EcbI and EcbR: homologs of LuxI and LuxR affecting antibiotic and exoenzyme production by Erwinia carotovora subsp. betavasculorum

Loci Identification of a N-acyl Homoserine Lactone Type Quorum Sensing System and a New LysR-type Transcriptional Regulator Associated with Antimicrobial Activity and Swarming in Burkholderia Gladioli UAPS07070

A random transposition mutant library of B. gladioli UAPS07070 was analyzed for searching mutants with impaired microbial antagonism. Three derivates showed diminished antimicrobial activity against a sensitive strain. The mutated loci showed high similarity to the quorum sensing genes of the AHL-synthase and its regulator. Another mutant was affected in a gene coding for a LysrR-type transcriptional regulator. The production of toxoflavin, the most well known antimicrobial-molecule and a major virulence factor of plant-pathogenic B. glumae and B. gladioli was explored. The absence of a yellowish pigment related to toxoflavin and the undetectable transcription of toxA in the mutants indicated the participation of the QS system and of the LysR-type transcriptional regulator in the regulation of toxoflavin. Additionally, those genes were found to be related to the swarming phenotype. Lettuce inoculated with the AHL synthase and the lysR mutants showed less severe symptoms. We present evidence of the participation of both, the quorum sensing and for the first time, of a LysR-type transcriptional regulator in antibiosis and swarming phenotype in a strain of B. gladioli

Attenuation and quantitation of virulence gene expression in quorum-quenched Dickeya chrysanthemi

N-Acyl-homoserine lactones (AHLs)-dependent quorum sensing (QS) system(s) is recruited by the soft rot bacterium Dickeya chrysanthemi for coordinating its social activities such as secretion of plant cell wall-degrading enzymes, while the main signal molecule and quantity dependence of virulence to QS in this bacterium have not been clarified. To do this end, the involvement of AHLs in African violet leaves and potato tuber maceration; swarming motility; pectate lyase and polygalacturonase enzymes production and in planta expression of virulence genes including pelE, pehX and pemA by electroporating two quorum-quenching vectors. The expression of two types of AHL-lactonase expressing vector caused dramatic decrease in swarming motility, production of pectinolytic enzymes and macerating of plant tissues. The maximum ability of quenching of QS in repression of D. chrysanthemi virulence was assessed quantitatively by q-RT-PCR, as expression of pelE, pehX and pemA genes were decreased 90.5-92.18 % in quenched cells. We also showed that virulence and pathogenicity of this bacterium was under the control of DHL-dependent QS system and that the existence of second DHL operating system is probable for this bacterium. Thus, this signal molecule would be the key point for future research to design DHL-specific lactonase enzymes using bioinformatics methods.

Microbial chemical signaling: a current perspective

Communication among microorganisms is mediated through quorum sensing. The latter is defined as cell-density linked, coordinated gene expression in microbial populations as a response to threshold signal concentrations followed by induction of a synchronized population response. This phenomenon is used by a variety of microbes to optimize their survival in a constantly challenging, dynamic milieu, by correlating individual cellular functions to community-based requirements. The synthesis, secretion, and perception of quorum-sensing molecules and their target response play a pivotal role in quorum sensing and are tightly controlled by complex, multilayered and interconnected signal transduction pathways that regulate diverse cellular functions. Quorum sensing exemplifies interactive social behavior innate to the microbial world that controls features such as, virulence, biofilm maturation, antibiotic resistance, swarming motility, and conjugal plasmid transfer. Over the past two decades, studies have been performed to rationalize bacterial cell-to-cell communication mediated by structurally and functionally diverse small molecules. This review describes the theoretical aspects of cellular and quorum-sensing mechanisms that affect microbial physiology and pathobiology.

Bacillus marcorestinctum sp. nov., a novel soil acylhomoserine lactone quorum-sensing signal quenching bacterium

A Gram-positive, facultatively anaerobic, endospore-forming and rod-shaped bacterium was isolated from soil samples and designated strain LQQ. This organism strongly quenches the acylhomoserine lactone quorum-sensing signal. The LQQ strain exhibits phenotypic characteristics consistent with its classification in the genus Bacillus. It is positive in catalase and no special growth factor is needed. It uses glucose as sole carbon source. The DNA G + C content is 39.8 mol %. The closest relatives based on the 16S rRNA gene sequence are Bacillus anthracis, Bacillus thuringiensis, and Brevibacillus brevis (syn. Bacillus brevis) with the similarity of 96.5%. The DNA–DNA hybridization data indicates a low level of genomic relatedness with the relative type strains of Bacillus thuringiensis (6.1%), Bacillus anthracis (10.5%) and Brevibacillus brevis (8.7%). On the basis of the phenotypic and phylogenetic data together with the genomic distinctiveness, the LQQ strain represents a novel species of the genus Bacillus, for which the name Bacillus marcorestinctum sp. nov. is proposed. The type strain is LQQ^T.

Messing with bacterial quorum sensing

Quorum sensing is widely recognized as an efficient mechanism to regulate expression of specific genes responsible for communal behavior in bacteria. Several bacterial phenotypes essential for the successful establishment of symbiotic, pathogenic, or commensal relationships with eukaryotic hosts, including motility, exopolysaccharide production, biofilm formation, and toxin production, are often regulated by quorum sensing. Interestingly, eukaryotes produce quorum-sensing-interfering (QSI) compounds that have a positive or negative influence on the bacterial signaling network. This eukaryotic interference could result in further fine-tuning of bacterial quorum sensing. Furthermore, recent work involving the synthesis of structural homologs to the various quorum-sensing signal molecules has resulted in the development of additional QSI compounds that could be used to control pathogenic bacteria. The creation of transgenic plants that express bacterial quorum-sensing genes is yet another strategy to interfere with bacterial behavior. Further investigation on the manipulation of quorum-sensing systems could provide us with powerful tools against harmful bacteria.

Restraining Erwinia virulence by expression of N-acyl homoserine lactonase gene pro3A-aiiA in Bacillus thuringiensis subsp leesis

To widen the biological control function of a genetically modified Bacillus thuringiensis subsp leesis strain BMB-005, an acyl homoserine lactonase (AHL lactonase) gene aiiA transcribed by the promoter of insecticidal crystal protein coding gene cry3A, was transformed into strain BMB-005. The amount of AHL lactonase protein produced by transformant BMB821A was 2.4-fold more than that produced by BMB-005. AHL-degradation assay showed that transformant BMB821A could degrade more AHLs molecules than the original strain BMB-005. The result of Erwinia carotovora pathogenicity test showed that the parental strain BMB-005 had no restraint of Erwinia infection, but the transformants exhibited strong restraint of E. carotovora infection on potato slices and cactus stems. Insecticidal bioassay against lepidopteran Spodoptera exigua showed that both strain BMB-005 and transformant BMB821A were toxic to S. exigua. The toxicity of transformant BMB821A (LC(50) was 3.8) was a little attenuated comparing with the toxicity of the original strain BMB-005 (LC(50) was 2.9). The B. thuringiensis strain BMB-005 has high toxicity against Helicoverpa armigera, Plutella xylostella, and S. exigua. This work provided new strategy for developing genetically engineered multi-functional B. thuringiensis strain that possesses insecticidal activity together with restraint of bacterial pathogenicity.

Comparative analysis of two classes of quorum-sensing signaling systems that control production of extracellular proteins and secondary metabolites in Erwinia carotovora subspecies

In Erwinia carotovora subspecies, N-acyl homoserine lactone (AHL) controls the expression of various traits, including extracellular enzyme/protein production and pathogenicity. We report here that E. carotovora subspecies possess two classes of quorum-sensing signaling systems defined by the nature of the major AHL analog produced as well as structural and functional characteristics of AHL synthase (AhlI) and AHL receptor (ExpR). Class I strains represented by E. carotovora subsp. atroseptica strain Eca12 and E. carotovora subsp. carotovora strains EC153 and SCC3193 produce 3-oxo-C8-HL (N-3-oxooctanoyl-l-homoserine lactone) as the major AHL analog as well as low but detectable levels of 3-oxo-C6-HL (N-3-oxohexanoyl-l-homoserine lactone). In contrast, the members of class II (i.e., E. carotovora subsp. betavasculorum strain Ecb168 and E. carotovora subsp. carotovora strains Ecc71 and SCRI193) produce 3-oxo-C6-HL as the major analog. ExpR species of both classes activate rsmA (Rsm, repressor of secondary metabolites) transcription and bind rsmA DNA. Gel mobility shift assays with maltose-binding protein (MBP)-ExpR(71) and MBP-ExpR(153) fusion proteins show that both bind a 20-mer sequence present in rsmA. The two ExpR functions (i.e., expR-mediated activation of rsmA expression and ExpR binding with rsmA DNA) are inhibited by AHL. The AHL effects are remarkably specific in that expR effect of EC153, a strain belonging to class I, is counteracted by 3-oxo-C8-HL but not by 3-oxo-C6-HL. Conversely, the expR effect of Ecc71, a strain belonging to class II, is neutralized by 3-oxo-C6-HL but not by 3-oxo-C8-HL. The AHL responses correlated with expR-mediated inhibition of exoprotein and secondary metabolite production.

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.

Involvement of bacterial quorum-sensing signals in spoilage of bean sprouts

Bacterial communication signals, acylated homoserine lactones (AHLs), were extracted from samples of commercial bean sprouts undergoing soft-rot spoilage. Bean sprouts produced in the laboratory did not undergo soft-rot spoilage and did not contain AHLs or AHL-producing bacteria, although the bacterial population reached levels similar to those in the commercial sprouts, 10(8) to 10(9) CFU/g. AHL-producing bacteria (Enterobacteriaceae and pseudomonads) were isolated from commercial sprouts, and strains that were both proteolytic and pectinolytic were capable of causing soft-rot spoilage in bean sprouts. Thin-layer chromatography and liquid chromatography-high-resolution mass spectrometry revealed the presence of N-3-oxo-hexanoyl-l-homoserine lactone in spoiled bean sprouts and in extracts from pure cultures of bacteria. During normal spoilage, the pH of the sprouts increased due to proteolytic activity, and the higher pH probably facilitated the activity of pectate lyase. The AHL synthetase gene (I gene) from a spoilage Pectobacterium was cloned, sequenced, and inactivated in the parent strain. The predicted amino acid sequence showed 97% homology to HslI and CarI in Erwinia carotovora. Spoilage of laboratory bean sprouts inoculated with the AHL-negative mutant was delayed compared to sprouts inoculated with the wild type, and the AHL-negative mutant did not cause the pH to rise. Compared to the wild-type strain, the AHL-negative mutant had significantly reduced protease and pectinase activities and was negative in an iron chelation (siderophore) assay. This is the first study demonstrating AHL regulation of iron chelation in Enterobacteriaceae. The present study clearly demonstrates that the bacterial spoilage of some food products is influenced by quorum-sensing-regulated phenotypes, and understanding these processes may be useful in the development of novel food preservation additives that specifically block the quorum-sensing systems.

Specificity and Enzyme Kinetics of the Quorum-quenching N-Acyl Homoserine Lactone Lactonase (AHL-lactonase)

N-Acyl homoserine lactone (AHL) quorum-sensing signals are the vital elements of bacterial quorum-sensing systems, which regulate diverse biological functions, including virulence. The AHL-lactonase, a quorumquenching enzyme encoded by aiiA from Bacillus sp., inactivates AHLs by hydrolyzing the lactone bond to produce corresponding N-acyl homoserines. To characterize the enzyme, the recombinant AHL-lactonase and its four variants were purified. Kinetic and substrate specificity analysis showed that AHL-lactonase had no or little residue activity to non-acyl lactones and noncyclic esters, but displayed strong enzyme activity toward all tested AHLs, varying in length and nature of the substitution at the C3 position of the acyl chain. The data also indicate that the amide group and the ketone at the C1 position of the acyl chain of AHLs could be important structural features in enzyme-substrate interaction. Surprisingly, although carrying a (104)HX- HXDH(109) short sequence identical to the zinc-binding motif of several groups of metallohydrolytic enzymes, AHL-lactonase does not contain or require zinc or other metal ions for enzyme activity. Except for the amino acid residue His-104, which was shown previously to not be required for catalysis, kinetic study and conformational analysis using circular dichroism spectrometry showed that substitution of the other key residues in the motif (His-106, Asp-108, and His-109), as well as His-169 with serine, respectively, caused conformational changes and significant loss of enzyme activity. We conclude that AHL-lactonase is a highly specific enzyme and that the (106)HXDH(109) approximately H(169) of AHL-lactonase represents a novel catalytic motif, which does not rely on zinc or other metal ions for activity.

Quorum sensing in plant-pathogenic bacteria

Quorum sensing (QS) allows bacteria to assess their local population density and/or physical confinement via the secretion and detection of small, diffusible signal molecules. This review describes how phytopathogenic bacteria have incorporated QS mechanisms into complex regulatory cascades that control genes for pathogenicity and colonization of host surfaces. Traits regulated by QS include the production of extracellular polysaccharides, degradative enzymes, antibiotics, siderophores, and pigments, as well as Hrp protein secretion, Ti plasmid transfer, motility, biofilm formation, and epiphytic fitness. Since QS regulatory systems are often required for pathogenesis, interference with QS signaling may offer a means of controlling bacterial diseases of plants. Several bacterial pathogens of plants that have been intensively studied and have revealed information of both fundamental and practical importance are reviewed here: Agrobacterium tumefaciens, Pantoea stewartii, Erwinia carotovora, Ralstonia solanacearum, Pseudomonas syringae, Pseudomonas aeruginosa, and Xanthomonas campestris.

Identification of quorum-quenching N-acyl homoserine lactonases from Bacillus species

A range of gram-negative bacterial species use N-acyl homoserine lactone (AHL) molecules as quorum-sensing signals to regulate different biological functions, including production of virulence factors. AHL is also known as an autoinducer. An autoinducer inactivation gene, aiiA, coding for an AHL lactonase, was cloned from a bacterial isolate, Bacillus sp. strain 240B1. Here we report identification of more than 20 bacterial isolates capable of enzymatic inactivation of AHLs from different sources. Eight isolates showing strong AHL-inactivating enzyme activity were selected for a preliminary taxonomic analysis. Morphological phenotypes and 16S ribosomal DNA sequence analysis indicated that these isolates probably belong to the species Bacillus thuringiensis. Enzymatic analysis with known Bacillus strains confirmed that all of the strains of B. thuringiensis and the closely related species B. cereus and B. mycoides tested produced AHL-inactivating enzymes but B. fusiformis and B. sphaericus strains did not. Nine genes coding for AHL inactivation were cloned either by functional cloning or by a PCR procedure from selected bacterial isolates and strains. Sequence comparison of the gene products and motif analysis showed that the gene products belong to the same family of AHL lactonases.

Quorum sensing as a population-density-dependent determinant of bacterial physiology

The discovery that bacterial cells can communicate with each other has led to the realization that bacteria are capable of exhibiting much more complex patterns of co-operative behaviour than would be expected for simple unicellular microorganisms. Now generically termed 'quorum sensing', bacterial cell-to-cell communication enables a bacterial population to mount a unified response that is advantageous to its survival by improving access to complex nutrients or environmental niches, collective defence against other competitive microorganisms or eukaryotic host defence mechanisms and optimization of population survival by differentiation into morphological forms better adapted to combating environmental threats. The principle of quorum sensing encompasses the production and release of signal molecules by bacterial cells within a population. Such molecules are released into the environment and, as cell numbers increase, so does the extracellular level of signal molecule, until the bacteria sense that a threshold has been reached and gene activation, or in some cases depression or repression, occurs via the activity of sensor-regulator systems. In this review, we will describe the biochemistry and molecular biology of a number of well-characterized N-acylhomoserine lactone quorum sensing systems to illustrate how bacteria employ cell-to-cell signalling to adjust their physiology in accordance with the prevailing high-population-density environment.

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.

Quenching quorum-sensing-dependent bacterial infection by an N-acyl homoserine lactonase

Bacterial cells sense their population density through a sophisticated cell-cell communication system and trigger expression of particular genes when the density reaches a threshold. This type of gene regulation, which controls diverse biological functions including virulence, is known as quorum sensing. Quorum-sensing signals, such as acyl-homoserine lactones (AHLs), are the essential components of the communication system. AHLs regulate virulence gene expression in a range of plant and animal (including human) bacterial pathogens. AHL-producing tobacco restored the pathogenicity of an AHL-negative mutant of Erwinia carotovora. Different bacterial species may produce different AHLs, which vary in the length and substitution of the acyl chain but contain the same homoserine lactone moiety. Here we show that the acyl-homoserine lactonase (AHL-lactonase), a new enzyme from Bacillus sp., inactivates AHL activity by hydrolysing the lactone bond of AHLs. Plants expressing AHL-lactonase quenched pathogen quorum-sensing signalling and showed significantly enhanced resistance to E. carotovora infection. Our results highlight a promising potential to use quorum-sensing signals as molecular targets for disease control, thereby broadening current approaches for prevention of bacterial infections.

Mutations in the extracellular protein secretion pathway genes (eps) interfere with rugose polysaccharide production in and motility of Vibrio cholerae

Vibrio cholerae is the causal organism of the diarrheal disease cholera. The rugose variant of V. cholerae is associated with the secretion of an exopolysaccharide. The rugose polysaccharide has been shown to confer increased resistance to a variety of agents, such as chlorine, bioacids, and oxidative and osmotic stresses. It also promotes biofilm formation, thereby increasing the survival of the bacteria in the aquatic environments. Here we show that the extracellular protein secretion system (gene designated eps) is involved directly or indirectly in the production of rugose polysaccharide. A TnphoA insertion in epsD gene of the eps operon abolished the production of rugose polysaccharide, reduced the secretion of cholera toxin and hemolysin, and resulted in a nonmotile phenotype. We have constructed defined mutations of the epsD and epsE genes that affected these phenotypes and complemented these defects by plasmid clones of the respective wild-type genes. These results suggest a major role for the eps system in pathogenesis and environmental survival of V. cholerae.

AiiA, an enzyme that inactivates the acylhomoserine lactone quorum-sensing signal and attenuates the virulence of Erwinia carotovora

N-acylhomoserine lactones, known as autoinducers (AIs), are widely conserved signal molecules present in quorum-sensing systems of many gram-negative bacteria. AIs are involved in the regulation of diverse biological functions, including expression of pathogenic genes in the plant pathogens Pseudomonas solanacearum, several Erwinia species, and the human pathogen Pseudomonas aeruginosa. A bacterial isolate, Bacillus sp. 240B1, is capable of enzymatic inactivation of AIs. The gene (aiiA) for AI inactivation from Bacillus sp. 240B1 has been cloned and shown to encode a protein of 250 amino acids. Sequence alignment indicates that AiiA contains a "HXHXDH" zinc-binding motif that is conserved in several groups of metallohydrolases. Site-directed mutagenesis showed that conserved aspartate and most histidine residues are required for AiiA activity. Expression of aiiA in transformed Erwinia carotovora strain SCG1 significantly reduces the release of AI, decreases extracellular pectolytic enzyme activities, and attenuates pathogenicity on potato, eggplant, Chinese cabbage, carrot, celery, cauliflower, and tobacco. Our results indicate that the AI-inactivation approach represents a promising strategy for prevention of diseases in which virulence is regulated by AIs.

AiiA, an enzyme that inactivates the acylhomoserine lactone quorum-sensing signal and attenuates the virulence of Erwinia carotovora

N-acylhomoserine lactones, known as autoinducers (AIs), are widely conserved signal molecules present in quorum-sensing systems of many gram-negative bacteria. AIs are involved in the regulation of diverse biological functions, including expression of pathogenic genes in the plant pathogens Pseudomonas solanacearum, several Erwinia species, and the human pathogen Pseudomonas aeruginosa. A bacterial isolate, Bacillus sp. 240B1, is capable of enzymatic inactivation of AIs. The gene (aiiA) for AI inactivation from Bacillus sp. 240B1 has been cloned and shown to encode a protein of 250 amino acids. Sequence alignment indicates that AiiA contains a "HXHXDH" zinc-binding motif that is conserved in several groups of metallohydrolases. Site-directed mutagenesis showed that conserved aspartate and most histidine residues are required for AiiA activity. Expression of aiiA in transformed Erwinia carotovora strain SCG1 significantly reduces the release of AI, decreases extracellular pectolytic enzyme activities, and attenuates pathogenicity on potato, eggplant, Chinese cabbage, carrot, celery, cauliflower, and tobacco. Our results indicate that the AI-inactivation approach represents a promising strategy for prevention of diseases in which virulence is regulated by AIs.