Cell-cell communication in gram-positive bacteria

Microbial dinner-party conversations: the role of LuxS in interspecies communication

Bacteria have a tendency to be gregarious by nature. Whether on abiotic surfaces in the environment or on the mucosal surfaces of humans, bacteria accumulate in complex multi-species communities. In these dynamic accretions, bacteria can be densely packed and often depend on each other for the provision of metabolic substrates. Under these circumstances, it will be advantageous for bacteria to be able to detect the presence of their neighbours, to communicate with them and to co-ordinate various physiological activities. Such cell-cell sensing and communication systems can be established through the release and detection of chemical signalling molecules. While originally considered a feature characteristic of eukaryotes, the exchange of chemical signals has now been demonstrated in many bacterial species and ecosystems. Indeed, it has even been suggested that assemblages of bacterial species can be considered as proto-multicellular organisms, whereby biological processes are controlled for the benefit of the entire community. Regardless of the extent to which bacterial communication represents a step on the road to multicellularity, it is becoming increasingly apparent that the signalling systems devised by bacteria are essential for successful relationships with other bacteria and with eukaryotic hosts.

Conjugative plasmid transfer in gram-positive bacteria

Conjugative transfer of bacterial plasmids is the most efficient way of horizontal gene spread, and it is therefore considered one of the major reasons for the increase in the number of bacteria exhibiting multiple-antibiotic resistance. Thus, conjugation and spread of antibiotic resistance represents a severe problem in antibiotic treatment, especially of immunosuppressed patients and in intensive care units. While conjugation in gram-negative bacteria has been studied in great detail over the last decades, the transfer mechanisms of antibiotic resistance plasmids in gram-positive bacteria remained obscure. In the last few years, the entire nucleotide sequences of several large conjugative plasmids from gram-positive bacteria have been determined. Sequence analyses and data bank comparisons of their putative transfer (tra) regions have revealed significant similarities to tra regions of plasmids from gram-negative bacteria with regard to the respective DNA relaxases and their targets, the origins of transfer (oriT), and putative nucleoside triphosphatases NTP-ases with homologies to type IV secretion systems. In contrast, a single gene encoding a septal DNA translocator protein is involved in plasmid transfer between micelle-forming streptomycetes. Based on these clues, we propose the existence of two fundamentally different plasmid-mediated conjugative mechanisms in gram-positive microorganisms, namely, the mechanism taking place in unicellular gram-positive bacteria, which is functionally similar to that in gram-negative bacteria, and a second type that occurs in multicellular gram-positive bacteria, which seems to be characterized by double-stranded DNA transfer.

Regulation of Vibrio vulnificus virulence by the LuxS quorum-sensing system

Vibrio vulnificus is a halophilic estuarine bacterium that causes fatal septicaemia and necrotizing wound infections. We tested whether V. vulnificus produces signalling molecules (autoinducer 1 and/or 2) stimulating Vibrio harveyi quorum-sensing system 1 and/or 2. Although there was no evidence for signalling system 1, we found that V. vulnificus produced a signalling activity in the culture supernatant that induced luminescence expression in V. harveyi through signalling system 2. Maximal autoinducer 2 (AI-2) activity was observed during mid-exponential to early stationary phase and disappeared in the late stationary phase when V. vulnificus was grown in heart infusion broth containing 2.5% NaCl. V. vulnificus showed increased signalling activity when it was cultured in the presence of glucose (0.5%) and at low pH (pH 6.0). From a cosmid library of V. vulnificus type strain ATCC 29307, we have identified the AI-2 synthase gene (luxSVv) showing 80% identity with that of V. harveyi (luxSVh) at the amino acid level. To investigate the pathogenic role of luxSVv, a deletion mutant of the clinical isolate V. vulnificus MO6-24/O was constructed. The luxSVv mutant showed a significant delay in protease production and an increase in haemolysin production. The decreased protease and increased haemolysin activities were restored to the isogenic wild-type level by complementation with the wild-type luxSVv allele. The change in phenotypes was also complemented by logarithmic phase spent media produced by the wild-type bacteria. Transcriptional activities of the haemolysin gene (vvhA) and protease gene (vvpE) were also observed in the mutant using chromosomal PvvhA::lacZ and PvvpE::lacZ transcriptional reporter constructs: transcription of vvhA was increased and of vvpE decreased by the mutation. The mutation resulted in an attenuation of lethality to mice. Intraperitoneal LD50 of the luxSVv mutant increased by 10- and 750-fold in ferric ammonium citrate-non-overloaded and ferric ammonium citrate-overloaded mice respectively. The time required for the death of mice was also significantly delayed in the luxSVv mutant. Cytotoxic activity of the organism against HeLa cells, measured by lactate dehydrogenase (LDH) release assay, was also decreased significantly by the mutation. Taken together, the V. vulnificus LuxS quorum-sensing system seems to play an important role in co-ordinating the expression of virulence factors.

Use of in-biofilm expression technology to identify genes involved in Pseudomonas aeruginosa biofilm development

Mature Pseudomonas aeruginosa biofilms form complex three-dimensional architecture and are tolerant of antibiotics and other antimicrobial compounds. In this work, an in vivo expression technology system, originally designed to study virulence-associated genes in complex mammalian environments, was used to identify genes up-regulated in P. aeruginosa grown to a mature (5-day) biofilm. Five unique cloned promoters unable to promote in vitro growth in the absence of purines after recovery from the biofilm environment were identified. The open reading frames downstream of the cloned promoter regions were identified, and knockout mutants were generated. Insertional mutation of PA5065, a homologue of Escherichia coli ubiB, was lethal, while inactivation of PA0240 (a porin homologue), PA3710 (a putative alcohol dehydrogenase), and PA3782 (a homologue of the Streptomyces griseus developmental regulator adpA) had no effect on planktonic growth but caused defects in biofilm formation in static and flowing systems. In competition experiments, mutants demonstrated reduced fitness compared with the parent strain, comprising less than 0.0001% of total biofilm cells after 5 days. Therefore, using in-biofilm expression technology, we have identified novel genes that do not affect planktonic growth but are important for biofilm formation, development, and fitness.

Mutation of luxS affects biofilm formation in Streptococcus mutans

Quorum sensing is a bacterial mechanism for regulating gene expression in response to changes in population density. Many bacteria are capable of acyl-homoserine lactone-based or peptide-based intraspecies quorum sensing and luxS-dependent interspecies quorum sensing. While there is good evidence about the involvement of intraspecies quorum sensing in bacterial biofilm, little is known about the role of luxS in biofilm formation. In this study, we report for the first time that luxS-dependent quorum sensing is involved in biofilm formation of Streptococcus mutans. S. mutans is a major cariogenic bacterium in the multispecies bacterial biofilm commonly known as dental plaque. An ortholog of luxS for S. mutans was identified using the data available in the S. mutans genome project (http://www.genome.ou.edu/smutans.html). Using an assay developed for the detection of the LuxS-associated quorum sensing signal autoinducer 2 (AI-2), it was demonstrated that this ortholog was able to complement the luxS negative phenotype of Escherichia coli DH5alpha. It was also shown that AI-2 is indeed produced by S. mutans. AI-2 production is maximal during mid- to late-log growth in batch culture. Mutant strains devoid of the luxS gene were constructed and found to be defective in producing the AI-2 signal. There are also marked phenotypic differences between the wild type and the luxS mutants. Microscopic analysis of in vitro-grown biofilm structure revealed that the luxS mutant biofilms adopted a much more granular appearance, rather than the relatively smooth, confluent layer normally seen in the wild type. These results suggest that LuxS-dependent signal may play an important role in biofilm formation of S. mutans.

Role of mobile DNA in the evolution of vancomycin-resistant Enterococcus faecalis

The complete genome sequence of Enterococcus faecalis V583, a vancomycin-resistant clinical isolate, revealed that more than a quarter of the genome consists of probable mobile or foreign DNA. One of the predicted mobile elements is a previously unknown vanB vancomycin-resistance conjugative transposon. Three plasmids were identified, including two pheromone-sensing conjugative plasmids, one encoding a previously undescribed pheromone inhibitor. The apparent propensity for the incorporation of mobile elements probably contributed to the rapid acquisition and dissemination of drug resistance in the enterococci.

Investigation of mycobacterial colonisation and invasion of the respiratory mucosa

BACKGROUND

The pathogenesis of Mycobacterium avium complex and Mycobacterium tuberculosis in the respiratory tract is poorly understood, as are the reasons for their differing virulence. We have previously shown that their initial adherence to the mucosa is identical.

METHODS

The interaction of M avium complex, M tuberculosis, and M smegmatis with human respiratory tissue was investigated in an organ culture model with an air interface. Tissue was infected for intervals up to 14 days and assessed by scanning electron microscopy for adherent bacteria or cultured for recoverable bacteria.

RESULTS

The mean number of adherent bacteria/mm(2) (and the viable count of macerated tissue, cfu/ml) at 15 minutes, 3 and 24 hours, 7 and 14 days were: M avium complex 168 (153), 209 (136), 289 (344), 193 (313), 14140 (16544); M tuberculosis 30 (37), 39 (23), 48 (53), 1 (760), 76 (2186); M smegmatis 108 (176), 49 (133), 97 (81), 114 (427), 34 (58), (n=6). There was no significant change in morphology between infected and uninfected tissue or tissue infected with the different species over 14 days. The number of M avium complex on the mucosa and recovered from tissue increased over time (p=0.03). M tuberculosis decreased on the surface, but recoverable bacteria increased (p=0.01). M smegmatis numbers on the mucosa and recovered from tissue decreased. Sectioned tissue showed M avium complex and M tuberculosis in submucosal mucus glands and M tuberculosis penetrating epithelial cells in one experiment.

CONCLUSIONS

The initial adherence to the mucosa of the three species was similar, but after 14 days they varied in their interaction with the tissue in a manner compatible with their pathogenicity.

LuxS-based signaling in Streptococcus gordonii: autoinducer 2 controls carbohydrate metabolism and biofilm formation with Porphyromonas gingivalis

Communication based on autoinducer 2 (AI-2) is widespread among gram-negative and gram-positive bacteria, and the AI-2 pathway can control the expression of genes involved in a variety of metabolic pathways and pathogenic mechanisms. In the present study, we identified luxS, a gene responsible for the synthesis of AI-2, in Streptococcus gordonii, a major component of the dental plaque biofilm. S. gordonii conditioned medium induced bioluminescence in an AI-2 reporter strain of Vibrio harveyi. An isogenic mutant of S. gordonii, generated by insertional inactivation of the luxS gene, was unaffected in growth and in its ability to form biofilms on polystyrene surfaces. In contrast, the mutant strain failed to induce bioluminescence in V. harveyi and was unable to form a mixed species biofilm with a LuxS-null strain of the periodontal pathogen Porphyromonas gingivalis. Complementation of the luxS mutation in S. gordonii restored normal biofilm formation with the luxS-deficient P. gingivalis. Differential display PCR demonstrated that the inactivation of S. gordonii luxS downregulated the expression of a number of genes, including gtfG, encoding glucosyltransferase; fruA, encoding extracellular exo-beta-D-fructosidase; and lacD encoding tagatose 1,6-diphosphate aldolase. However, S. gordonii cell surface expression of SspA and SspB proteins, previously implicated in mediating adhesion between S. gordonii and P. gingivalis, was unaffected by inactivation of luxS. The results suggest that S. gordonii produces an AI-2-like signaling molecule that regulates aspects of carbohydrate metabolism in the organism. Furthermore, LuxS-dependent intercellular communication is essential for biofilm formation between nongrowing cells of P. gingivalis and S. gordonii.

Inducible bacteriocin production in Lactobacillus is regulated by differential expression of the pln operons and by two antagonizing response regulators, the activity of which is enhanced upon phosphorylation

Expression of the five (pln) operons involved in the bacteriocin production of Lactobacillus plantarum C11 is regulated by a so-called pheromone-based signal-transducing network, in which the peptide pheromone (PlnA) induces bacteriocin production through the action of a histidine protein kinase (PlnB) and two antagonizing response regulators (PlnC as an activator and PlnD as a negative regulator). All pln-regulated promoters contain a conserved pair of direct repeats that serve as binding sites for PlnC and PlnD. In the present work, we show that the five PlnA-responsive operons are differentially expressed with regard to both timing and strength, and that the pheromone triggers a strong autoactivating loop of the regulatory unit (plnABCD) during an early stage of induction that gradually leads to enhanced activation of the other operons. The transport operon (plnGHSTUV), which is involved in the secretion of the pheromone and bacteriocins, is also expressed relatively early upon induction, but is quickly turned off soon after peak expression. Further investigation of the various promoters revealed that, although subtle differences within the promoter regions could account for the observed differential regulation, the presence of a downstream promoter-proximal sequence in one promoter was found to cause delayed peak activity. How phosphorylation regulates the activity of the pln response regulators was also accessed by direct mutagenesis at their phosphorylation sites. It was found that the two response regulators exert activity at two different levels: a low level when they are not phosphorylated and an elevated level when they are phosphorylated. The present data demonstrate that bacteriocin production in L. plantarum C11 is a highly regulated process, in which different regulatory mechanisms are applied to fine tune the timing and strength of expression of the five pln operons.

The contribution of microbial virulence to wound infection

This qualitative study explored the perceptions and feelings of patients with pressure ulcers. Five participants who had suffered from pressure ulcers extending into the subcutaneous tissue and deep fascia were interviewed. A semi-structured interview technique was used, in which participants were asked a series of open questions based on the reviewed literature and the author's clinical experience. Content analysis was used to analyse the interview transcripts. The following themes emerged: pain, exudate levels, loss of independence, emotional factors, worry about healing, relationships, body image and social isolation.

Ammonia pulses and metabolic oscillations guide yeast colony development

On solid substrate, growing yeast colonies alternately acidify and alkalinize the medium. Using morphological, cytochemical, genetic, and DNA microarray approaches, we characterized six temporal steps in the "acid-to-alkali" colony transition. This transition is connected with the production of volatile ammonia acting as starvation signal between colonies. We present evidence that the three membrane proteins Ato1p, Ato2p, and Ato3p, members of the YaaH family, are involved in ammonia production in Saccharomyces cerevisiae colonies. The acid-to-alkali transition is connected with decrease of mitochondrial oxidative catabolism and by peroxisome activation, which in parallel with activation of biosynthetic pathways contribute to decrease the general stress level in colonies. These metabolic features characterize a novel survival strategy used by yeast under starvation conditions prevalent in nature.

Enterococcal plasmid transfer: sex pheromones, transfer origins, relaxases, and the Staphylococcus aureus issue

Certain conjugative plasmids in Enterococcus faecalis encode a mating response to peptide sex pheromones encoded on the chromosome of potential recipient (plasmid-free) strains. The pheromone precursors correspond to the precursors of surface lipoproteins with the mature peptides coming from the last 7-8 residues of the related signal sequences. Processing that gives rise to the pAD1-related peptide involves a chromosome-encoded metalloprotease (Eep) that is believed to operate within the cytoplasmic membrane. Mutations in the determinants for cAD1 and cAM373, cad and camE, respectively, do not affect cell viability; and when the related plasmid is present, the pheromone response is normal. A cAM373-like activity is produce by Staphylococcus aureus, but the corresponding lipoprotein determinant (camS) is unrelated to the enterococcal determinant (camE). pAD1 has two origins of transfer, oriT1 and oriT2 and encodes a relaxase (TraX), which has been shown to specifically nick in oriT2. pAM373 has a site, oriT, that is similar to oriT2 of pAD1. Both sites (oriT2 of pAD1 and oriT of pAM373) have a series of short direct repeats (5-6 bp with 5-6 bp-spacings) adjacent to a long inverted repeat (140 bp). The direct repeats differ significantly and confer specificity to the two systems. pAD1 and pAM373 are both able to mobilize the nonconjugative plasmid pAMalpha1, which encodes two relaxases that are involved in transfer. Relevant information concerning the possible movement of vancomycin resistance from E. faecalis to S. aureus in a clinical environment is discussed.

Controlling infection by tuning in and turning down the volume of bacterial small-talk

As the prevalence of bacterial resistance to multiple antibiotics increases it is becoming progressively more difficult to treat infections and, in many cases, the available therapeutic options are severely limited. Hence, there is a growing urgency to the search for novel targets and the development of new antimicrobials. To infect a host and cause disease bacteria produce an array of virulence determinants that contribute to pathogenesis. It is now known that many different Gram-positive and Gram-negative pathogens communicate via the production and sensing of small, diffusible signal molecules, to coordinate virulence determinant production. As a consequence, this event, now termed quorum sensing, represents a novel therapeutic target offering the opportunity to attenuate virulence, and thus control infection, by blocking cell-to-cell communication.

A conserved mechanism for extracellular signaling in eukaryotes and prokaryotes

Epidermal growth factor receptor (EGFr) is a key mediator of cell communication during animal development and homeostasis. In Drosophila, the signaling event is commonly regulated by the polytopic membrane protein Rhomboid (RHO), which mediates the proteolytic activation of EGFr ligands, allowing the secretion of the active signal. Until very recently, the biochemical function of RHO had remained elusive. It is now believed that Drosophila RHO is the founder member of a previously undescribed family of serine proteases, and that it could be directly responsible for the unusual, intramembranous cleavage of EGFr ligands. Here we show that the function of RHO is conserved in Gram-negative bacteria. AarA, a Providencia stuartii RHO-related protein, is active in Drosophila on the fly EGFr ligands. Vice versa, Drosophila RHO-1 can effectively rescue the bacterium's ability to produce or release the signal that activates density-dependent gene regulation (or quorum sensing). This study provides the first evidence that prokaryotic and eukaryotic RHOs could have a conserved role in cell communication and that their biochemical properties could be more similar than previously anticipated.

Listening in on bacteria: acyl-homoserine lactone signalling

Bacterial cell-to-cell signalling has emerged as a new area in microbiology. Individual bacterial cells communicate with each other and co-ordinate group activities. Although a lot of detail is known about the mechanisms of a few well-characterized bacterial communication systems, other systems have been discovered only recently. Bacterial intercellular communication has become a target for the development of new anti-virulence drugs.

DegS and YaeL participate sequentially in the cleavage of RseA to activate the sigma(E)-dependent extracytoplasmic stress response

All cells have stress response pathways that maintain homeostasis in each cellular compartment. In the Gram-negative bacterium Escherichia coli, the sigma(E) pathway responds to protein misfolding in the envelope. The stress signal is transduced across the inner membrane to the cytoplasm via the inner membrane protein RseA, the anti-sigma factor that inhibits the transcriptional activity of sigma(E). Stress-induced activation of the pathway requires the regulated proteolysis of RseA. In this report we show that RseA is degraded by sequential proteolytic events controlled by the inner membrane-anchored protease DegS and the membrane-embedded metalloprotease YaeL, an ortholog of mammalian Site-2 protease (S2P). This is consistent with the mechanism of activation of ATF6, the mammalian unfolded protein response transcription factor by Site-1 protease and S2P. Thus, mammalian and bacterial cells employ a conserved proteolytic mechanism to activate membrane-associated transcription factors that initiate intercompartmental cellular stress responses.

The Enterococcus faecalis fsrB gene, a key component of the fsr quorum-sensing system, is associated with virulence in the rabbit endophthalmitis model

We used a rabbit endophthalmitis model to explore the role of fsrB, a gene required for the function of the fsr quorum-sensing system of Enterococcus faecalis, in pathogenicity. A nonpolar deletion mutant of fsrB had significantly reduced virulence compared to wild type. Complementation of mutation restored virulence. These data corroborate the role of fsrB in E. faecalis pathogenesis and suggest that the rabbit endophthalmitis model can be used to study the in vivo role of quorum sensing.

pfs-dependent regulation of autoinducer 2 production in Salmonella enterica serovar Typhimurium

Bacterial intercellular communication provides a mechanism for signal-dependent regulation of gene expression to promote coordinated population behavior. Salmonella enterica serovar Typhimurium produces a non-homoserine lactone autoinducer in exponential phase as detected by a Vibrio harveyi reporter assay for autoinducer 2 (AI-2) (M. G. Surette and B. L. Bassler, Proc. Natl. Acad. Sci. USA 95:7046-7050, 1998). The luxS gene product mediates the production of AI-2 (M. G. Surette, M. B. Miller, and B. L. Bassler, Proc. Natl. Acad. Sci. USA 96:1639-1644, 1999). Environmental cues such as rapid growth, the presence of preferred carbon sources, low pH, and/or high osmolarity were found to influence the production of AI-2 (M. G. Surette and B. L. Bassler, Mol. Microbiol. 31:585-595, 1999). In addition to LuxS, the pfs gene product (Pfs) is required for AI-2 production, as well as S-adenosylhomocysteine (SAH) (S. Schauder, K. Shokat, M. G. Surette, and B. L. Bassler, Mol. Microbiol. 41:463-476, 2001). In bacterial cells, Pfs exhibits both 5'-methylthioadenosine (MTA) and SAH nucleosidase functions. Pfs is involved in methionine metabolism, regulating intracellular MTA and SAH levels (elevated levels of MTA and SAH are potent inhibitors of polyamine synthetases and S-adenosylmethionine dependent methyltransferase reactions, respectively). To further investigate regulation of AI-2 production in Salmonella, we constructed pfs and luxS promoter fusions to a luxCDABE reporter in a low-copy-number vector, allowing an examination of transcription of the genes in the pathway for signal synthesis. Here we report that luxS expression is constitutive but that the transcription of pfs is tightly correlated to AI-2 production in Salmonella serovar Typhimurium 14028. Neither luxS nor pfs expression appears to be regulated by AI-2. These results suggest that AI-2 production is regulated at the level of LuxS substrate availability and not at the level of luxS expression. Our results indicate that AI-2-dependent signaling is a reflection of metabolic state of the cell and not cell density.

The autoregulatory role of EsaR, a quorum-sensing regulator in Pantoea stewartii ssp. stewartii: evidence for a repressor function

Capsular polysaccharide synthesis and virulence in the plant pathogenic bacterium Pantoea stewartii ssp. stewartii requires the quorum-sensing regulatory proteins, EsaR and EsaI, and the diffusible inducer N-(3-oxo-hexanoyl)-L-homoserine lactone. Prior mutational studies suggested that EsaR might function as a repressor of quorum sensing in the control of capsular polysaccharide synthesis. Further, a lux box-like palindromic sequence coinciding with the putative -10 element of the esaR promoter suggested a possible negative autoregulatory role for EsaR. This report presents genetic evidence that EsaR represses the esaR gene under inducer-limiting conditions, and that addition of inducer promotes rapid, dose-dependent derepression. DNA mobility-shift assays and analyses by surface plasmon resonance refractometry show that EsaR binds target DNAs in a ligand-free state, and that inducer alters the binding characteristics of EsaR. Physical measurements indicate that the EsaR protein binds N-(3-oxo-hexanoyl)-L-homoserine lactone, in a 1:1 protein:ligand ratio, and that inducer binding enhances the thermal stability of the EsaR protein. These combined genetic and biochemical data establish that EsaR regulates its own expression by signal-independent repression and signal-dependent derepression. Additionally, we provide evidence that EsaR does not govern the expression of the linked esaI gene, thus EsaR has no role in controlling coinducer synthesis.

Conjugal transfer of the virulence plasmid of Salmonella enterica is regulated by the leucine-responsive regulatory protein and DNA adenine methylation

Host-encoded functions that regulate the transfer operon (tra) in the virulence plasmid of Salmonella enterica (pSLT) were identified with a genetic screen. Mutations that decreased tra operon expression mapped in the lrp gene, which encodes the leucine-responsive regulatory protein (Lrp). Reduced tra operon expression in an Lrp- background is caused by lowered transcription of the traJ gene, which encodes a transcriptional activator of the tra operon. Gel retardation assays indicated that Lrp binds a DNA region upstream of the traJ promoter. Deletion of the Lrp binding site resulted in lowered and Lrp-independent traJ transcription. Conjugal transfer of pSLT decreased 50-fold in a Lrp- background. When a FinO- derivative of pSLT was used, conjugal transfer from an Lrp- donor decreased 1000-fold. Mutations that derepressed tra operon expression mapped in dam, the gene encoding Dam methyltransferase. Expression of the tra operon and conjugal transfer remain repressed in an Lrp- Dam- background. These observations support the model that Lrp acts as a conjugation activator by promoting traJ transcription, whereas Dam methylation acts as a conjugation repressor by activating FinP RNA synthesis. This dual control of conjugal transfer may also operate in other F-like plasmids such as F and R100.

A quorum-sensing signaling system essential for genetic competence in Streptococcus mutans is involved in biofilm formation

In a previous study, a quorum-sensing signaling system essential for genetic competence in Streptococcus mutans was identified, characterized, and found to function optimally in biofilms (Li et al., J. Bacteriol. 183:897-908, 2001). Here, we demonstrate that this system also plays a role in the ability of S. mutans to initiate biofilm formation. To test this hypothesis, S. mutans wild-type strain NG8 and its knockout mutants defective in comC, comD, comE, and comX, as well as a comCDE deletion mutant, were assayed for their ability to initiate biofilm formation. The spatial distribution and architecture of the biofilms were examined by scanning electron microscopy and confocal scanning laser microscopy. The results showed that inactivation of any of the individual genes under study resulted in the formation of an abnormal biofilm. The comC mutant, unable to produce or secrete a competence-stimulating peptide (CSP), formed biofilms with altered architecture, whereas the comD and comE mutants, which were defective in sensing and responding to the CSP, formed biofilms with reduced biomass. Exogenous addition of the CSP and complementation with a plasmid containing the wild-type comC gene into the cultures restored the wild-type biofilm architecture of comC mutants but showed no effect on the comD, comE, or comX mutant biofilms. The fact that biofilms formed by comC mutants differed from the comD, comE, and comX mutant biofilms suggested that multiple signal transduction pathways were affected by CSP. Addition of synthetic CSP into the culture medium or introduction of the wild-type comC gene on a shuttle vector into the comCDE deletion mutant partially restored the wild-type biofilm architecture and further supported this idea. We conclude that the quorum-sensing signaling system essential for genetic competence in S. mutans is important for the formation of biofilms by this gram-positive organism.

Identification and characterization of genes encoding sex pheromone cAM373 activity in Enterococcus faecalis and Staphylococcus aureus

The sex pheromone cAM373 of Enterococcus faecalis and the related staph-cAM373 of Staphylococcus aureus were found to correspond to heptapeptides located within the C-termini of the signal sequences of putative prelipoproteins. The deduced mature forms of the lipoproteins share no detectable homology and presumably serve unrelated functions in the cells. The chromosomally encoded genetic determinants for production of the pheromones have been identified and designated camE (encoding cAM373) and camS (encoding staph-cAM373). Truncated and full-length clones of camE were generated in Escherichia coli, in which cAM373 activity was expressed. In E. faecalis, insertional inactivation in the middle of camE had no detectable phenotypic effects on the pheromone system. Establishment of an in frame translation stop codon within the signal sequence resulted in reduction of cAM373 activity to 3% of normal levels. The camS determinant has homologues in Staphylococcus epidermidis, Bacillus subtilis and Listeria monocytogenes; however, corresponding heptapeptides present within those sequences do not resemble staph-cAM373 closely. The particular significance of staph-cAM373 as a potential intergeneric inducer of transfer-proficient genetic elements is discussed.

Virulence genes of the phytopathogen Rhodococcus fascians show specific spatial and temporal expression patterns during plant infection

The phytopathogenic bacterium Rhodococcus fascians provokes shoot meristem formation and malformations on aerial plant parts, mainly at the axils. The interaction is accompanied by bacterial colonization of the plant surface and tissues. Upon infection, the two bacterial loci required for full virulence, fas and att, were expressed only at the sites of symptom development, although their expression profiles differed both spatially and temporally. The att locus was expressed principally in bacteria located on the plant surface at early stages of infection. Expression of the fas locus occurred throughout infection, mainly in bacteria that were penetrating, or had penetrated, the plant tissues and coincided with sites of meristem initiation and proliferation. The implications for the regulation of virulence genes of R. fascians during plant infection are discussed.

Two targets in pCF10 DNA for PrgX binding: their role in production of Qa and prgX mRNA and in regulation of pheromone-inducible conjugation

PrgX is the primary cytoplasmic protein involved in negative control of pheromone-inducible conjugation functions of the Enterococcus faecalis plasmid pCF10. PrgX is believed to act in concert with an antisense RNA called Qa to inhibit readthrough of transcription from the prgQ promoter into the pCF10 genes mediating conjugation functions; PrgX also positively regulates its own expression, as well as that of Qa. We found two DNA target sites for PrgX binding in the intergenic region between the prgX and prgQ genes of pCF10. The primary binding site near prgX includes an 11 bp palindromic sequence and showed relatively high affinity for His-tagged PrgX (His-PrgX). The secondary binding site is between the -35 and -10 regions of the prgQ promoter, and contains only a half of the palindromic sequence; this binding site showed weaker affinity. A region of pCF10 including the prgQ promoter and the secondary binding site reduced Qa RNA levels greatly and this reduction was overcome by the presence of the primary binding site and PrgX. In constructs where the binding sites were mutated individually or in combination, the intracellular levels of PrgX protein and Qa RNA were reduced significantly. On the basis of these results, we propose that both DNA binding sites are required for the autoregulation of PrgX expression and for positive regulation of Qa RNA.

ccfA, the genetic determinant for the cCF10 peptide pheromone in Enterococcus faecalis OG1RF

The nosocomial pathogen Enterococcus faecalis has a unique pheromone-inducible conjugative mating system. Conjugative transfer of the E. faecalis plasmid pCF10 is specifically induced by the cCF10 peptide pheromone (LVTLVFV). Genomic sequence information has recently allowed the identification of putative structural genes coding for the various enterococcal pheromones (D. B. Clewell et al., Mol. Microbiol. 35:246-247, 2000). The cCF10 pheromone sequence LVTLVFV was found within an open reading frame designated ccfA, encoding a putative lipoprotein precursor. Several other pheromone sequences were found in similar locations within other predicted lipoproteins. CcfA shows significant sequence relatedness to the Escherichia coli protein YidC, an inner membrane protein translocase, as well as to a large number of homologs identified in gram-positive and in gram-negative bacteria. Analysis of the deduced CcfA amino acid sequence suggested that mature cCF10 peptide could be formed from the proteolytic degradation of its signal peptide. Expression of the cloned ccfA gene with an inducible expression vector dramatically increased cCF10 production by E. faecalis and also resulted in cCF10 production by Lactococcus lactis, a non-pheromone producer. Site-directed mutagenesis of the ccfA sequence encoding the cCF10 peptide confirmed that ccfA was a functional genetic determinant for cCF10.

Trypsin mediates growth phase-dependent transcriptional tegulation of genes involved in biosynthesis of ruminococcin A, a lantibiotic produced by a Ruminococcus gnavus strain from a human intestinal microbiota

Ruminococcin A (RumA) is a trypsin-dependent lantibiotic produced by Ruminococcus gnavus E1, a gram-positive strict anaerobic strain isolated from a human intestinal microbiota. A 12.8-kb region from R. gnavus E1 chromosome, containing the biosynthetic gene cluster of RumA, has been cloned and sequenced. It consisted of 13 open reading frames, organized in three operons with predicted functions in lantibiotic biosynthesis, signal transduction regulation, and immunity. One unusual feature of the locus is the presence of three almost identical structural genes, all of them encoding the RumA precursor. In order to determine the role of trypsin in RumA production, the transcription of the rum genes has been investigated under inducing and noninducing conditions. Trypsin activity is needed for the growth phase-dependent transcriptional activation of RumA operons. Our results suggest that bacteriocin production by R. gnavus E1 is controlled through a complex signaling mechanism involving the proteolytic processing of a putative extracellular inducer-peptide by trypsin, a specific environmental cue of the digestive ecosystem.

Cell density modulates acid adaptation in Streptococcus mutans: implications for survival in biofilms

Streptococcus mutans normally colonizes dental biofilms and is regularly exposed to continual cycles of acidic pH during ingestion of fermentable dietary carbohydrates. The ability of S. mutans to survive at low pH is an important virulence factor in the pathogenesis of dental caries. Despite a few studies of the acid adaptation mechanism of this organism, little work has focused on the acid tolerance of S. mutans growing in high-cell-density biofilms. It is unknown whether biofilm growth mode or high cell density affects acid adaptation by S. mutans. This study was initiated to examine the acid tolerance response (ATR) of S. mutans biofilm cells and to determine the effect of cell density on the induction of acid adaptation. S. mutans BM71 cells were first grown in broth cultures to examine acid adaptation associated with growth phase, cell density, carbon starvation, and induction by culture filtrates. The cells were also grown in a chemostat-based biofilm fermentor for biofilm formation. Adaptation of biofilm cells to low pH was established in the chemostat by the acid generated from excess glucose metabolism, followed by a pH 3.5 acid shock for 3 h. Both biofilm and planktonic cells were removed to assay percentages of survival. The results showed that S. mutans BM71 exhibited a log-phase ATR induced by low pH and a stationary-phase acid resistance induced by carbon starvation. Cell density was found to modulate acid adaptation in S. mutans log-phase cells, since pre-adapted cells at a higher cell density or from a dense biofilm displayed significantly higher resistance to the killing pH than the cells at a lower cell density. The log-phase ATR could also be induced by a neutralized culture filtrate collected from a low-pH culture, suggesting that the culture filtrate contained an extracellular induction component(s) involved in acid adaptation in S. mutans. Heat or proteinase treatment abolished the induction by the culture filtrate. The results also showed that mutants defective in the comC, -D, or -E genes, which encode a quorum sensing system essential for cell density-dependent induction of genetic competence, had a diminished log-phase ATR. Addition of synthetic competence stimulating peptide (CSP) to the comC mutant restored the ATR. This study demonstrated that cell density and biofilm growth mode modulated acid adaptation in S. mutans, suggesting that optimal development of acid adaptation in this organism involves both low pH induction and cell-cell communication.

A two-component signal-transduction cascade in Carnobacterium piscicola LV17B: two signaling peptides and one sensor-transmitter

In the lactic acid bacterium Carnobacterium piscicola LV17B a peptide-pheromone dependent quorum-sensing mode is involved in the regulation of bacteriocin production. Bacteriocin CB2 was identified as an environmental signal that induces bacteriocin production. Here, we demonstrate that a second 24 amino acid peptide (CS) also induces bacteriocin production. Transcription activation of several carnobacteriocin operons is triggered by CB2 or CS via a two-component signal transduction system composed of CbnK and CbnR.

Staphylococcus aureus and Staphylococcus epidermidis peptide pheromones produced by the accessory gene regulator agr system

The accessory gene regulator (agr) system of staphylococci regulates the expression of virulence factors in response to cell density. The extracellular signaling molecule encoded by this system is a thiolactone-containing pheromone peptide whose primary sequence varies among staphylococcal strains. A post-translational modification of the peptide is believed to be carried out by an enzyme with a novel function, AgrB. Staphylococcal pheromones show cross-inhibiting properties: Pheromones of self and pheromones of non-self induce and suppress the agr response, respectively, and have therefore been proposed as novel anti-staphylococcal drugs. As inhibition of agr leads to diminished expression of toxins, but to increased expression of colonization factors and biofilm formation, their therapeutic potential remains yet to be evaluated in depth.

Peptide pheromone-dependent regulation of antimicrobial peptide production in Gram-positive bacteria: a case of multicellular behavior

Quorum sensing enables unicellular organisms to behave in a multicellular way by allowing population-wide synchronized adaptive responses that involve modulation of a wide range of physiological responses in a cell density-, cell proximity- or growth phase-dependent manner. Examples of processes modulated by quorum sensing are the development of genetic competence, conjugative plasmid transfer, sporulation and cell differentiation, biofilm formation, virulence response, production of antibiotics, antimicrobial peptides and toxins, and bioluminescence (for reviews see [38]). The cell-to-cell communication strategies involved in these processes are based on the utilization of small signal molecules produced and released into the environment by the microorganisms. These communication molecules are referred to as pheromones and act as chemical messengers that transmit information across space. The extracellular pheromones accumulate in the environment and trigger a response in the target cells when its concentration reaches a certain threshold value. Elucidation of the chemical nature of the pheromones modulating the processes mentioned above reveals that most of them are unmodified peptides, post-translationally modified peptides, N-acyl homoserine lactones, or butyrolactones. Lactone-based pheromones are the preferred communication signals in Gram-negative bacteria (for review see [47,48]), whereas peptide-based pheromones are the predominant extracellular signals among Gram-positive bacteria (for review see [37,61]). However, lactone-based pheromones are utilized as signals that modulate differentiation and secondary metabolism production in Streptomyces (for review see [20]). This review focuses on the major advances and current views of the peptide-pheromone dependent regulatory circuits involved in production of antimicrobial peptides in Gram-positive bacteria.

Chemical synthesis and biological activity of the gelatinase biosynthesis-activating pheromone of Enterococcus faecalis and its analogs

An 11-residue peptide lactone, termed the gelatinase biosynthesis-activating pheromone (GBAP), triggers the production of the pathogenicity-related extracellular proteases, gelatinase and serine protease, in Enterococcus faecalis. In this study, we synthesized GBAP and its analogs and examined their gelatinase biosynthesis-inducing activity. This study on the structure-activity relationship shows that a lactone ring was indispensable for the activity.

The intracellular function of extracellular signaling peptides

A novel class of extracellular signaling peptides has been identified in Gram-positive bacteria that are actively transported into the cell to interact with intracellular receptors. The defining members of this novel class of signaling peptides are the Phr peptides of Bacillus subtilis and the mating pheromones of Enterococcus faecalis. These peptides are small and unmodified, gene encoded, and secreted by the bacterium. Most of these peptides diffuse into the extracellular medium, and when their concentration is sufficiently high, they are then actively transported into the cell by an oligopeptide permease (Opp). Once inside the cell, these peptides interact with an array of intracellular receptors. In B. subtilis, the Phr peptides regulate development of environmentally resistant spores and genetically competent cells (i.e. the natural ability to take up exogenous DNA). In E. faecalis, the mating pheromones regulate cell-cell transfer of plasmids, many of which encode antibiotic resistance or virulence factors. At least one component of the signaling pathway for these peptides is conserved in many bacteria, Opp. Opp is a non-specific transporter that transports peptides for use as carbon and nitrogen sources. The possibility that other bacteria could possess similar intracellularly functioning signaling peptides is discussed.

Peptide pheromone-induced transfer of plasmid pCF10 in Enterococcus faecalis: probing the genetic and molecular basis for specificity of the pheromone response

The tetracycline resistance plasmid pCF10 represents a class of unique mobile genetic elements of the bacterial genus Enterococcus, whose conjugative transfer functions are inducible by peptide sex pheromones excreted by potential recipient cells. These plasmids play a significant role in the dissemination of virulence and antibiotic resistance genes among the enterococci, which have become major nosocomial pathogens. Pheromone response by plasmid-carrying donor cells involves specific import of the peptide signal molecule, and subsequent interaction of the signal with one or more intracellular regulatory gene products. The pheromones are chromosomally encoded hydrophobic octa- or hepta-peptides, and different families of homologous plasmids encode the ability to respond to each pheromone. Among the four pheromone-responsive plasmids that have been characterized in some detail, there is considerable conservation in the genes encoding pheromone sensing and regulatory functions, and the peptides themselves show considerable similarity. In spite of this, there is extremely high specificity of response to each peptide, with virtually no "cross-induction" of transfer of non-cognate pheromone plasmids by the pheromones. This communication reviews the evidence for this specificity and discusses current molecular and genetic approaches to defining the basis for specificity.

The att locus of Rhodococcus fascians strain D188 is essential for full virulence on tobacco through the production of an autoregulatory compound

The ability of Rhodococcus fascians strain D188 to provoke leafy gall formation on a variety of plant species is correlated with the linear plasmid pFiD188, on which different pathogenicity loci were identified. The att locus affects the severity of symptom development on tobacco, whereas the fas locus is essential for virulence. To gain insight into the function of the att locus, sequence and expression analyses were performed. The att locus contains nine open reading frames homologous to arginine and beta-lactam biosynthetic genes. att gene expression is transcriptionally induced by leafy gall extracts, but not by extracts of uninfected plants, and depends on the attR gene that encodes a LysR-type transcriptional regulator. The att locus proves to be essential for the formation of inducing factors (IFs) that are present in gall extracts. Because the induction of the fas locus also requires the presence of IFs in gall extracts, the att locus is proposed to play an important role in regulating the expression of the virulence loci of R. fascians.

Crystal structure of the quorum-sensing protein LuxS reveals a catalytic metal site

The ability of bacteria to regulate gene expression in response to changes in cell density is termed quorum sensing. This behavior involves the synthesis and recognition of extracellular, hormone-like compounds known as autoinducers. Here we report the structure of an autoinducer synthase, LuxS from Bacillus subtilis, at 1.6-A resolution (R(free) = 0.204; R(work) = 0.174). LuxS is a homodimeric enzyme with a novel fold that incorporates two identical tetrahedral metal-binding sites. This metal center is composed of a Zn(2+) atom coordinated by two histidines, a cysteine, and a solvent molecule, and is reminiscent of active sites found in several peptidases and amidases. Although the nature of the autoinducer synthesized by LuxS cannot be deduced from the crystal structure, features of the putative active site suggest that LuxS might catalyze hydrolytic, but not proteolytic, cleavage of a small substrate. Our analysis represents a test of structure-based functional assignment.

Quorum sensing is a global regulatory mechanism in enterohemorrhagic Escherichia coli O157:H7

Enterohemorrhagic Escherichia coli (EHEC) O157:H7 is responsible for outbreaks of bloody diarrhea and hemolytic-uremic syndrome in many countries. EHEC virulence mechanisms include the production of Shiga toxins (Stx) and formation of attaching and effacing (AE) lesions on intestinal epithelial cells. We recently reported that genes involved in the formation of the AE lesion were regulated by quorum sensing through autoinducer-2, which is synthesized by the product of the luxS gene. In this study we hybridized an E. coli gene array with cDNA synthesized from RNA that was extracted from EHEC strain 86-24 and its isogenic luxS mutant. We observed that 404 genes were regulated by luxS at least fivefold, which comprises approximately 10% of the array genes; 235 of these genes were up-regulated and 169 were down-regulated in the wild-type strain compared to in the luxS mutant. Down-regulated genes included several involved in cell division, as well as ribosomal and tRNA genes. Consistent with this pattern of gene expression, the luxS mutant grows faster than the wild-type strain (generation times of 37.5 and 60 min, respectively, in Dulbecco modified Eagle medium). Up-regulated genes included several involved in the expression and assembly of flagella, motility, and chemotaxis. Using operon::lacZ fusions to class I, II, and III flagellar genes, we were able to confirm this transcriptional regulation. We also observed fewer flagella by Western blotting and electron microscopy and decreased motility halos in semisolid agar in the luxS mutant. The average swimming speeds for the wild-type strain and the luxS mutant are 12.5 and 6.6 microm/s, respectively. We also observed an increase in the production of Stx due to quorum sensing. Genes encoding Stx, which are transcribed along with lambda-like phage genes, are induced by an SOS response, and genes involved in the SOS response were also regulated by quorum sensing. These results indicate that quorum sensing is a global regulatory mechanism for basic physiological functions of E. coli as well as for virulence factors.

Evidence for dual functionality of the operon plnABCD in the regulation of bacteriocin production in Lactobacillus plantarum

The regulatory operon (plnABCD) involved in bacteriocin production in Lactobacillus plantarum C11 encodes four different proteins: a cationic prepeptide (PlnA); a histidine protein kinase (PlnB); and two highly homologous response regulators (PlnC and PlnD; over 75% sequence similarity). The mature product of PlnA, plantaricin A, serves as an extracellular pheromone that induces bacteriocin production. The exact roles of plnBCD in bacteriocin production have not been established experimentally. A reporter system containing the gusA gene fused with the plnA promoter was used to study plnABCD. We demonstrated that the plnABCD operon codes for an autoregulatory unit capable of activating its own promoter. Deletion analyses, performed in a heterologous expression host to define the roles of the individual genes, confirmed that both the inducer gene (plnA) and the kinase gene (plnB) are required for autoactivation. Apparently, the latter gene encodes a protein that serves as a receptor for the pheromone peptide. It was also demonstrated conclusively that the two regulators PlnC and PlnD, which have been shown previously to bind specifically to the DNA regulatory repeats of the plnA promoter, possess differential activities on the plnA promoter, with PlnC being much more active than PlnD. The functions of the response regulators were investigated further in the bacteriocin producer strain C11 in order to reveal their roles in bacteriocin production. Surprisingly, the two response regulators display totally opposite functions: although overexpression of plnC activated transcription and bacteriocin production, the overexpression of plnD repressed both processes, thus strongly suggesting that PlnD plays a role in the downregulation of bacteriocin synthesis. To our knowledge, this is the first evidence for a protein involved directly in negative regulation of bacteriocin production, and also it was shown for the first time that two highly homologous response regulators, with opposite functions, are encoded by genes located on the same operon.

Acylated homoserine lactone detection in Pseudomonas aeruginosa biofilms by radiolabel assay

We describe the development of a new radioactive assay for acyl-HSL production by bacterial cultures. The assay is based on the uptake of radiolabeled methionine and conversion of the radiolabel into SAM. The radiolabeled SAM is then incorporated into acyl-HSL by an acyl-HSL synthase. This assay is faster than previously used bioassays and shows no bias for the detection of acyl-HSLs of a particular length or side chain substitution. Acyl-HSL production can be monitored over a wide range of growth conditions in liquid culture. This assay can also be used in conjunction with a tube biofilm reactor to monitor acyl-HSL production by biofilm cultures. Ultimately this assay will allow comparison of acyl-HSL production by cells subjected to a variety of physiological conditions.

Reversible transition between active and dormant microbial states in soil

The rate of respiration obtained in the substrate-induced respiration (SIR) method can be divided into the respiration rate of growing (r) and non-growing (K) microorganisms. The fraction of r is generally small (5-20%) in soils with no recent addition of substrates, but can be 100% in soils with high substrate availability. This suggests that substrate availability determines the proportion of biomass between these groups, and implies that transitions between them can take place reversibly. These hypotheses were tested by adding three different amounts of glucose which induced first-order, zero-order, and growth-associated respiration kinetics to three soils at four pre-incubation times (4, 12, 27, and 46 days) before the SIR measurement. An abiotic flush of CO(2) in the SIR measurement was detected and corrected for before data analysis. Accumulated CO(2)-C over 4 days after glucose addition, corrected for the respiration in unamended controls, corresponded to 41-50% mineralization of the glucose-C, and the relative amount mineralized by each soil was independent of the glucose amount added. The high glucose concentration gave an increased SIR, which reverted to the initial value within 27-46 days. In a specific sample, the maximum respiration rate induced during the pre-incubation, and the amount of organisms transformed from the K to the r state, as quantified in respiration rate units in the SIR measurement, were identical to each other, and these parameters were also highly correlated to the initial glucose concentration. The K-->r transition was very fast, probably concurrent with the instantaneous increase in the respiration rate obtained by the glucose amendment. Thereafter, a slow first-order back-transition from the r to the K state ensued, with half-lives of 12, 23, and 70 days for the three soils. The results suggest the existence of community-level controls by which growth within or of the whole biomass is inhibited until it has been completely transformed into the r state. The data also suggest that the microbial specific activity is not related to the availability of exogenous substrate in a continuous fashion, rather it responds as a sharp transition between dormant and fully active. Furthermore, the inherent physiological state of the microbial biomass is strongly related to its history. It is proposed that the normal dynamics of the soil microbial biomass is an oscillation between active and dormant physiological states, while significant growth occurs only at substantial substrate amendment.

Intra- and interspecies signaling between Streptococcus salivarius and Streptococcus pyogenes mediated by SalA and SalA1 lantibiotic peptides

Streptococcus salivarius 20P3 produces a 22-amino-acid residue lantibiotic, designated salivaricin A (SalA), that inhibits the growth of a range of streptococci, including all strains of Streptococcus pyogenes. Lantibiotic production is associated with the sal genetic locus comprising salA, the lantibiotic structural gene; salBCTX genes encoding peptide modification and export machinery proteins; and salYKR genes encoding a putative immunity protein and two-component sensor-regulator system. Insertional inactivation of salB in S. salivarius 20P3 resulted in abrogation of SalA peptide production, of immunity to SalA, and of salA transcription. Addition of exogenous SalA peptide to salB mutant cultures induced dose-dependent expression of salA mRNA (0.2 kb), demonstrating that SalA production was normally autoregulated. Inactivation of salR encoding the response regulator of the SalKR two-component system led to reduced production of, and immunity to, SalA. The sal genetic locus was also present in S. pyogenes SF370 (M type 1), but because of a deletion across the salBCT genes, the corresponding lantibiotic peptide, designated SalA1, was not produced. However, in S. pyogenes T11 (M type 4) the sal locus gene complement was apparently complete, and active SalA1 peptide was synthesized. Exogenously added SalA1 peptide from S. pyogenes T11 induced salA1 transcription in S. pyogenes SF370 and in an isogenic S. pyogenes T11 salB mutant and salA transcription in S. salivarius 20P3 salB. Thus, SalA and SalA1 are examples of streptococcal lantibiotics whose production is autoregulated. These peptides act as intra- and interspecies signaling molecules, modulating lantibiotic production and possibly influencing streptococcal population ecology in the oral cavity.

Gelatinase biosynthesis-activating pheromone: a peptide lactone that mediates a quorum sensing in Enterococcus faecalis

Biosynthesis of gelatinase, a virulence factor of Enterococcus faecalis, was found to be regulated in a cell density-dependent fashion in which its production is active in late log to early stationary phase. Addition of early stationary phase culture filtrate to medium shifted the onset of gelatinase production to that of mid-log phase, suggesting that E. faecalis secretes a gelatinase biosynthesis-activating pheromone (GBAP). GBAP was isolated from culture supernatant of E. faecalis OG1S-P. Structural analysis suggested GBAP to be an 11-residue cyclic peptide containing a lactone structure, in which the alpha-carboxyl group of the C-terminal amino acid is linked to a hydroxyl group of the serine of the third residue. A synthetic peptide possessing the deduced structure showed GBAP activity at nanomolar concentrations as did natural GBAP. Database searches revealed that GBAP corresponds to a C-terminal part of a 242-residue FsrB protein. Northern analysis showed that GBAP slowly induces the transcription of two operons, fsrB-fsrC encoding FsrB and a putative histidine kinase FsrC and gelE-sprE encoding gelatinase GelE and serine protease SprE. Strains with an insertion mutation in either fsrC or a putative response regulator gene fsrA failed to respond to GBAP, suggesting that the GBAP signal is transduced by a two-component regulatory system.

Resuscitation of dormant Mycobacterium tuberculosis by phospholipids or specific peptides

The presence of dormant tubercle bacilli presents a major problem for tuberculosis treatment. The culture supernatant of Mycobacterium tuberculosis was previously shown to resuscitate dormant bacilli in vitro. Here we report identification of active components as phospholipids and a tuberculosis protein Rv1174c. Remarkably, dormant bacilli from a one year old culture which failed to form any colonies could be resuscitated with peptides derived from Rv1174c and formed 10(5-7) colonies/ml. This finding represents the first unambiguous demonstration of resuscitation of dormant tubercle bacilli in vitro and may have implication for the study of mycobacterial dormancy and the design of novel strategies for improved treatment of tuberculosis.

Kinetics of the AHL regulatory system in a model biofilm system: how many bacteria constitute a "quorum"?

Acylated homoserine lactones (AHLs) regulate a wide variety of phenotypes in Gram-negative bacteria. Most research suggests that AHL-mediated phenotypes are not expressed in populations until late logarithmic phase or stationary phase. Here, we model how the concentration of AHLs inside bacterial cells and in a biofilm changes over time as a function of population growth rate, diffusion of AHLs and the rate of autoinduction. Our theoretical results show that the concentration of AHLs inside a single bacterium (and by implication induction of a phenotype) has a non-trivial behaviour over time, and often exhibits a rapid increase early in population growth. This rapid increase is followed by a plateau, followed by another rise in the concentration of AHLs, to a second plateau. High concentrations of AHLs inside the bacterial cell early in population growth are positively affected by slow diffusion rates out of the cell and the biofilm, slow bacterial growth rates and fast autoinduction. In contrast, fast growth rates, slow autoinduction rates and high diffusion rates result in a high concentration plateau in stationary phase. More generally, the density-dependent nature of AHL regulation can be viewed as a trade-off between factors that dilute intracellular concentrations of AHLs (diffusion out of the cell, cell division), and those that increase concentrations (a slowing or restriction of diffusion or growth, or autoinduction). These results suggest that expression of AHL-mediated phenotypes can occur at relatively low cell densities and low external/environmental AHL concentrations.

Processing and export of peptide pheromones and bacteriocins in Gram-negative bacteria

Cell-density-dependent gene expression is widespread in bacteria and is mediated by extracellular communication molecules. Gram-negative bacteria often use N-acyl homoserine lactones, whereas cell-cell signaling in Gram-positive bacteria is accomplished using post-translationally processed peptide pheromones. In many Gram-positive bacteria, export of these peptides requires the activity of a dedicated ATP-binding cassette (ABC) transporter, which cleaves off a typical leader peptide termed the double-glycine leader sequence concomitant with translocation across the membrane. Inspection of bacterial genome sequences has revealed the presence of similar ABC transporters, as well as genes encoding peptides with double-glycine-type leader sequences in Gram-negative bacteria, and it is suggested that the postulated transported peptides could perform a signaling function.

Peptides and ATP binding cassette peptide transporters

In this review our knowledge of ATP binding cassette (ABC) transporters specific for peptides is discussed. Besides serving a role in nutrition of the cell, the systems participate in various signaling processes that allow (micro)organisms to monitor the local environment. In bacteria, these include regulation of gene expression, competence development, sporulation, DNA transfer by conjugation, chemotaxis, and virulence development, and the role of ABC transporters in each of these processes is discussed. Particular attention is paid to the specificity determinants of peptide receptors and transporters in relation to their structure and to the mechanisms of peptide binding.

Dominant-negative mutants of prgX: evidence for a role for PrgX dimerization in negative regulation of pheromone-inducible conjugation

PrgX negatively regulates prgQ transcriptional readthrough in the pheromone-inducible enterococcal conjugative plasmid pCF10. We isolated and characterized 13 dominant-negative prgX mutants, all of which mapped in either the N- or the C-terminus of PrgX. In all mutants, the in vivo level of Qa RNA, an antisense RNA to prgQ RNA, was greatly reduced. When oligomerization of PrgX was tested with a phage lambda cI repressor fusion system, the oligomerization domain was found to be between amino acid residues 78 and 280. When histidine-tagged PrgX (His-PrgX) was purified by nickel column chromatography from a strain also expressing PrgX, PrgX was co-purified with His-PrgX. Although PrgX was expressed at a much higher level than His-PrgX, an approximately equal amount of PrgX was co-purified. Pheromone induction greatly decreased the co-purification of PrgX. Based on these data, we propose that both the N- and the C-terminal domains of PrgX are required for PrgX positive autoregulation and for the repression of prgQ transcription readthrough. In vivo, PrgX exists as a dimer, and dimerization is mediated by the central region of PrgX.