and functional in both blackleg and soft rotting species of Erwinia.

Quorum sensing: cell-to-cell communication in bacteria

Bacteria communicate with one another using chemical signal molecules. As in higher organisms, the information supplied by these molecules is critical for synchronizing the activities of large groups of cells. In bacteria, chemical communication involves producing, releasing, detecting, and responding to small hormone-like molecules termed autoinducers . This process, termed quorum sensing, allows bacteria to monitor the environment for other bacteria and to alter behavior on a population-wide scale in response to changes in the number and/or species present in a community. Most quorum-sensing-controlled processes are unproductive when undertaken by an individual bacterium acting alone but become beneficial when carried out simultaneously by a large number of cells. Thus, quorum sensing confuses the distinction between prokaryotes and eukaryotes because it enables bacteria to act as multicellular organisms. This review focuses on the architectures of bacterial chemical communication networks; how chemical information is integrated, processed, and transduced to control gene expression; how intra- and interspecies cell-cell communication is accomplished; and the intriguing possibility of prokaryote-eukaryote cross-communication.

Variations on a theme: diverse N-acyl homoserine lactone-mediated quorum sensing mechanisms in gram-negative bacteria

Many Gram-negative bacteria employ a mechanism of cell-cell communication known as quorum sensing (QS). The role of QS is to enable the cells in a culture to coordinate their gene expression profile with changes in the population cell density. The best characterized mechanisms of QS employ N-acylated homoserine lactones (AHLs) as signalling molecules. These AHLs are made by enzymes known as LuxI homologs, and accumulate in the culture supernatant at a rate proportional to the increase in cell density. Once the AHL concentration exceeds a certain threshold value, these ligands bind to intracellular receptors known as LuxR homologs. The latter are transcriptional regulators, whose activity alters upon binding the AHL ligand, thereby eliciting a change in gene transcription. Over the last five years, it has become increasingly obvious that this is a rather simplistic view of AHL-dependent QS, and that in fact, there is considerable diversity in the way in which LuxI-R homologs operate. The aim of the current review is to describe these variations on the basic theme, and to show how functional genomics is revolutionizing our understanding of QS-controlled regulons.

ExpR, a LuxR homolog of Erwinia carotovora subsp. carotovora, activates transcription of rsmA, which specifies a global regulatory RNA-binding protein

N-acyl homoserine lactone (AHL) is required by Erwinia carotovora subspecies for the expression of various traits, including extracellular enzyme and protein production and pathogenicity. Previous studies with E. carotovora subsp. carotovora have shown that AHL deficiency causes the production of high levels of RsmA, an RNA binding protein that functions as a global negative regulator of extracellular enzymes and proteins and secondary metabolites (Rsm, regulator of secondary metabolites). We document here that ExpR, a putative AHL receptor belonging to the LuxR family of regulators, activates RsmA production. In the absence of AHL, an ExpR(+) E. carotovora subsp. carotovora strain compared to its ExpR(-) mutant, produces higher levels of rsmA RNA and better expresses an rsmA-lacZ transcriptional fusion. Moreover, the expression of the rsmA-lacZ fusion in Escherichia coli is much higher in the presence of expR(71) (the expR gene of E. carotovora subsp. carotovora strain Ecc71) than in its absence. We also show that purified preparation of MBP-ExpR(71) binds (MBP, maltose binding protein) rsmA DNA. By contrast, MBP-ExpR(71) does not bind ahlI (gene for AHL synthase), pel-1 (gene for pectate lyase), or rsmB (gene for regulatory RNA that binds RsmA), nor does ExpR(71) activate expression of these genes. These observations strongly suggest transcriptional activation of rsmA resulting from a direct and specific interaction between ExpR(71) and the rsmA promoter. Several lines of evidence establish that N-3-oxohexanoyl-L-homoserine lactone (3-oxo-C6-HL), the major AHL analog produced by E. carotovora subsp. carotovora strain Ecc71, inhibits ExpR(71)-mediated activation of rsmA expression. These findings for the first time establish that the expR effect in E. carotovora subsp. carotovora is channeled via RsmA, a posttranscriptional regulator of E. carotovora subspecies, and AHL neutralizes this ExpR effect.

Carbapenem antibiotic biosynthesis in Erwinia carotovora is regulated by physiological and genetic factors modulating the quorum sensing-dependent control pathway

Erwinia carotovora produces the beta-lactam antibiotic, carbapenem, in response to a quorum sensing signalling molecule, N-(3-oxohexanoyl)-L-homoserine lactone (OHHL). We have mapped the OHHL-dependent promoter upstream of the first of the biosynthetic genes, carA. We have also analysed the effect on this promoter of the known genetic regulators of carbapenem expression, carR, carI (encoding homologues of LuxR and LuxI respectively) and hor (encoding a SlyA/MarR-like transcriptional regulator). We describe a previously unknown promoter located within the carA-H operon. This promoter does not respond to CarR and is required for quorum sensing-independent expression of the carbapenem resistance determinants encoded by the carFG genes. We have mapped the carR, carI and hor transcription start points, shown that CarR is positively autoregulated in the presence of OHHL, and have demonstrated negative feedback affecting transcription of carI. In addition, various environmental and physiological factors were shown to impinge on the transcription of the car biosynthetic genes. The nature of the carbon source and the temperature of growth influence carbapenem production by modulating the level of the OHHL signalling molecule, and thereby physiologically fine-tune the quorum sensing regulatory system.

Regulation and biosynthesis of carbapenem antibiotics in bacteria

Carbapenem antibiotics are members of the beta-lactam family of antibiotics, the most important class of antibiotics currently in clinical use. They are active against many important Gram-positive and Gram-negative pathogens. One important feature of carbapenem antibiotics is their resistance to several beta-lactamases. Thienamycin, isolated from Streptomyces cattleya, was the first carbapenem described. Other well-studied carbapenems were isolated from the Gram-negative bacteria Erwinia carotovora subsp. carotovora, Serratia sp. strain ATCC39006 and Photorhabdus luminescens strain TT01. Here, we review the genetics and biochemistry of carbapenem production in these bacteria. Research into carbapenems could uncover a new repertoire of bioactive molecules and biosynthetic enzymes, and exploiting these novel enzymes could lead to development of new classes of antibiotics with useful chemotherapeutic activities.

Insecticidal Bacillus thuringiensis silences Erwinia carotovora virulence by a new form of microbial antagonism, signal interference

It is commonly known that bacteria may produce antibiotics to interfere with the normal biological functions of their competitors in order to gain competitive advantages. Here we report that Bacillus thuringiensis suppressed the quorum-sensing-dependent virulence of plant pathogen Erwinia carotovora through a new form of microbial antagonism, signal interference. E. carotovora produces and responds to acyl-homoserine lactone (AHL) quorum-sensing signals to regulate antibiotic production and expression of virulence genes, whereas B. thuringiensis strains possess AHL-lactonase, which is a potent AHL-degrading enzyme. B. thuringiensis did not seem to interfere with the normal growth of E. carotovora; rather, it abolished the accumulation of AHL signal when they were cocultured. In planta, B. thuringiensis significantly decreased the incidence of E. carotovora infection and symptom development of potato soft rot caused by the pathogen. The biocontrol efficiency is correlated with the ability of bacterial strains to produce AHL-lactonase. While all the seven AHL-lactonase-producing B. thuringiensis strains provided significant protection against E. carotovora infection, Bacillus fusiformis and Escherichia coli strains that do not process AHL-degradation enzyme showed little effect in biocontrol. Mutation of aiiA, the gene encoding AHL-lactonase in B. thuringiensis, resulted in a substantial decrease in biocontrol efficacy. These results suggest that signal interference mechanisms existing in natural ecosystems could be explored as a new version of antagonism for prevention of bacterial infections.

Lsr-mediated transport and processing of AI-2 in Salmonella typhimurium

The LuxS-dependent autoinducer AI-2 is proposed to function in interspecies cell-cell communication in bacteria. In Salmonella typhimurium, AI-2 is produced and released during exponential growth and is subsequently imported into the bacteria via the Lsr (luxS regulated) ATP binding cassette (ABC) transporter. AI-2 induces transcription of the lsrACDBFGE operon, the first four genes of which encode the Lsr transport apparatus. In this report, we identify and characterize LsrK, a new protein that is required for the regulation of the lsr operon and the AI-2 uptake process. LsrK is a kinase that phosphorylates AI-2 upon entry into the cell. Our data indicate that phosphorylation of AI-2 results in its sequestration in the cytoplasm. We suggest that phospho-AI-2 is the inducer responsible for inactivation of LsrR, the repressor of the lsr operon. We also show that two previously uncharacterized members of the lsr operon, LsrF and LsrG, are necessary for the further processing of phospho-AI-2. Transport and processing of AI-2 could be required for removing the quorum-sensing signal, conveying the signal to an internal detector and/or scavenging boron.

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

Edwardsiella tarda is a gram-negative bacterium that causes septicaemia in fish and serious damage to the aquaculture industry. The virulence factors of this pathogen and control mechanisms of the expression of virulence genes have not yet been clearly elucidated. A number of gram-negative pathogenic bacteria have a quorum-sensing system. These bacteria produce N-acyl-L-homoserine lactone (AHL) that they use them as a quorum-sensing signal molecule. In this study, we found that E. tarda isolated from deceased flounder produces AHLs. Thin layer chromatography analysis indicated that the two kinds of AHL produced by E. tarda seemed to be N-hexanoyl-L-homoserine lactone (C6-HSL) and N-heptanoyl-L-homoserine lactone (C7-HSL). We have cloned and sequenced the quorum-sensing genes, luxI homolog (edwI) and luxR homolog (edwR). EdwI and EdwR showed high identity with CarI/CarR and ExpI/ExpR from Erwinia carotovora, respectively. SDS-PAGE analysis of extracellular proteins revealed that the expression of the 55-kDa protein, which was reported as a virulent-strain-specific protein, is controlled by AHLs. These results suggest that some virulence factors are regulated by the quorum-sensing system in E. tarda.

Influence of food preservation parameters and associated microbiota on production rate, profile and stability of acylated homoserine lactones from food-derived Enterobacteriaceae

Quorum-dependent regulation is mediated by N-acyl-L-homoserine lactones (AHLs) in several Gram-negative bacteria. The production of AHLs has typically been studied using pure bacteria cultures grown in nutrient-rich media at optimal temperature. AHLs are produced in several chill-stored foods by Gram-negative bacteria participating in spoilage. As part of our investigation of the role of AHLs in food quality, we studied the AHL production in two Enterobacteriaceae isolated from cold-smoked salmon under growth conditions typical of those found in cold-smoked salmon. We tested the influence of carbon source (glucose, sucrose, xylose, arabinose, mannose, mannitol and sorbitol), temperature (5 and 25 degrees C), salt concentration (0-7%), pH (6, 7 and 8) and co-existing lactic acid bacteria microflora on the AHL profile and production rate from Serratia proteamaculans strain B5a and Enterobacter agglomerans strain B6a. The two strains produced the same types of AHLs under all conditions tested. The specific AHL concentrations (moles/liter/OD(450)) changed slightly for both strains at the various conditions. S. proteamaculans strain B5a produced approximately 150 nM/OD(450) N-3-oxo-hexanoyl homoserine lactone (OHHL) and E. agglomerans strain B6a produced two major signals, OHHL and N-3-oxo-octanoyl homoserine lactone (OOHL) in a 1:9 ratio with a total concentration of approximately 3000 nM/OD(450). The AHL signal molecules became unstable with increasing pH (>7.5). In cold-smoked salmon, pH is approximately 6 and therefore only a low degree of pH-induced turnover is expected to occur in this product. Overall, our study demonstrates that food-derived Enterobacteriaceae produce AHLs of the same type and in the same magnitude when grown under food-relevant conditions as when grown in laboratory media at high temperature. Also, the AHLs produced in foods will be relatively stable and their regulatory impact lasting during storage.

N-acylhomoserine lactones undergo lactonolysis in a pH-, temperature-, and acyl chain length-dependent manner during growth of Yersinia pseudotuberculosis and Pseudomonas aeruginosa

In gram-negative bacterial pathogens, such as Pseudomonas aeruginosa and Yersinia pseudotuberculosis, cell-to-cell communication via the N-acylhomoserine lactone (AHL) signal molecules is involved in the cell population density-dependent control of genes associated with virulence. This phenomenon, termed quorum sensing, relies upon the accumulation of AHLs to a threshold concentration at which target structural genes are activated. By using biosensors capable of detecting a range of AHLs we observed that, in cultures of Y. pseudotuberculosis and P. aeruginosa, AHLs accumulate during the exponential phase but largely disappear during the stationary phase. When added to late-stationary-phase, cell-free culture supernatants of the respective pathogen, the major P. aeruginosa [N-butanoylhomoserine lactone (C4-HSL) and N-(3-oxododecanoyl)homoserine lactone (3-oxo-C12-HSL)] and Y. pseudotuberculosis [N-(3-oxohexanoyl)homoserine lactone (3-oxo-C6-HSL) and N-hexanoylhomoserine lactone (C6-HSL)] AHLs were inactivated. Short-acyl-chain compounds (e.g., C4-HSL) were turned over more extensively than long-chain molecules (e.g., 3-oxo-C12-HSL). Little AHL inactivation occurred with cell extracts, and no evidence for inactivation by specific enzymes was apparent. This AHL turnover was discovered to be due to pH-dependent lactonolysis. By acidifying the growth media to pH 2.0, lactonolysis could be reversed. By using carbon-13 nuclear magnetic resonance spectroscopy, we found that the ring opening of homoserine lactone (HSL), N-propionyl HSL (C3-HSL), and C4-HSL increased as pH increased but diminished as the N-acyl chain was lengthened. At low pH levels, the lactone rings closed but not via a simple reversal of the ring opening reaction mechanism. Ring opening of C4-HSL, C6-HSL, 3-oxo-C6-HSL, and N-octanoylhomoserine lactone (C8-HSL), as determined by the reduction of pH in aqueous solutions with time, was also less rapid for AHLs with more electron-donating longer side chains. Raising the temperature from 22 to 37 degrees C increased the rate of ring opening. Taken together, these data show that (i) to be functional under physiological conditions in mammalian tissue fluids, AHLs require an N-acyl side chain of at least four carbons in length and (ii) that the longer the acyl side chain the more stable the AHL signal molecule.

Genetic control of quorum-sensing signal turnover in Agrobacterium tumefaciens

A signal turnover system is an essential component of many genetic regulatory mechanisms. The best-known example is the ubiquitin-dependent protein degradation system that exists in many organisms. We found that Agrobacterium tumefaciens adopts a unique signal turnover system to control exiting from a quorum-sensing mode. A. tumefaciens regulates Ti plasmid conjugal transfer by a quorum-sensing signal, N-3-oxo-octanoyl homoserine lactone (3OC8HSL), also known as Agrobacterium autoinducer. By using Tn5 mutagenesis and a functional cloning approach, we identified two genes that are involved in switching from a conjugal quorum-sensing mode to a nonconjugal mode at the onset of stationary phase. First, we located attJ, which codes for an IclR-type suppressor that regulates the second gene attM. The latter encodes a homologue of N-acylhomoserine lactone (AHL)-lactonase. Mass spectrometry analysis shows that the enzyme encoded by attM is an AHL-lactonase that hydrolyzes the lactone ring of 3OC8HSL. In wild-type A. tumefaciens, attM expression is initially suppressed by AttJ but significantly elevated at the stationary phase accompanied a sharp decline in 3OC8HSL. DNA gel retardation analysis shows that AttJ specifically binds to the promoter that controls AHL-lactonase expression. Mutation of attJ resulted in constitutive production of AHL-lactonase that abolishes 3OC8HSL accumulation and Ti plasmid transfer. These data suggest that A. tumefaciens has a sophisticated multicomponent quorum-sensing signal turnover system, allowing the cell to sense a change in growth and adjust cellular activities accordingly.

Nonenzymatic turnover of an Erwinia carotovora quorum-sensing signaling molecule

The production of virulence factors and carbapenem antibiotic in the phytopathogen Erwinia carotovora is under the control of quorum sensing. The quorum-sensing signaling molecule, N-(3-oxohexanoyl)-L-homoserine lactone (OHHL), accumulates in log-phase culture supernatants of E. carotovora but diminishes in concentration during the stationary phase. In this study, we show that the diminution in OHHL was not due to sequestration of the ligand by the cells, although some partitioning did occur. Rather, it was caused by degradation of the molecule. The rate of stationary-phase degradation of OHHL was as rapid as the rate of log-phase accumulation of the ligand, but it was nonenzymatic and led to a decrease in the expression of selected genes known to be under the control of quorum sensing. The degradation of OHHL was dependent on the pH of the supernatant, which increased as the growth curve progressed in cultures grown in Luria-Bertani medium from pH 7 to approximately 8.5. OHHL became unstable over a narrow pH range (pH 7 to 8). Instability was increased at high temperatures even at neutral pH but could be prevented at the growth temperature (30 degrees C) by buffering the samples at pH 6.8. These results may provide a rationale for the observation that an early response of plants which are under attack by Erwinia is to activate a proton pump which alkalizes the site of infection to a pH of >8.2.

Quorum-sensing in Gram-negative bacteria

It has become increasingly and widely recognised that bacteria do not exist as solitary cells, but are colonial organisms that exploit elaborate systems of intercellular communication to facilitate their adaptation to changing environmental conditions. The languages by which bacteria communicate take the form of chemical signals, excreted from the cells, which can elicit profound physiological changes. Many types of signalling molecules, which regulate diverse phenotypes across distant genera, have been described. The most common signalling molecules found in Gram-negative bacteria are N-acyl derivatives of homoserine lactone (acyl HSLs). Modulation of the physiological processes controlled by acyl HSLs (and, indeed, many of the non-acyl HSL-mediated systems) occurs in a cell density- and growth phase-dependent manner. Therefore, the term 'quorum-sensing' has been coined to describe this ability of bacteria to monitor cell density before expressing a phenotype. In this paper, we review the current state of research concerning acyl HSL-mediated quorum-sensing. We also describe two non-acyl HSL-based systems utilised by the phytopathogens Ralstonia solanacearum and Xanthomonas campestris.

The evolution of bacterial LuxI and LuxR quorum sensing regulators

Quorum sensing is a widespread form of bacterial communication in which individual cells produce and respond to specific N-acyl homoserine lactone signal metabolites. The different autoinducer synthases that generate these signals and the receptor/activator proteins that mediate the cell's response to them constitute evolutionarily conserved families of regulatory proteins known as the LuxI and LuxR families, respectively. We have performed a phylogenetic analysis of 76 individual LuxI and LuxR homologues present in diverse members of the Gram-negative Proteobacteria. The results were consistent with an early origin for these regulators during the evolution of the Proteobacteria, with functional pairs of luxI and luxR genes possibly coevolving as regulatory cassettes. In many cases, specific LuxI and LuxR family members appeared to have been inherited horizontally. In particular, those species containing multiple LuxI and/or LuxR homologues usually appeared to have obtained each individual homologue or functional pair of homologues from an independent source. Because multiple homologues interact to form regulatory cascades, this finding suggests that hierarchical signalling pathways can potentially evolve by the sequential integration of pre-existing regulatory circuits acquired from diverse sources.

Biosynthesis of carbapenem antibiotic and prodigiosin pigment in Serratia is under quorum sensing control

Serratia sp. ATCC 39006 produces the carbapenem antibiotic, carbapen-2-em-3-carboxylic acid and the red pigment, prodigiosin. We have previously reported the characterization of a gene, carR, controlling production of carbapenem in this strain. We now describe further characterization of the carR locus to locate the genes encoding carbapenem biosynthetic and resistance functions. A novel family of diverse proteins showing sequence similarity to the C-terminal domain of CarF (required for carbapenem resistance) is described. We also report the isolation of the locus involved in the biosynthesis of the red pigment, prodigiosin. A cosmid containing approximately 35 kb of the Serratia chromosome encodes synthesis of the pigment in the heterologous host, Erwinia carotovora, demonstrating, for the first time, that the complete prodigiosin biosynthetic gene cluster had been cloned and functionally expressed. We report the isolation of a third locus in Serratia, containing convergently transcribed genes, smaI and smaR, encoding LuxI and LuxR homologues respectively. SmaI directs the synthesis of N-acyl homoserine lactones involved in the quorum sensing process. We demonstrate that biosynthesis of the two secondary metabolites, carbapenem antibiotic and prodigiosin pigment, is under pheromone-mediated transcriptional regulation in this bacterium. Finally, we describe a new prodigiosin-based bioassay for detection of some N-acyl homoserine lactones.

N-acyl homoserine lactone binding to the CarR receptor determines quorum-sensing specificity in Erwinia

Quorum sensing via an N-acyl homoserine lactone (HSL) pheromone controls the biosynthesis of a carbapenem antibiotic in Erwinia carotovora. Transcription of the carbapenem biosynthetic genes is dependent on the LuxR-type activator protein, CarR. Equilibrium binding of a range of HSL molecules, which are thought to activate CarR to bind to its DNA target sequence, was examined using fluorescence quenching, DNA bandshift analysis, limited proteolysis and reporter gene assays. CarR bound the most physiologically relevant ligand, N-(3-oxohexanoyl)-L-homoserine lactone, with a stoichiometry of two molecules of ligand per dimer of protein and a dissociation constant of 1.8 microM, in good agreement with the concentration of HSL required to activate carbapenem production in vivo. In the presence of HSL, CarR formed a very high molecular weight complex with its target DNA, indicating that the ligand causes the protein to multimerize. Chemical cross-linking analysis supported this interpretation. Our data show that the ability of a given HSL to facilitate CarR binding to its target DNA sequence is directly proportional to the affinity of the HSL for the protein.