A hierarchical quorum-sensing system in Yersinia pseudotuberculosis is involved in the regulation of motility and clumping

Cell-cell signalling in bacteria: not simply a matter of quorum

Bacterial signalling known as quorum sensing (QS) relies on the synthesis of autoinducing signals throughout growth; when a threshold concentration is reached, these signals interact with a transcriptional regulator, allowing the expression of specific genes at a high cell density. One of the most studied intraspecies signalling is based on the use of N-acyl-homoserine lactones (AHL). Many factors other than cell density were shown to affect AHL accumulation and interfere with the QS signalling process. At the cellular level, the genetic determinants of QS are integrated in a complex regulatory network, including QS cascades and various transcriptional and post-transcriptional regulators that affect the synthesis of the AHL signal. In complex environments where bacteria exist, AHL do not accumulate at a constant rate; the diffusion and perception of the AHL signal outside bacterial cells can be compromised by abiotic environmental factors, by members of the bacterial community such as AHL-degrading bacteria and also by compounds produced by eukaryotes acting as an AHL mimic or inhibitor. This review aims to present all factors interfering with the AHL-mediated signalling process, at the levels of signal production, diffusion and perception.

Functional characterization of FlgM in the regulation of flagellar synthesis and motility in Yersinia pseudotuberculosis

We describe here the functional characterization of the flgM gene in Yersinia pseudotuberculosis. Direct interaction of FlgM with the alternative sigma factor σ 28 (FliA) was first confirmed. A conserved region in the C-terminus of FlgM was found which included the σ 28 binding domain. By site-directed mutagenesis, bacterial two-hybrid analysis and Western blotting, the primary FlgM binding sites with σ 28 were shown to be Ile85, Ala86 and Leu89. A role for FlgM in swimming motility was demonstrated by inactivation of flgM and subsequent complementation in trans. Transcriptional fusion analyses showed differential gene expression of flhDC, fliA, flgM and fliC in the fliA and flgM mutants compared with the wild-type. flhDC expression was not influenced by σ 28 or FlgM while fliA expression was abolished in the fliA mutant and considerably reduced in the flgM mutant when compared to the wild-type, indicating that both FliA and FlgM can activate fliA transcription. Conversely, flgM transcription was higher in the fliA mutant when compared to the wild-type, suggesting that flgM transcription was repressed by σ 28. Interestingly, fliC expression was markedly increased in the flgM mutant, suggesting a negative regulatory role for FlgM in fliC expression. The transcription of other σ-dependent genes (cheW, flgD, flaA, csrA and fliZ) was also examined in fliA and flgM mutant backgrounds and this revealed that other σ-factors apart from σ 28 may be involved in flagellar biogenesis in Y. pseudotuberculosis. Taking together the motility phenotypes and effects of flgM mutation on the regulation of these key motility genes, we propose that the mechanisms regulating flagellar biogenesis in Y. pseudotuberculosis may differ from those described for other bacteria.

OmpR positively regulates urease expression to enhance acid survival of Yersinia pseudotuberculosis

Yersinia pseudotuberculosis is an enteric bacterium which must overcome the acidic stress in host organs for successful colonization, but how this bacterium survives in acidic conditions remains largely unknown. In the present study, the importance of OmpR in acid survival of Y. pseudotuberculosis YpIII was confirmed by the fact that mutation of ompR (strain DeltaompR) greatly reduced cell survival at pH 4.5 or lower. To characterize the regulatory role of OmpR in this acid survival process, proteomic analysis was carried out to compare YpIII at pH 7.0 and pH 4.5 with DeltaompR at pH 7.0, and urease components were revealed to be the main targets for OmpR regulation. Addition of urea to the culture medium also enhanced acid survival of YpIII but not DeltaompR and urease activity was significantly induced by acid in YpIII but not in DeltaompR. Each of the seven components of the YpIII urease gene cluster was fused to a lacZ reporter and their expression was dramatically decreased in a DeltaompR background; this supports the notion that OmpR positively regulates urease expression. Furthermore, gel shift analysis revealed that OmpR binds to the deduced promoter regions of three polycistronic transcriptional units (ureABC, ureEF and ureGD) in the urease cluster, suggesting that the regulation of OmpR to urease components is direct. Taken together, these data strongly suggest that OmpR activates urease expression to enhance acid survival in Y. pseudotuberculosis.

Future research trends in the major chemical language of bacteria

Microbiology was revolutionized in the 1990's by the discovery that many different bacterial species coordinate their behavior when they form a group. In fact, bacteria are now considered multicellular organisms capable of communicating and changing behavior in relation to their cell-density; since 1994 this has been called quorum sensing. This group behavior ensures survival and propagation of the community in many natural environments. Bacterial intercellular communication is mediated by different chemical signals that are synthesized by bacteria which are then either secreted or diffused in the external environment. Bacteria are then able to detect the type and concentration of the signal resulting in regulation of gene expression and, consequently, a synchronized response by the community. The predominant signalling molecules produced by Gram-negative bacteria are N-acyl derivatives of homoserine lactone (AHLs) which have been shown to be produced by over seventy bacterial species. In this essay we discuss the importance of quorum sensing via AHLs and highlight current and future trends in this important field of research.

Mutation in the S-ribosylhomocysteinase (luxS) gene involved in quorum sensing affects biofilm formation and virulence in a clinical isolate of Aeromonas hydrophila

A diarrheal isolate SSU of Aeromonas hydrophila produces a cytotoxic enterotoxin (Act) with cytotoxic, enterotoxic, and hemolytic activities. Our laboratory has characterized from the above Aeromonas strain, in addition to Act, the type 3- and T6-secretion systems and their effectors, as well as the genes shown to modulate the production of AI-1-like autoinducers, N-acylhomoserine lactones (AHLs) involved in quorum sensing (QS). In this study, we demonstrated the presence of an S-ribosylhomocysteinase (LuxS)-based autoinducer (AI)-2 QS system in A. hydrophila SSU and its contribution to bacterial virulence. The luxS isogenic mutant of A. hydrophila, which we prepared by marker exchange mutagenesis, showed an alteration in the dynamics and architecture of the biofilm formation, a decrease in the motility of the bacterium, and an enhanced virulence in the septicemic mouse model. Moreover, these effects of the mutation could be complemented. Enhanced production of the biofilm exopolysaccharide and filaments in the mutant strain were presumably the major causes of the observed phenotype. Our earlier studies indicated that the wild-type A. hydrophila with overproduction of DNA adenine methyltransferase (Dam) had significantly reduced motility, greater hemolytic activity associated with Act, and an enhanced ability to produce AI-1 lactones. Furthermore, such a Dam-overproducing strain was not lethal to mice. On the contrary, the luxS mutant with Dam overproduction showed an increased motility and had no effect on lactone production. In addition, the Dam-overproducing luxS mutant strain was not altered in its ability to induce lethality in a mouse model of infection when compared to the parental strain which overproduced Dam. We suggested that an altered gene expression in the luxS mutant of A. hydrophila SSU, as it related to biofilm formation and virulence, might be linked with the interruption of the bacterial metabolic pathway, specifically of methionine synthesis.

Insights into the role of quorum sensing in food spoilage

Food spoilage is a consequence of the degrading enzymatic activity of some food-associated bacteria. Several proteolytic, lipolytic, chitinolytic, and pectinolytic activities associated with the deterioration of goods are regulated by quorum sensing, suggesting a potential role of such cell-to-cell communication in food spoilage. Here we review quorum sensing signaling molecules and methods of their detection and quantification, and we provide insights into the role of quorum sensing in food spoilage and address potential quorum sensing inhibitors that might be used as biopreservatives.

Agr-mediated dispersal of Staphylococcus aureus biofilms

The agr quorum-sensing system of Staphylococcus aureus modulates the expression of virulence factors in response to autoinducing peptides (AIPs). Recent studies have suggested a role for the agr system in S. aureus biofilm development, as agr mutants exhibit a high propensity to form biofilms, and cells dispersing from a biofilm have been observed displaying an active agr system. Here, we report that repression of agr is necessary to form a biofilm and that reactivation of agr in established biofilms through AIP addition or glucose depletion triggers detachment. Inhibitory AIP molecules did not induce detachment and an agr mutant was non-responsive, indicating a dependence on a functional, active agr system for dispersal. Biofilm detachment occurred in multiple S. aureus strains possessing divergent agr systems, suggesting it is a general S. aureus phenomenon. Importantly, detachment also restored sensitivity of the dispersed cells to the antibiotic rifampicin. Proteinase K inhibited biofilm formation and dispersed established biofilms, suggesting agr-mediated detachment occurred in an ica-independent manner. Consistent with a protease-mediated mechanism, increased levels of serine proteases were detected in detaching biofilm effluents, and the serine protease inhibitor PMSF reduced the degree of agr-mediated detachment. Through genetic analysis, a double mutant in the agr-regulated Aur metalloprotease and the SplABCDEF serine proteases displayed minimal extracellular protease activity, improved biofilm formation, and a strongly attenuated detachment phenotype. These findings indicate that induction of the agr system in established S. aureus biofilms detaches cells and demonstrate that the dispersal mechanism requires extracellular protease activity.

A biofilm is a surface-attached community of cells bound together by an extracellular matrix. In a bacterial infection, biofilm-encased cells are protected from antibiotic therapy and host immune response, and these encased cells can develop into a chronic infection. Staphylococcus aureus is a prominent bacterial pathogen known to form biofilms on many medical implants and host tissues. In this report, we demonstrate that repression of the S. aureus quorum-sensing system is required to form a biofilm, and quorum-sensing reactivation in established biofilms disperses the cells. Genetic and molecular analysis demonstrates that quorum-sensing is activated before and required for the detachment mechanism. Detachment is protease-mediated, as established biofilms are sensitive to a non-specific protease and quorum-sensing activation increases the production of extracellular proteases. Using mutations in the protease genes, we show that these secreted enzymes are required for the detachment mechanism. These findings denote that S. aureus quorum-sensing can function as a dispersal mechanism to colonize new sites, and our results suggest this mechanism could be modulated to treat recalcitrant biofilms.

Functional quorum sensing systems affect biofilm formation and protein expression in Yersinia pestis

Gram-negative bacteria predominantly use two types of quorum sensing (QS) systems--LuxI-LuxR, responsible for synthesis of N-acylhomoserine lactones (AHL or AI-1 signal molecule), and LuxS, which makes furanones (AI-2 signal molecule). We showed that LuxS and two LuxI-LuxR (YtbIR and YpsIR) systems are functional in Y. pestis. Four different AHL molecules were detected in Y. pestis extracts using TLC bioassays. Our data suggest that YtbIR is responsible for the production of long chain AHLs. Confocal laser scanning microscopy showed that biofilm formation is decreased in an ytbIR ypsIR luxS mutant. Two-dimensional gel electrophoresis revealed altered levels of protein expression in a Y. pestis triple QS mutant at 26 degrees C and 37 degrees C.

Relevance of N-acyl-L-homoserine lactone production by Yersinia enterocolitica in fresh foods

AIMS

Determination of the food matrix impact on the potential for N-acyl-L-homoserine lactones (AHLs) production by Yersinia enterocolitica.

METHODS AND RESULTS

Induction and inhibition of a sensor strain and a fluorescent assay were used to investigate Y. enterocolitica AHL production in artificial media, as well as in different food extracts. All Y. enterocolitica strains tested produced AHLs in artificial media. Thin Layer Chromatography analysis of Y. enterocolitica strains indicated the production of 3-oxo-hexanoyl homoserine lactone and hexanoyl homoserine lactone. Yersinia enterocolitica produced AHL principally in fish and meat extracts.

CONCLUSIONS

AHL production by Y. enterocolitica was observed in products of animal origin, but were inhibited by some vegetables extracts.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study suggests that quorum sensing systems in Y. enterocolitica is significant in foods but depends upon the type of food. Determination of physiological functions in Y. enterocolitica which are regulated by quorum sensing and their relation to the production of AHLs in foods need to be further assessed.

Quorum sensing by enteric pathogens

PURPOSE OF REVIEW

This review presents advances in our understanding of how pathogenic, enteric bacteria use quorum sensing to regulate several traits that allow them to establish and maintain infection in their host, including motility, biofilm formation, and virulence-specific genes.

RECENT FINDINGS

Quorum sensing in enteric bacteria has been elusive for a long time. Recent data indicate that enteric bacteria use several quorum-sensing mechanisms including the LuxR-I quorum-sensing system, the LuxS/AI-2 system, and the AI-3/epinephrine/norepinephrine system to assess their environment and to recognize the host environment. These systems allow bacteria to communicate across species boundaries, and the AI-3/epinephrine/norepinephrine system is involved in interkingdom signaling.

SUMMARY

Recent developments in our understanding of the molecular and biochemical mechanisms involved in quorum sensing as well as the chemical signal(s) to which bacteria respond and their corresponding physiological responses will improve our understanding of bacterial pathogenesis and microbial flora-host interactions, and potentially lead to novel strategies for combating infection.

Characterization of a luxI/luxR-type quorum sensing system and N-acyl-homoserine lactone-dependent regulation of exo-enzyme and antibacterial component production in Serratia plymuthica RVH1

Quorum sensing by means of N-acyl-l-homoserine lactones (AHLs) is widespread in Gram-negative bacteria, where diverse AHLs influence a wide variety of functions, even in a single genus such as Serratia. Here we report the identification and characterization of the quorum sensing system of Serratia plymuthica strain RVH1. This strain isolated from a raw vegetable processing line produces at least three AHLs which were identified as N-butanoyl- (C4-HSL), N-hexanoyl- (C6-HSL) and N-(3-oxo-hexanoyl)-homoserine lactone (3-oxo-C6-HSL). The identified LuxI homolog SplI synthesizes 3-oxo-C6-HSL, and influences the production of C4-HSL and C6-HSL, as splI gene inactivation resulted in loss of 3-oxo-C6-HSL production and smaller amounts of C4-HSL and C6-HSL produced. SplI-dependent quorum sensing controls 2,3-butanediol fermentation (previously reported) and the production of an extracellular chitinase, nuclease, protease and antibacterial compound. The identity of the latter is not yet elucidated, but appears to be different from the known antibacterial compounds produced by Serratia strains. SplR, the homolog of the LuxR regulator, appears to act as a repressor of synthesis of extracellular enzymes and antibacterial compound and to autorepress its own expression, probably by binding to a 21bp lux box sequence.

Quorum sensing regulates dpsA and the oxidative stress response in Burkholderia pseudomallei

Burkholderia pseudomallei is the causative agent of melioidosis, a fatal human tropical disease. The non-specific DNA-binding protein DpsA plays a key role in protecting B. pseudomallei from oxidative stress mediated, for example, by organic hydroperoxides. The regulation of dpsA expression is poorly understood but one possibility is that it is regulated in a cell population density-dependent manner via N-acylhomoserine lactone (AHL)-dependent quorum sensing (QS) since a lux-box motif has been located within the dpsA promoter region. Using liquid chromatography and tandem mass spectrometry, it was first established that B. pseudomallei strain PP844 synthesizes AHLs. These were identified as N-octanoylhomoserine lactone (C8-HSL), N-(3-oxooctanoyl)homoserine lactone (3-oxo-C8-HSL), N-(3-hydroxyoctanoyl)-homoserine lactone (3-hydroxy-C8-HSL), N-decanoylhomoserine lactone (C10-HSL), N-(3-hydroxydecanoyl) homoserine lactone (3-hydroxy-C10-HSL) and N-(3-hydroxydodecanoyl)homoserine lactone (3-hydroxy-C12-HSL). Mutation of the genes encoding the LuxI homologue BpsI or the LuxR homologue BpsR resulted in the loss of C8-HSL and 3-oxo-C8-HSL synthesis, demonstrating that BpsI was responsible for directing the synthesis of these AHLs only and that bpsI expression and hence C8-HSL and 3-oxo-C8-HSL production depends on BpsR. In bpsI, bpsR and bpsIR mutants, dpsA expression was substantially down-regulated. Furthermore, dpsA expression in Escherichia coli required both BpsR and C8-HSL. bpsIR-deficient mutants exhibited hypersensitivity to the organic hydroperoxide tert-butyl hydroperoxide by displaying a reduction in cell viability which was restored by provision of exogenous C8-HSL (bpsI mutant only), by complementation with the bpsIR genes or by overexpression of dpsA. These data indicate that in B. pseudomallei, QS regulates the response to oxidative stress at least in part via the BpsR/C8-HSL-dependent regulation of DpsA.

Comprehensive profiling of N-acylhomoserine lactones produced by Yersinia pseudotuberculosis using liquid chromatography coupled to hybrid quadrupole–linear ion trap mass spectrometry

A method for the comprehensive profiling of the N-acylhomoserine lactone (AHL) family of bacterial quorum-sensing molecules is presented using liquid chromatography (LC) coupled to hybrid quadrupole-linear ion trap (QqQLIT) mass spectrometry. Information-dependent acquisition (IDA), using triggered combinations of triple-quadrupole and linear ion trap modes in the same LC-MS/MS run, was used to simultaneously screen, quantify and identify multiple AHLs in a single sample. This MS method uses common AHL fragment ions attributed to the homoserine moiety and the 3-oxo-, 3-hydroxy- or unsubstituted acyl side chains, to identify unknown AHLs in cell-free culture supernatants in an unbiased manner. This LC-MS technique was applied to determine the relative molar ratios of AHLs produced by Yersinia pseudotuberculosis and the consequences of inactivating by mutation either or both of the AHL synthase genes (ypsI and ytbI) on AHL profile and concentration. The Y. pseudotuberculosis wild type but not the ypsI ytbI double mutant produced at least 24 different AHLs with acyl chains ranging from C4 to C15 with or without 3-oxo or 3-hydroxy substituents. YtbI, in contrast to YpsI, could direct the synthesis of all of the AHLs identified. The most abundant and hence most biologically relevant Y. pseudotuberculosis AHLs were found to be the 3-oxo-substituted C6, C7 and C8 AHLs and the unsubstituted C6 and C8 compounds. The LC-QqQLIT methodology is broadly applicable to quorum-sensing signal molecule analysis and can provide comprehensive AHL profiles and concentrations from a single sample and simultaneously collect confirmatory spectra for each AHL identified.

New insights into the regulatory mechanisms of the LuxR family of quorum sensing regulators

Bacteria use small signal molecules, referred to as autoinducers, in order to monitor their population density and coordinate gene expression in a process named quorum sensing. In Gram-negative bacteria, acylated homoserine lactones are the most common autoinducer used for cell-to-cell communication. Increasing evidence that many different functions are controlled by acylated homoserine lactone quorum sensing has stimulated intensive investigations into the physiology, molecular biology and biochemistry that underlie this process. Here we review our current understanding of the molecular mechanisms used by the transcriptional regulators responsive to acylated homoserine lactone autoinducers to control gene expression and the structural modifications induced by acylated homoserine lactones binding specifically on these regulators.

Cooperation of two distinct ExpR regulators controls quorum sensing specificity and virulence in the plant pathogen Erwinia carotovora

Quorum sensing, the population density-dependent regulation mediated by N-acylhomoserine lactones (AHSL), is essential for the control of virulence in the plant pathogen Erwinia carotovora ssp. carotovora (Ecc). In Erwinia carotovora ssp. the AHSL signal with an acyl chain of either 6 or 8 carbons is generated by an AHSL synthase, the expI gene product. This work demonstrates that the AHSL receptor, ExpR1, of Ecc strain SCC3193 has strict specificity for the cognate AHSL 3-oxo-C8-HSL. We have also identified a second AHSL receptor (ExpR2) and demonstrate a novel quorum sensing mechanism, where ExpR2 acts synergistically with the previously described ExpR1 to repress virulence gene expression in Ecc. We show that this repression is released by addition of AHSLs and appears to be largely mediated via the negative regulator RsmA. Additionally we show that ExpR2 has the novel property to sense AHSLs with different acyl chain lengths. The expI expR1 double mutant is able to act in response to a number of different AHSLs, while the expI expR2 double mutant can only respond to the cognate signal of Ecc strain SCC3193. These results suggest that Ecc is able to react both to the cognate AHSL signal and the signals produced by other bacterial species.

Quorum sensing affects virulence-associated proteins F1, LcrV, KatY and pH6 etc. of Yersinia pestis as revealed by protein microarray-based antibody profiling

Protein microarray that consists of virulence-associated proteins of Yersinia pestis is used to compare antibody profiles elicited by the wild-type and quorum sensing (QS) mutant strain of this bacterium to define the immunogens that are impacted by QS. The results will lead the way for future functional proteomics studies. The antibody profile that was induced by the QS mutant differed from that of the parent strain. Detailed comparison of the antibody profiles, according to the proteins' functional annotations, showed that QS affects the expression of many virulence-associated proteins of Y. pestis. The antibodies to many virulence-associated proteins were not detected or lower titers of antibodies to many proteins were detected in the sera of rabbits immunized with the QS mutant, relative to those of the wild type, which indicated that these proteins were not expressed or expressed at relatively lower levels in the QS mutant. The results demonstrated that antibody profiling by protein microarrays is a promising high-throughput method for revealing the interactions between pathogens and the host immune system.

The twin arginine translocation system is essential for virulence of Yersinia pseudotuberculosis

Yersinia species pathogenic to humans have been extensively characterized with respect to type III secretion and its essential role in virulence. This study concerns the twin arginine translocation (Tat) pathway utilized by gram-negative bacteria to secrete folded proteins across the bacterial inner membrane into the periplasmic compartment. We have shown that the Yersinia Tat system is functional and required for motility and contributes to acid resistance. A Yersinia pseudotuberculosis mutant strain with a disrupted Tat system (tatC) was, however, not affected in in vitro growth or more susceptible to high osmolarity, oxidative stress, or high temperature, nor was it impaired in type III secretion. Interestingly, the tatC mutant was severely attenuated via both the oral and intraperitoneal routes in the systemic mouse infection model and highly impaired in colonization of lymphoid organs like Peyer's patches and the spleen. Our work highlights that Tat secretion plays a key role in the virulence of Y. pseudotuberculosis.

Anti-quorum sensing activity of medicinal plants in southern Florida

Bacterial intercellular communication, or quorum sensing (QS), controls the pathogenesis of many medically important organisms. Anti-QS compounds are known to exist in marine algae and have the ability to attenuate bacterial pathogenicity. We hypothesized that terrestrial plants traditionally used as medicines may also produce anti-QS compounds. To test this hypothesis, 50 medicinal plants from southern Florida were screened for anti-QS activity using two biomonitor strains, Chromobacterium violaceum and Agrobacterium tumefaciens. Of these plants, six showed QS inhibition: Conocarpus erectus L. (Combretaceae), Chamaecyce hypericifolia (L.) Millsp. (Euphorbiaceae), Callistemon viminalis (Sol. ex Gaertn.) G. Don (Myrtaceae), Bucida burceras L. (Combretaceae), Tetrazygia bicolor (Mill.) Cogn. (Melastomataceae), and Quercus virginiana Mill. (Fagaceae). This study introduces not only a new mode of action and possible validation for traditional plant use, but also a potentially new therapeutic direction for the treatment of bacterial infections.

Identification of QuiP, the product of gene PA1032, as the second acyl-homoserine lactone acylase of Pseudomonas aeruginosa PAO1

The relevance of the acyl homoserine lactone (acyl-HSL) quorum signals N-3-oxododecanoyl-homoserine lactone (3OC12HSL) and N-butanoyl-homoserine lactone to the biology and virulence of Pseudomonas aeruginosa is well investigated. Previously, P. aeruginosa was shown to degrade long-chain, but not short-chain, acyl-HSLs as sole carbon and energy sources (J. J. Huang, J.-I. Han, L.-H. Zhang, and J. R. Leadbetter, Appl. Environ. Microbiol. 69:5941-5949, 2003). A gene encoding an enzyme with acyl-HSL acylase activity, pvdQ (PA2385), was identified, but it was not required for acyl-HSL utilization. This indicated that P. aeruginosa encodes another acyl-HSL acylase, which we identify here. A comparison of total cell proteins of cultures grown with long-acyl acyl-HSLs versus other substrates implicated the involvement of a homolog of PvdQ, the product of gene PA1032, for which we propose the name QuiP. Transposon mutants of quiP were defective for growth when P. aeruginosa was cultured in medium containing decanoyl-HSL as a sole carbon and energy source. Complementation with a functional copy of quiP rescued this growth defect. When P. aeruginosa was grown in buffered lysogeny broth, constitutive expression of QuiP in P. aeruginosa led to decreased accumulations of the quorum signal 3OC12HSL, relative to the wild type. Heterologous expression of QuiP was sufficient to confer long-chain acyl-HSL acylase activity upon Escherichia coli. Examination of gene expression patterns during acyl-HSL-dependent growth of P. aeruginosa further supported the involvement of quiP in signal decay and revealed other genes also possibly involved. It is not yet known under which "natural" conditions quiP is expressed or how P. aeruginosa balances the expression of its quorum-sensing systems with the expression of its acyl-HSL acylase activities.

Quorum-sensing signal synthesis by the Yersinia pestis acyl-homoserine lactone synthase YspI

The acyl-homoserine lactone molecular species (AHLs) produced by the Yersinia pestis AHL synthase YspI were identified by biochemical and physical/chemical techniques. Bioassays of extracts from culture supernatants of the recombinant YspI and wild-type Yersinia pestis showed similar profiles of AHLs. Analysis by liquid chromatography-mass spectrometry revealed that the predominant AHLs were N-3-oxooctanoyl-L-homoserine lactone and N-3-oxo-hexanoyl-L-homoserine lactone.

Quorum sensing in Yersinia enterocolitica controls swimming and swarming motility

The Yersinia enterocolitica LuxI homologue YenI directs the synthesis of N-3-(oxohexanoyl)homoserine lactone (3-oxo-C6-HSL) and N-hexanoylhomoserine lactone (C6-HSL). In a Y. enterocolitica yenI mutant, swimming motility is temporally delayed while swarming motility is abolished. Since both swimming and swarming are flagellum dependent, we purified the flagellin protein from the parent and yenI mutant. Electrophoresis revealed that in contrast to the parent strain, the yenI mutant grown for 17 h at 26 degrees C lacked the 45-kDa flagellin protein FleB. Reverse transcription-PCR indicated that while mutation of yenI had no effect on yenR, flhDC (the motility master regulator) or fliA (the flagellar sigma factor) expression, fleB (the flagellin structural gene) was down-regulated. Since 3-oxo-C6-HSL and C6-HSL did not restore swimming or swarming in the yenI mutant, we reexamined the N-acylhomoserine lactone (AHL) profile of Y. enterocolitica. Using AHL biosensors and mass spectrometry, we identified three additional AHLs synthesized via YenI: N-(3-oxodecanoyl)homoserine lactone, N-(3-oxododecanoyl)homoserine lactone (3-oxo-C12-HSL), and N-(3-oxotetradecanoyl)homoserine lactone. However, none of the long-chain AHLs either alone or in combination with the short-chain AHLs restored swarming or swimming in the yenI mutant. By investigating the transport of radiolabeled 3-oxo-C12-HSL and by introducing an AHL biosensor into the yenI mutant we demonstrate that the inability of exogenous AHLs to restore motility to the yenI mutant is not related to a lack of AHL uptake. However, both AHL synthesis and motility were restored by complementation of the yenI mutant with a plasmid-borne copy of yenI.

Comparative genomics reveals what makes an enterobacterial plant pathogen

The bacterial family Enterobacteriaceae contains some of the most devastating human and animal pathogens, including Escherichia coli, Salmonella enterica and species of Yersinia and Shigella. These are among the best-studied of any organisms, yet there is much to be learned about the nature and evolution of interactions with their hosts and with the wider environment. Comparative and functional genomics have fundamentally improved our understanding of their modes of adaptation to different ecological niches and the genes that determine their pathogenicity. In addition to animal pathogens, Enterobacteriaceae include important plant pathogens, such as Erwinia carotovora subsp. atroseptica (Eca), the first plant-pathogenic enterobacterium to be sequenced. This review focuses on genomic comparisons between Eca and other enterobacteria, with particular emphasis on the differences that exemplify or explain the plant-associated lifestyle(s) of Eca. Horizontal gene transfer in Eca may directly have led to the acquisition of a number of determinants that mediate its interactions, pathogenic or otherwise, with plants, offering a glimpse into its evolutionary divergence from animal-pathogenic enterobacteria.

The LuxR receptor: the sites of interaction with quorum-sensing signals and inhibitors

The function of LuxR homologues as quorum sensors is mediated by the binding of N-acyl-L-homoserine lactone (AHL) signal molecules to the N-terminal receptor site of the proteins. In this study, site-directed mutagenesis was carried out of the amino acid residues comprising the receptor site of LuxR from Vibrio fischeri, and the ability of the L42A, L42S, Y62F, W66F, D79N, W94D, V109D, V109T and M135A LuxR mutant proteins to activate green fluorescent protein expression from a P(luxI) promoter was measured. X-ray crystallographic studies of the LuxR homologue TraR indicated that residues Y53 and W57 form hydrogen bonds to the 1-carbonyl group and the ring carbonyl group, respectively, of the cognate AHL signal. Based on the activity and signal specificity of the LuxR mutant proteins, and on molecular modelling, a model is suggested in which Y62 (corresponding to Y53 in TraR) forms a hydrogen bond with the ring carbonyl group rather than the 1-carbonyl group, while W66 (corresponding to W57 in TraR) forms a hydrogen bond to the 1-carbonyl group. This flips the position of the acyl side chain in the LuxR/signal molecule complex compared to the TraR/signal molecule complex. Halogenated furanones from the marine alga Delisea pulchra and the synthetic signal analogue N-(sulfanylacetyl)-L-homoserine lactone can block quorum sensing. The LuxR mutant proteins were insensitive to inhibition by N-(propylsulfanylacetyl)-L-homoserine lactone. In contrast, the mutations had only a minor effect on the sensitivity of the proteins to halogenated furanones, and the data strongly suggest that these compounds do not compete in a 'classic' way with N-3-oxohexanoyl-L-homoserine lactone for the binding site. Based on modelling and experimental data it is suggested that these compounds bind in a non-agonist fashion.

Elevated temperature enhances virulence of Erwinia carotovora subsp. carotovora strain EC153 to plants and stimulates production of the quorum sensing signal, N-acyl homoserine lactone, and extracellular proteins

Erwinia carotovora subsp. atroseptica, E. carotovora subsp. betavasculorum, and E. carotovora subsp. carotovora produce high levels of extracellular enzymes, such as pectate lyase (Pel), polygalacturonase (Peh), cellulase (Cel), and protease (Prt), and the quorum-sensing signal N-acyl-homoserine lactone (AHL) at 28 degrees C. However, the production of these enzymes and AHL by these bacteria is severely inhibited during growth at elevated temperatures (31.2 degrees C for E. carotovora subsp. atroseptica and 34.5 degrees C for E. carotovora subsp. betavasculorum and most E. carotovora subsp. carotovora strains). At elevated temperatures these bacteria produce high levels of RsmA, an RNA binding protein that promotes RNA decay. E. carotovora subsp. carotovora strain EC153 is an exception in that it produces higher levels of Pel, Peh, Cel, and Prt at 34.5 degrees C than at 28 degrees C. EC153 also causes extensive maceration of celery petioles and Chinese cabbage leaves at 34.5 degrees C, which correlates with a higher growth rate and higher levels of rRNA and AHL. The lack of pectinase production by E. carotovora subsp. carotovora strain Ecc71 at 34.5 degrees C limits the growth of this organism in plant tissues and consequently impairs its ability to cause tissue maceration. Comparative studies with ahlI (the gene encoding a putative AHL synthase), pel-1, and peh-1 transcripts documented that at 34.5 degrees C the RNAs are more stable in EC153 than in Ecc71. Our data reveal that overall metabolic activity, AHL levels, and mRNA stability are responsible for the higher levels of extracellular protein production and the enhanced virulence of EC153 at 34.5 degrees C compared to 28 degrees C.

Sociomicrobiology: the connections between quorum sensing and biofilms

In the past decade, significant debate has surrounded the relative contributions of genetic determinants versus environmental conditions to certain types of human behavior. While this debate goes on, it is with a certain degree of irony that microbiologists studying aspects of bacterial community behavior face the same questions. Information regarding two social phenomena exhibited by bacteria, quorum sensing and biofilm development, is reviewed here. These two topics have been inextricably linked, possibly because biofilms and quorum sensing represent two areas in which microbiologists focus on social aspects of bacteria. We will examine what is known about this linkage and discuss areas that might be developed. In addition, we believe that these two aspects of bacterial behavior represent a small part of the social repertoire of bacteria. Bacteria exhibit many social activities and they represent a model for dissecting social behavior at the genetic level. Therefore, we introduce the term 'sociomicrobiology'.

Quorum sensing regulates exopolysaccharide production, motility, and virulence in Pseudomonas syringae

The N-acyl homoserine lactone (AHL)-mediated quorum-sensing system in the phytopathogen Pseudomonas syringae pv. syringae requires the AHL synthase AhlI and the regulator AhlR, and is additionally subject to regulation by AefR. The contribution of quorum sensing to the expression of a variety of traits expected to be involved in epiphytic fitness and virulence of P syringae were examined. Both an aefR- mutant and an ahlI- ahlR- double mutant, deficient in AHL production, were significantly impaired in alginate production and had an increased susceptibility to hydrogen peroxide compared with the wild-type strain. These mutants were hypermotile in culture, invaded leaves more rapidly, and caused an increased incidence of brown spot lesions on bean leaves after a 48-h moist incubation. Interestingly, an aefR- mutant was both the most motile and virulent. Like the wild-type strain, the AHL-deficient mutant strains incited water-soaked lesions on bean pods. However, lesions caused by an ahlI- ahlR- double mutant were larger, whereas those incited by an aefR- mutant were smaller. In contrast, tissue maceration of pods, which occurs at a later stage of infection, was completely abolished in the AHL-deficient mutants. Both the incidence of disease and in planta growth of P syringae pv. tabaci were greatly reduced in transgenic tobacco plants that produced AHL compared with wild-type plants. These results demonstrate that quorum sensing in E syringae regulates traits that contribute to epiphytic fitness as well as to distinct stages of disease development during plant infection.

Potential use of a Yersinia ruckeri O1 auxotrophic aroA mutant as a live attenuated vaccine

The aroA gene of Yersinia ruckeri, which encodes 5-enolpyruvylshikimate 3-phosphate synthase, was insertionally inactivated with a DNA fragment containing a kanamycin resistance determinant and reintroduced by allelic exchange into the chromosome of Y. ruckeri 21102 O1 by means of the suicide vector pIVET8. The Y. ruckeri aroA::Kan(r) mutant was highly attenuated when inoculated intraperitoneally into rainbow trout, with a 50% lethal dose of >5 x 10(7) CFU. The mutants were not recoverable from the internal organs 48 h post-inoculation or later. The vaccination of rainbow trout with the AroA mutant as a live vaccine conferred significant protection (relative percentage survival = 90%) against the pathogenic wild-type strain of Y. ruckeri.

Biofilm formation and dispersal in Xanthomonas campestris

Xanthomonas campestris pathovar campestris is the causal agent of black rot disease of cruciferous plants. A cell-cell signalling system encoded by genes within the rpf cluster is required for the full virulence of this plant pathogen. This system has recently been implicated in regulation of the formation and dispersal of Xanthomonas biofilms.

Bacterial populations in the rhizosphere of tobacco plants producing the quorum-sensing signals hexanoyl-homoserine lactone and 3-oxo-hexanoyl-homoserine lactone

A tobacco line genetically modified to produce two N-acyl homoserine lactones and its non-transformed parental line were grown in non-sterile soil. Microbial populations inhabiting the bulk soil, and those colonizing the root system of the two tobacco lines, were analyzed using cultivation-independent (phospholipid fatty acid and denaturing gradient gel electrophoresis) and cultivation-based assays. The cell density of total cultivable bacteria, fluorescent pseudomonads, sporulated, and thermotolerant bacteria was also determined in a time-course experiment (15 weeks). A possible "rhizosphere effect" related to the development of the plant was seen. However, no dissimilarities in cell population densities or population ratios of the microbial groups were detected in the rhizosphere of the two plant lines. Similarly, bacterial communities that either produced N-acyl homoserine lactone or degraded the signal hexanoyl homoserine lactone were enumerated from the two plant lines. No noticeable differences were evidenced from one plant genotype to the other. Whilst the transgenic plants released detectable amounts of the quorum-sensing signal molecules and efficiently cross-talked with the surrounding microbial populations, the bias generated by these signals in the reported experimental conditions therefore appears to remain weak, if not non-existent.

Quorum quenching: enzymatic disruption of N-acylhomoserine lactone-mediated bacterial communication in Burkholderia thailandensis

Many species of gram-negative bacteria communicate by synthesizing, secreting, and responding to N-acylhomoserine lactones (AHLs), a mechanism termed quorum sensing. Several investigations have characterized numerous AHL-degrading enzymes (AiiA lactonases) encoded by environmental isolates of Bacillus spp. The Burkholderia thailandensis quorum system is comprised of at least three AHL synthases (AHSs) and five transcriptional regulators belonging to the LuxIR class of proteins. Expression of the Bacillus anthracis (Ames strain) AiiA lactonase in B. thailandensis completely abolished the accumulation of N-decanoylhomoserine lactone (C(10)-HSL) and N-octanoylhomoserine lactone (C(8)-HSL), reduced N-hexanoylhomoserine lactone (C(6)-HSL) levels, altered both swarming and twitching motility, caused a significant increase in generation time, and affected carbon metabolism. In contrast, heterologous expression of the Bacillus cereus strain A24 AiiA lactonase in B. thailandensis reduced the concentrations of C(6)-HSL, C(8)-HSL, and C(10)-HSL to nondetectable levels; altered both swarming and twitching motility; and caused fluctuations in carbon utilization. Individual disruption of the B. thailandensis AHSs, specifically disruption of the btaI1 and btaI3 genes, which encode the proteins that direct the synthesis of C(8)-HSL and C(6)-HSL, respectively, caused the hyper-beta-hemolysis of sheep erythrocytes on blood agar plates. In contrast, AHL cleavage in B. thailandensis by the Bacillus AiiA lactonases failed to enhance beta-hemolytic activity. The results of this study demonstrate that heterologous expression of Bacillus sp. AiiA lactonases in B. thailandensis reduced AHL accumulation, affected both swarming and twitching motility, increased generation time, altered substrate utilization, and prevented the beta-hemolysis of sheep erythrocytes.

Real-time characterization of virulence factor expression in Yersinia pestis using a GFP reporter system

A real-time reporter system was developed to monitor the thermal induction of virulence factors in Yersinia pestis, the etiological agent of plague. The reporter system consists of a plasmid in Y. pestis in which the expression of green fluorescent protein (GFP) is under the control of the promoters for six virulence factors, yopE, sycE, yopK, yopT, yscN, and lcrE yopN, which are all components of the Type III secretion virulence mechanism of Y. pestis. Induction of the expression of these genes in vivo was determined by the increase in fluorescence intensity of GFP in real time, in 96-well format. Different basal levels of expression at 26 degrees C were observed for the Y. pestis promoters. Expressed as percentages of the level measured for the lac promoter (positive control), the basal expression levels before temperature shift were: yopE (15%), sycE (15%), yopK (13%), yopT (4%), lcrE (3.3%), and yscN (0.8%). Following the shift in temperature from 26 to 37 degrees C, the rates of expression of these genes increased with the yopE reporter showing the strongest degree of induction. The rates of induction of the other virulence factors after the temperature, expressed as percentages of yopE induction, were: yopK (57%), sycE (9%), yscN (3%), lcrE (3%), and yopT (2%). The thermal induction of each of these promoter fusions was repressed by calcium, and the ratios of the initial rates of thermal induction without calcium supplementation compared to the rate with calcium supplementation were: yopE (11-fold), yscN (7-fold), yopK (6-fold), lcrE (3-fold), yopT (2-fold), and sycE (1-fold). This work demonstrates a novel approach to quantify gene induction and provides a method to rapidly determine the effects of external stimuli on expression of Y. pestis virulence factors in real time, in living cells, as a means to characterize virulence determinants.

Involvement of N-acylhomoserine lactones throughout plant infection by Erwinia carotovora subsp. atroseptica (Pectobacterium atrosepticum)

Erwinia carotovora subsp. atroseptica is responsible for potato blackleg disease in the field and tuber soft rot during crop storage. The process leading to the disease occurs in two phases: a primary invasion step followed by a maceration step. Bacteria-to-bacteria communication is associated with a quorum-sensing (QS) process based on the production of N-acylhomoserine lactones (HSL). The role of HSL throughout plant infection was analyzed. To this purpose, HSL produced by a specific E. carotovora subsp. atroseptica wild-type strain, which was particularly virulent on potato, were identified. A derivative of this strain that expressed an HSL lactonase gene and produced low amounts of HSL was generated. The comparison of these strains allowed the evaluation of the role of HSL and QS in disease establishment and development. Bacterial growth and motility; activity of proteins secreted by type I, II, and III systems; and hypersensitive and maceration reactions were evaluated. Results indicated that HSL production and QS regulate only those traits involved in the second stage of the host plant infection (i.e., tissue maceration) and hypersensitive response in nonhost tobacco plants. Therefore, the use of QS quenching strategies for biological control in E. carotovora subsp. atroseptica cannot prevent initial infection and multiplication of this pathogen.

Presence of acylated homoserine lactones (AHLs) and AHL-producing bacteria in meat and potential role of AHL in spoilage of meat

Quorum-sensing (QS) signals (N-acyl homoserine lactones [AHLs]) were extracted and detected from five commercially produced vacuum-packed meat samples. Ninety-six AHL-producing bacteria were isolated, and 92 were identified as Enterobacteriaceae. Hafnia alvei was the most commonly identified AHL-producing bacterium. Thin-layer chromatographic profiles of supernatants from six H. alvei isolates and of extracts from spoiling meat revealed that the major AHL species had an R(f) value and shape similar to N-3-oxo-hexanoyl homoserine lactone (OHHL). Liquid chromatography-mass spectrometry (MS) (high-resolution MS) analysis confirmed the presence of OHHL in pure cultures of H. alvei. Vacuum-packed meat spoiled at the same rate when inoculated with the H. alvei wild type compared to a corresponding AHL-lacking mutant. Addition of specific QS inhibitors to the AHL-producing H. alvei inoculated in meat or to naturally contaminated meat did not influence the spoilage of vacuum-packed meat. An extracellular protein of approximately 20 kDa produced by the H. alvei wild-type was not produced by the AHL-negative mutant but was restored in the mutant when complemented by OHHL, thus indicating that AHLs do have a regulatory role in H. alvei. Coinoculation of H. alvei wild-type with an AHL-deficient Serratia proteamaculans B5a, in which protease secretion is QS regulated, caused spoilage of liquid milk. By contrast, coinoculation of AHL-negative strains of H. alvei and S. proteamaculans B5a did not cause spoilage. In conclusion, AHL and AHL-producing bacteria are present in vacuum-packed meat during storage and spoilage, but AHL does not appear to influence the spoilage of this particular type of conserved meat. Our data indicate that AHL-producing H. alvei may induce food quality-relevant phenotypes in other bacterial species in the same environment. H. alvei may thus influence spoilage of food products in which Enterobacteriaceae participate in the spoilage process.

Mutational analysis and biochemical characterization of the Burkholderia thailandensis DW503 quorum-sensing network

Numerous gram-negative bacteria communicate and regulate gene expression through a cell density-responsive mechanism termed quorum sensing (QS), which involves the synthesis and perception of diffusible N-acyl-homoserine lactones (AHL). In this study we genetically and physiologically characterized the Burkholderia thailandensis DW503 QS network. In silico analysis of the B. thailandensis genome revealed the presence of at least three AHL synthases (AHS) and five transcriptional regulators belonging to the LuxIR family of proteins. Mass spectrometry demonstrated that wild-type B. thailandensis synthesizes N-hexanoyl-homoserine lactone (C6-HSL), N-octanoyl-homoserine lactone (C8-HSL), and N-decanoyl-homoserine lactone (C10-HSL). Mutation of the btaI1 (luxI) AHS gene prevented accumulation of C8-HSL in culture supernatants, enhanced beta-hemolysis of sheep erythrocytes, increased lipase production, and altered colony morphology on swarming and twitching motility plates. Disruption of the btaI3 (luxI) AHS prevented biosynthesis of C6-HSL and increased lipase production and beta-hemolysis, whereas mutagenesis of the btaI2 (luxI) allele eliminated C10-HSL accumulation and reduced lipase production. Complementation of the btaI1 and btaI3 mutants fully restored the synthesis of C8-HSL and C6-HSL to parental levels. In contrast, mutagenesis of the btaR1, btaR3, btaR4, and btaR5 (luxR) transcriptional regulators had no effect on AHL accumulation, enhanced lipase production, and resulted in extensive beta-hemolysis on sheep blood agar plates. Furthermore, interruption of the btaI1, btaR1, and btaR3 genes altered colony morphology on twitching and swarming motility plates and induced pigmentation. Additionally, phenotypic microarray analysis indicated that QS in B. thailandensis both positively and negatively affects the metabolism of numerous substrates, including citric acid, formic acid, glucose 6-phosphate, capric acid, gamma-hydroxybutyric acid, and d-arabinose. These results demonstrate that mutagenesis of the B. thailandensis QS system affects various cellular processes, including lipase production, swarming and twitching motility, beta-hemolysis of sheep erythrocytes, and carbon metabolism and/or transport.

Quorum sensing: a primer for food microbiologists

Quorum sensing is a signaling mechanism through which bacteria modulate a number of cellular functions (genes), including sporulation, biofilm formation, bacteriocin production, virulence responses, as well as others. Quorum sensing is a mechanism of cell-to-cell communication and is mediated by extracellular chemical signals generated by the bacteria when specific cell densities are reached. When the concentration of the signal (and cell population) is sufficiently high, the target gene or genes are either activated or repressed. Quorum sensing increases the ability of the bacteria to have access to nutrients or to more favorable environmental niches and enhances bacterial defenses against eukaryotic hosts, competing bacteria, and environmental stresses. The physiological and clinical aspects of quorum sensing have received considerable attention and have been studied at the molecular level. Little is known, however, on the role of quorum sensing in food spoilage or in the growth and/or toxin production of pathogens present in food. A number of compounds have been isolated or synthesized that antagonize quorum sensors, and application of these antagonists may potentially be useful in inhibiting the growth or virulence mechanisms of bacteria in different environments, including food. It is important that food microbiologists have an awareness and an understanding of the mechanisms involved in bacterial quorum sensing, since strategies targeting quorum sensing may offer a means to control the growth of undesirable bacteria in foods.

The Burkholderia cepacia epidemic strain marker is part of a novel genomic island encoding both virulence and metabolism-associated genes in Burkholderia cenocepacia

The Burkholderia cepacia epidemic strain marker (BCESM) is a useful epidemiological marker for virulent B. cenocepacia strains that infect patients with cystic fibrosis. However, there was no evidence that the original marker, identified by random amplified polymorphic DNA fingerprinting, contributed to pathogenicity. Here we demonstrate that the BCESM is part of a novel genomic island encoding genes linked to both virulence and metabolism. The BCESM was present on a 31.7-kb low-GC-content island that encoded 35 predicted coding sequences (CDSs): an N-acyl homoserine lactone (AHL) synthase gene (cciI) and corresponding transcriptional regulator (cciR), representing the first time cell signaling genes have been found on a genomic island; fatty acid biosynthesis genes; an IS66 family transposase; transcriptional regulator CDSs; amino acid metabolism genes; and a group of hypothetical genes. Mutagenesis of the AHL synthase, amidase (amiI), and porin (opcI) genes on the island was carried out. Testing of the isogenic mutants in a rat model of chronic lung infection demonstrated that the amidase played a role in persistence, while the AHL synthase and porin were both involved in virulence. The island, designated the B. cenocepacia island (cci), is the first genomic island to be defined in the B. cepacia complex and its discovery validates the original epidemiological correlation of the BCESM with virulent CF strains. The features of the cci, which overlap both pathogenicity and metabolism, expand the concept of bacterial pathogenicity islands and illustrate the diversity of accessory functions that can be acquired by lateral gene transfer in bacteria.

A Caenorhabditis elegans model of Yersinia infection: biofilm formation on a biotic surface

To investigate Yersinia pathogenicity and the evolutionary divergence of the genus, the effect of pathogenic yersiniae on the model organism Caenorhabditis elegans was studied. Three strains of Yersinia pestis, including a strain lacking pMT1, caused blockage and death of C. elegans; one strain, lacking the haemin storage (hms) locus, caused no effect. Similarly, 15 strains of Yersinia enterocolitica caused no effect. Strains of Yersinia pseudotuberculosis showed different levels of pathogenicity. The majority of strains (76 %) caused no discernible effect; 5 % caused a weak infection, 9.5 % an intermediate infection, and 9.5 % a severe infection. There was no consistent relationship between serotype and severity of infection; nor was there any relationship between strains causing infection of C. elegans and those able to form a biofilm on an abiotic surface. Electron microscope and cytochemical examination of infected worms indicated that the infection phenotype is a result of biofilm formation on the head of the worm. Seven transposon mutants of Y. pseudotuberculosis strain YPIII pIB1 were completely or partially attenuated; mutated genes included genes encoding proteins involved in haemin storage and lipopolysaccharide biosynthesis. A screen of 15 defined C. elegans mutants identified four where mutation caused (complete) resistance to infection by Y. pseudotuberculosis YPIII pIB1. These mutants, srf-2, srf-3, srf-5 and the dauer pathway gene daf-1, also exhibit altered binding of lectins to the nematode surface. This suggests that biofilm formation on a biotic surface is an interactive process involving both bacterial and invertebrate control mechanisms.

Detection of N-(3-oxohexanoyl)-L-homoserine lactone in mice infected with Yersinia enterocolitica serotype O8

Yersinia enterocolitica synthesizes N-acyl-L-homoserine lactone (AHL) signal molecules via the LuxR-LuxI homologues YenR-YenI. In this study we checked two prototypes of mouse-virulent Y. enterocolitica serotype O8 strains WA-314 and 8081 for AHL production in vitro and in vivo (mouse infection model). We used thin-layer chromatography in combination with the Escherichia coli AHL biosensor to identify the AHL species produced. We detected only OHHL [N-(3-oxohexanoyl)-L-homoserine lactone] and not HHL (N-hexanoyl-L-homoserine lactone) produced by Y. enterocolitica O8 in culture supernatant or infected mouse tissue. This is the first report demonstrating AHL production by yersiniae during infection.

Biofilm dispersal in Xanthomonas campestris is controlled by cell-cell signaling and is required for full virulence to plants

The rpf gene cluster of Xanthomonas campestris pathovar campestris (Xcc) is required for the pathogenesis of this bacterium to plants. Several rpf genes are involved in the coordinate positive regulation of the production of virulence factors mediated by the small diffusible molecule DSF (for diffusible signal factor). RpfF directs the synthesis of DSF, and a two-component sensory transduction system comprising RpfC and RpfG has been implicated in the perception of the DSF signal and signal transduction. In L medium, rpfF, rpfG, rpfC, and rpfGHC mutants grew as matrix-enclosed aggregates, whereas the wild type grew in a dispersed planktonic fashion. Synthesis of the extracellular polysaccharide xanthan was required for aggregate formation. Addition of DSF triggered dispersion of the aggregates formed by the rpfF strain, but not those of rpf strains defective in DSF signal transduction. An extracellular enzyme from Xcc whose synthesis was positively controlled by the DSF/rpf system could disperse the aggregates produced by all rpf strains. The enzyme was identified as the single endo-beta-1,4-mannanase encoded by the Xcc genome. This enzyme had no detectable activity against soluble xanthan. The endo-beta-1,4-mannanase was required for the full virulence of Xcc to plants. On the basis of this model system, we propose that one role of the beta-mannanase during disease is to promote transitions from an aggregated or biofilm lifestyle to a planktonic lifestyle in response to the DSF signal.

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.

Proteome analysis of extracellular proteins regulated by the las and rhl quorum sensing systems in Pseudomonas aeruginosa PAO1

The las and rhl quorum sensing (QS) systems regulate the expression of several genes in response to cell density changes in Pseudomonas aeruginosa. Many of these genes encode surface-associated or secreted virulence factors. Proteins from stationary phase culture supernatants were collected from wild-type and P. aeruginosa PAO1 mutants deficient in one or more of the lasRI, rhlRI and vfr genes and analysed using two-dimensional gel electrophoresis. All mutants released significantly lower amounts of protein than the wild-type. Protein spot patterns from each strain were compared using image analysis and visible spot differences were identified using mass spectrometry. Several previously unknown QS-regulated proteins were characterized, including an aminopeptidase (PA2939), an endoproteinase (PrpL) and a unique 'hypothetical' protein (PA0572), which could not be detected in the culture supernatants of Deltalas mutants, although they were unaffected in Deltarhl mutants. Chitin-binding protein (CbpD) and a hypothetical protein (PA4944) with similarity to host factor I (HF-I) could not be detected when any of the lasRI or rhlRI genes were disrupted. Fourteen proteins were present at significantly greater levels in the culture supernatants of QS mutants, suggesting that QS may also negatively control the expression of some genes. Increased levels of two-partner secretion exoproteins (PA0041 and PA4625) were observed and may be linked to increased stability of their cognate transporters in a QS-defective background. Known QS-regulated extracellular proteins, including elastase (lasB), LasA protease (lasA) and alkaline metalloproteinase (aprA) were also detected.

Quorum-sensing-directed protein expression in Serratia proteamaculans B5a

N-Acyl-L-homoserine-lactone-producing Serratia species are frequently encountered in spoiling foods of vegetable and protein origin. The role of quorum sensing in the food spoiling properties of these bacteria is currently being investigated. A set of luxR luxI homologous genes encoding a putative quorum sensor was identified in the N-(3-oxo-hexanoyl)-L-homoserine lactone (3-oxo-C6-HSL)-producing Serratia proteamaculans strain B5a. The 3-oxo-C6-HSL synthase SprI showed 79 % similarity with EsaI from Pantoea stewartii and the putative regulatory protein SprR was 86 % similar to the SpnR of Serratia marcescens. Proteome analysis suggested that the presence of at least 39 intracellular proteins was affected by the 3-oxo-C6-HSL-based quorum sensing system. The lipB-encoded secretion system was identified as one target gene of the quorum sensing system. LipB was required for the production of extracellular lipolytic and proteolytic activities, thus rendering the production of food-deterioration-relevant exoenzymes indirectly under the control of quorum sensing. Strain B5a caused quorum-sensing-controlled spoilage of milk. Furthermore, chitinolytic activity was controlled by quorum sensing. This control appeared to be direct and not mediated via LipB. The data presented here demonstrate that quorum-sensing-controlled exoenzymic activities affect food quality.

Phosphate availability regulates biosynthesis of two antibiotics, prodigiosin and carbapenem, in Serratia via both quorum-sensing-dependent and -independent pathways

Serratia sp. ATCC 39006 produces two secondary metabolite antibiotics, 1-carbapen-2-em-3-carboxylic acid (Car) and the red pigment, prodigiosin (Pig). We have previously reported that production of Pig and Car is controlled by N-acyl homoserine lactone (N-AHL) quorum sensing, with synthesis of N-AHLs directed by the LuxI homologue SmaI, and is also regulated by Rap, a member of the SlyA family. We now describe further characterization of the SmaI quorum-sensing system and its connection with other regulatory mechanisms. We show that the genes responsible for biosynthesis of Pig, pigA-O, are transcribed as a single polycistronic message in an N-AHL-dependent manner. The smaR gene, transcribed convergently with smaI and predicted to encode the LuxR homologue partner of SmaI, was shown to possess a negative regulatory function, which is uncommon among the LuxR-type transcriptional regulators. SmaR represses transcription of both the pig and car gene clusters in the absence of N-AHLs. Specifically, we show that SmaIR exerts its effect on car gene expression via transcriptional control of carR, encoding a pheromone-independent LuxR homologue. Transcriptional activation of the pig and car gene clusters also requires a functional Rap protein, but Rap dependency can be bypassed by secondary mutations. Transduction of these suppressor mutations into wild-type backgrounds confers a hyper-Pig phenotype. Multiple mutations cluster in a region upstream of the pigA gene, suggesting this region may represent a repressor target site. Two mutations mapped to genes encoding pstS and pstA homologues, which are parts of a high-affinity phosphate transport system (Pst) in Escherichia coli. Disruption of pstS mimicked phosphate limitation and caused concomitant hyper-production of Pig and Car, which was mediated, in part, through increased transcription of the smaI gene. The Pst and SmaIR systems define distinct, yet overlapping, regulatory circuits which form part of a complex regulatory network controlling the production of secondary metabolites in Serratia ATCC 39006.