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

Competence-induced fratricide in streptococci

Competence for natural genetic transformation in Streptococcus pneumoniae is controlled by the extracellular concentration of the competence-stimulating peptide (CSP), an exported peptide pheromone. Upon entering the competent state, pneumococci start transcribing a number of CSP-responsive genes, termed the early and late competence (com) genes. Some of the proteins encoded by these com genes are absolutely required for DNA uptake and transformation, but most of them are dispensable. This finding indicates that the majority of CSP-regulated proteins in S. pneumoniae is involved in processes unrelated to natural genetic transformation. Recently, however, it became clear that the biological role of a few of the dispensable proteins might be linked to the transformation process. Although these proteins are not needed for transformation per se, they constitute a killing mechanism that could be used by competent cells to acquire DNA from non-competent pneumococci. This mechanism, termed fratricide, has so far only been described for pneumococci. In this manuscript, we review evidence that suggests the conservation of fratricide as well as the independent evolution of its genetic control and of its effectors in several species of the genus Streptococcus, and discuss its possible biological significance in relation to natural transformation.

Phenotypic and functional characterization of Bacillus anthracis biofilms

Biofilms, communities of micro-organisms attached to a surface, are responsible for many chronic diseases and are often associated with environmental reservoirs or lifestyles. Bacillus anthracis is a Gram-positive, endospore-forming bacterium and is the aetiological agent of pulmonary, gastrointestinal and cutaneous anthrax. Anthrax infections are part of the natural lifecycle of many ruminants in North America, including cattle and bison, and B. anthracis is thought to be a central part of this ecosystem. However, in endemic areas in which humans and livestock interact, chronic cases of cutaneous anthrax are commonly reported. This suggests that biofilms of B. anthracis exist in the environment and are part of the ecology associated with its lifecycle. Currently, there are few data that account for the importance of the biofilm mode of life in B. anthracis, yet biofilms have been characterized in other pathogenic and non-pathogenic Bacillus species, including Bacillus cereus and Bacillus subtilis, respectively. This study investigated the phenotypic and functional role of biofilms in B. anthracis. The results demonstrate that B. anthracis readily forms biofilms which are inherently resistant to commonly prescribed antibiotics, and that antibiotic resistance is not solely the function of sporulation.

Comparative genome hybridization of Streptococcus mutans strains

The basis for genotypic and phenotypic variation within Streptococcus mutans is poorly understood but the availability of the genome sequence of strain UA159 provides an opportunity for comparative studies. Genomic DNA prepared from nine strains of S. mutans was used to probe a microarray consisting of oligonucleotides representing 1948 open reading frames of S. mutans UA159. A total of 385 (20%) of the UA159 open reading frames were found to be absent from one or more of the test strains. Absent open reading frames frequently occurred in blocks of adjacent open reading frames and represented regions previously experimentally detected by polymerase chain reaction, predicted genomic islands and insertion sequence elements as well as novel open reading frames. Approximately half appear to involve foreign DNA acquired by horizontal transmission. The results indicate the existence of distinct core and dispensable genomes and may help explain the phenotypic and genotypic variation within S. mutans.

Genetic variation incomC, the gene encoding competence-stimulating peptide (CSP) inStreptococcus mutans

The genetic variability in comC, the gene encoding the quorum-sensing molecule, competence-stimulating peptide (CSP) in Streptococcus mutans is reported. Seven comC alleles encoding three distinct mature CSPs were identified among 36 geographically diverse strains, although, compared with Streptococcus pneumoniae, the amount of predicted amino acid sequence variation is low. In agreement with other studies, significant variation was found in the natural competence for DNA uptake in these strains. However, there was no correlation between the CSP genotype and the ability to transform these strains. Representative strains encoding each of the CSP variants became competent in response to synthetic CSPs of each type. Therefore, in contrast to S. pneumoniae, comC alleles in S. mutans are functionally equivalent and there is no evidence of pherotype specificity.

Structure-activity analysis of quorum-sensing signaling peptides from Streptococcus mutans

Streptococcus mutans secretes and utilizes a 21-amino-acid signaling peptide pheromone to initiate quorum sensing for genetic competence, biofilm formation, stress responses, and bacteriocin production. In this study, we designed and synthesized a series of truncated peptides and peptides with amino acid substitutions to investigate their structure-activity relationships based on the three-dimensional structures of S. mutans wild-type signaling peptide UA159sp and C-terminally truncated peptide TPC3 from mutant JH1005 defective in genetic competence. By analyzing these peptides, we demonstrated that the signaling peptide of S. mutans has at least two functional domains. The C-terminal structural motif consisting of a sequence of polar hydrophobic charged residues is crucial for activation of the signal transduction pathway, while the core alpha-helical structure extending from residue 5 to the end of the peptide is required for receptor binding. Peptides in which three or more residues were deleted from the C terminus did not induce genetic competence but competitively inhibited quorum sensing activated by UA159sp. Disruption of the amphipathic alpha-helix by replacing the Phe-7, Phe-11, or Phe-15 residue with a hydrophilic residue resulted in a significant reduction in or complete loss of the activity of the peptide. In contrast to the C-terminally truncated peptides, these peptides with amino acid substitutions did not compete with UA159sp to activate quorum sensing, suggesting that disruption of the hydrophobic face of the alpha-helical structure results in a peptide that is not able to bind to the receptor. This study is the first study to recognize the importance of the signaling peptide C-terminal residues in streptococcal quorum sensing.

The detection of environmental autoinducing peptide quorum-sensing genes from an uncultured Clostridium sp. in landfill leachate reactor biomass

AIMS

To elucidate whether a dominant uncultured clostridial (Clostridium thermocellum-like) species in an environmental sample (landfill leachate), possesses an autoinducing peptide (AIP) quorum-sensing (QS) gene, although it may not be functional.

METHODS AND RESULTS

A modified AIP accessory gene regulator (agr)C PCR protocol was performed on extracted DNA from a landfill leachate sample (also characterized by 16S rRNA gene cloning) and the PCR products were cloned, sequenced and phylogenetically analysed. It appeared that two agrC gene phylotypes existed, most closely related to the C. thermocellum agrC gene, differing by only 1 bp.

CONCLUSIONS

It is possible to specifically identify and characterize the agrC AIP QS gene from uncultured Firmicutes (C. thermocellum-like) bacteria derived from environmental (landfill leachate) sample.

SIGNIFICANCE AND IMPACT OF THE STUDY

This is the first successful attempt at identifying AIP QS genes from a cellulolytic environment (landfill). The agrC gene was identified as being most closely related to the C. thermocellum agrC gene, the same bacterium identified as being dominant, according to 16S rRNA gene cloning and subsequently fluorescence in situ hybridization analyses, in the same biomass.

Purification and functional studies of a potent modified quorum-sensing peptide and a two-peptide bacteriocin in Streptococcus mutans

Bacteria use quorum-sensing signals or autoinducers to communicate. The signals in Gram-positive bacteria are often peptides activated by proteolytic removal of an N-terminal leader sequence. While investigating stimulation of antimicrobial peptide production by the Streptococcus mutans synthetic competence stimulating peptide signal (21-CSP), we found a peptide similar to the 21-CSP, but lacking the three C-terminal amino acid residues (18-CSP). The 18-CSP was more potent in inducing competence, biofilm formation, and antimicrobial activity than the 21-CSP. Our results indicate that cleavage of the three C-terminal residues occurred post export, and was not regulated by the CSP-signalling system. Deletion of comD encoding the CSP receptor abolished the competence and biofilm responses to the 21-CSP and the 18-CSP, suggesting that signal transduction via the ComD receptor is involved in the responses to both CSPs. In S. mutans GS5, beside the 18-CSP we also purified to homogeneity a two-peptide bacteriocin which production was stimulated by the 18-CSP and the 21-CSP. Partial sequence of the two-peptide bacteriocin revealed the product of the smbAB genes recently described. We found that the peptide SmbB was slightly different from the deduced sequence, and confirmed the prediction that both peptides constituting SmbAB bacteriocin are post-translationally modified. SmbAB exhibited antimicrobial activity against 11 species of streptococci, Enterococcus faecalis and Staphylocococcus epidermidis. Taken together, the findings support the involvement of the CSP response in bacteriocin production by streptococci and suggest a novel strategy to potentiate autoinducer activity.

Switch from planktonic to sessile life: a major event in pneumococcal pathogenesis

Two main patterns of gene expression of Streptococcus pneumoniae were observed during infection in the host by quantitative real time RT-PCR; one was characteristic of bacteria in blood and one of bacteria in tissue, such as brain and lung. Gene expression in blood was characterized by increased expression of pneumolysin, pspA and hrcA, while pneumococci in tissue infection showed increased expression of neuraminidases, metalloproteinases, oxidative stress and competence genes. In vitro situations with similar expression patterns were detected in liquid culture and in a newly developed pneumococcal model of biofilm respectively. The biofilm model was dependent on addition of synthetic competence stimulating peptide (CSP) and no biofilm was formed by CSP receptor mutants. As one of the differentially expressed gene sets in vivo were the competence genes, we exploited competence-specific tools to intervene on pneumococcal virulence during infection. Induction of the competence system by the quorum-sensing peptide, CSP, not only induced biofilm formation in vitro, but also increased virulence in pneumonia in vivo. In contrast, a mutant for the ComD receptor, which did not form biofilm, also showed reduced virulence in pneumonia. These results were opposite to those found in a bacteraemic sepsis model of infection, where the competence system was downregulated. When pneumococci in the different physiological states were used directly for challenge, sessile cells grown in a biofilm were more effective in inducing meningitis and pneumonia, while planktonic cells from liquid culture were more effective in inducing sepsis. Our data enable us, using in vivo gene expression and in vivo modulation of virulence, to postulate the distinction – from the pneumococcal point of view – between two main types of disease. During bacteraemic sepsis pneumococci resemble planktonic growth, while during tissue infection, such as pneumonia or meningitis, pneumococci are in a biofilm-like state.

Messing with bacterial quorum sensing

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

Assessment of genes associated with Streptococcus mutans biofilm morphology

Streptococcus mutans, the major pathogen responsible for dental caries in humans, is a biofilm-forming bacterium. In the present study, 17 different pulsed-field gel electrophoresis patterns of genomic DNA were identified in S. mutans organisms isolated clinically from whole saliva. The S. mutans isolates showed different abilities to form biofilms on polystyrene surfaces in semidefined minimal medium cultures. Following cultivation in a flow cell system in tryptic soy broth with 0.25% sucrose and staining using a BacLight LIVE/DEAD system, two strains, designated FSC-3 and FSC-4, showed the greatest and least, respectively, levels of biofilm formation when examined with confocal laser scanning microscopy. Further, image analyses of spatial distribution and architecture were performed to quantify the merged green (live cells) and red (dead cells) light. The light intensity of the FSC-3 biofilm was greater than that of the FSC-4 biofilm in the bottom area but not in the top area. S. mutans whole-genome array results showed that approximately 3.8% of the genes were differentially expressed in the two strains, of which approximately 2.2%, including bacitracin transport ATP-binding protein gene glrA and a BLpL-like putative immunity protein gene, were activated in FSC-3. In addition, about 1.6% of the genes, including those associated with phosphotransferase system genes, were repressed. Analyses of the glrA-deficient strains and reverse transcription-PCR confirmed the role of the gene in biofilm formation. Differential assessment of biofilm-associated genes in clinical strains may provide useful information for understanding the morphological development of streptococcal biofilm, as well as for colonization of S. mutans.

Membrane composition changes and physiological adaptation by Streptococcus mutans signal recognition particle pathway mutants

Previously, we presented evidence that the oral cariogenic species Streptococcus mutans remains viable but physiologically impaired and sensitive to environmental stress when genes encoding the minimal conserved bacterial signal recognition particle (SRP) elements are inactivated. Two-dimensional gel electrophoresis of isolated membrane fractions from strain UA159 and three mutants (Deltaffh, DeltascRNA, and DeltaftsY) grown at pH 7.0 or pH 5.0 allowed us to obtain insight into the adaptation process and the identities of potential SRP substrates. Mutant membrane preparations contained increased amounts of the chaperones DnaK and GroES and ClpP protease but decreased amounts of transcription- and translation-related proteins, the beta subunit of ATPase, HPr, and several metabolic and glycolytic enzymes. Therefore, the acid sensitivity of SRP mutants might be caused in part by diminished ATPase activity, as well as the absence of an efficient mechanism for supplying ATP quickly at the site of proton elimination. Decreased amounts of LuxS were also observed in all mutant membranes. To further define physiological changes that occur upon disruption of the SRP pathway, we studied global gene expression in S. mutans UA159 (parent strain) and AH333 (Deltaffh mutant) using microarray analysis. Transcriptome analysis revealed up-regulation of 81 genes, including genes encoding chaperones, proteases, cell envelope biosynthetic enzymes, and DNA repair and replication enzymes, and down-regulation of 35 genes, including genes concerned with competence, ribosomal proteins, and enzymes involved in amino acid and protein biosynthesis. Quantitative real-time reverse transcription-PCR analysis of eight selected genes confirmed the microarray data. Consistent with a demonstrated defect in competence and the suggested impairment of LuxS-dependent quorum sensing, biofilm formation was significantly decreased in each SRP mutant.

Genetic diversity of competence gene loci in clinical genotypes of Streptococcus mutans

The frequencies of 21 competence genes were analyzed in 94 genotypes of Streptococcus mutans. These include those of a main regulatory system (comCDE), structural, and other regulatory orthologues identified in the genome of strain UA159. PCR and Southern blot analysis revealed that all genes are widespread within the species.

The atlA operon of Streptococcus mutans: role in autolysin maturation and cell surface biogenesis

The Smu0630 protein (AtlA) was recently shown to be involved in cell separation, biofilm formation, and autolysis. Here, transcriptional studies revealed that atlA is part of a multigene operon under the control of at least three promoters. The morphology and biofilm-forming capacity of a nonpolar altA mutant could be restored to that of the wild-type strain by adding purified AtlA protein to the medium. A series of truncated derivatives of AtlA revealed that full activity required the C terminus and repeat regions. AtlA was cell associated and readily extractable from with sodium dodecyl sulfate. Of particular interest, the surface protein profile of AtlA-deficient strains was dramatically altered compared to the wild-type strain, as was the nature of the association of the multifunctional adhesin P1 with the cell wall. In addition, AtlA-deficient strains failed to develop competence as effectively as the parental strain. Mutation of thmA, which can be cotranscribed with atlA and encodes a putative pore-forming protein, resulted in a phenotype very similar to that of the AtlA-deficient strain. ThmA was also shown to be required for efficient processing of AtlA to its mature form, and treatment of the thmA mutant strain with full-length AtlA protein did not restore normal cell separation and biofilm formation. The effects of mutating other genes in the operon on cell division, biofilm formation, or AtlA biogenesis were not as profound. This study reveals that AtlA is a surface-associated protein that plays a critical role in the network connecting cell surface biogenesis, biofilm formation, genetic competence, and autolysis.

Proteomic analysis of Escherichia coli biofilms reveals the overexpression of the outer membrane protein OmpA

Bacterial colonisation and biofilm formation on the surface of urinary catheters is a common cause of nosocomial infection, and as such is a major impediment to their long-term use. Understanding the mechanisms of biofilm formation on urinary catheters is critical to their control and will aid the future development of materials used in their manufacture. In this report we have used proteomic analysis coupled with immunoassays to show that the major outer membrane protein (OmpA) of Escherichia coli is overexpressed during biofilm formation. A series of synthetic hydrogels being developed for potential use as catheter coatings were used as the substrata and OmpA expression was increased in biofilms on all these surfaces, as well as being a feature of both a laboratory and a clinical strain of E. coli. Up-regulation of OmpA may, therefore, be a common feature of E. coli biofilms. These findings present OmpA as a potential target for biofilm inhibition and may contribute to the rational design of biofilm inhibiting hydrogel coatings for urinary catheters.

Biofilms, a new approach to the microbiology of dental plaque

Dental plaque has the properties of a biofilm, similar to other biofilms found in the body and the environment. Modern molecular biological techniques have identified about 1000 different bacterial species in the dental biofilm, twice as many as can be cultured. Oral biofilms are very heterogeneous in structure. Dense mushroom-like structures originate from the enamel surface, interspersed with bacteria-free channels used as diffusion pathways. The channels are probably filled with an extracellular polysaccharide (EPS) matrix produced by the bacteria. Bacteria in biofilms communicate through signaling molecules, and use this "quorum-sensing" system to optimize their virulence factors and survival. Bacteria in a biofilm have a physiology different from that of planktonic cells. They generally live under nutrient limitation and often in a dormant state. Such "sleepy" bacteria respond differently to antibiotics and antimicrobials, because these agents were generally selected in experiments with metabolically active bacteria. This is one of the explanations as to why antibiotics and antimicrobials are not as successful in the clinic as could be expected from laboratory studies. In addition, it has been found that many therapeutic agents bind to the biofilm EPS matrix before they even reach the bacteria, and are thereby inactivated. Taken together, these fundings highlight why the study of bacteria in the oral cavity is now taken on by studying the biofilms rather than individual species.

A pleiotropic regulator, Frp, affects exopolysaccharide synthesis, biofilm formation, and competence development in Streptococcus mutans

Exopolysaccharide synthesis, biofilm formation, and competence are important physiologic functions and virulence factors for Streptococcus mutans. In this study, we report the role of Frp, a transcriptional regulator, on the regulation of these traits crucial to pathogenesis. An Frp-deficient mutant showed decreased transcription of several genes important in virulence, including those encoding fructosyltransferase (Ftf), glucosyltransferase B (GtfB), and GtfC, by reverse transcription and quantitative real-time PCR. Expression of Ftf was decreased in the frp mutant, as assessed by Western blotting as well as by the activity assays. Frp deficiency also inhibited the production of GtfB in the presence of glucose and sucrose as well as the production of GtfC in the presence of glucose. As a consequence of the effects on GtfB and -C, sucrose-induced biofilm formation was decreased in the frp mutant. The expression of competence mediated by the competence-signaling peptide (CSP) system, as assessed by comC gene transcription, was attenuated in the frp mutant. As a result, the transformation efficiency was decreased in the frp mutant but was partially restored by adding synthetic CSP. Transcription of the frp gene was significantly increased in the frp mutant under all conditions tested, indicating that frp transcription is autoregulated. Furthermore, complementation of the frp gene in the frp mutant restored transcription of the affected genes to levels similar to those in the wild-type strain. These results suggest that Frp is a novel pleiotropic effector of multiple cellular functions and is involved in the modulation of exopolysaccharide synthesis, sucrose-dependent biofilm formation, and competence development.

Effective prevention of microbial biofilm formation on medical devices by low-energy surface acoustic waves

Low-energy surface acoustic waves generated from electrically activated piezo elements are shown to effectively prevent microbial biofilm formation on indwelling medical devices. The development of biofilms by four different bacteria and Candida species is prevented when such elastic waves with amplitudes in the nanometer range are applied. Acoustic-wave-activated Foley catheters have all their surfaces vibrating with longitudinal and transversal dispersion vectors homogeneously surrounding the catheter surfaces. The acoustic waves at the surface are repulsive to bacteria and interfere with the docking and attachment of planktonic microorganisms to solid surfaces that constitute the initial phases of microbial biofilm development. FimH-mediated adhesion of uropathogenic Escherichia coli to guinea pig erythrocytes was prevented at power densities below thresholds that activate bacterial force sensor mechanisms. Elevated power densities dramatically enhanced red blood cell aggregation. We inserted Foley urinary catheters attached with elastic-wave-generating actuators into the urinary tracts of male rabbits. The treatment with the elastic acoustic waves maintained urine sterility for up to 9 days compared to 2 days in control catheterized animals. Scanning electron microscopy and bioburden analyses revealed diminished biofilm development on these catheters. The ability to prevent biofilm formation on indwelling devices and catheters can benefit the implanted medical device industry.

Biofilm formation by Streptococcus pneumoniae: role of choline, extracellular DNA, and capsular polysaccharide in microbial accretion

Streptococcus pneumoniae colonizes the human upper respiratory tract, and this asymptomatic colonization is known to precede pneumococcal disease. In this report, chemically defined and semisynthetic media were used to identify the initial steps of biofilm formation by pneumococcus during growth on abiotic surfaces such as polystyrene or glass. Unencapsulated pneumococci adhered to abiotic surfaces and formed a three-dimensional structure about 25 microm deep, as observed by confocal laser scanning microscopy and low-temperature scanning electron microscopy. Choline residues of cell wall teichoic acids were found to play a fundamental role in pneumococcal biofilm development. The role in biofilm formation of choline-binding proteins, which anchor to the teichoic acids of the cell envelope, was determined using unambiguously characterized mutants. The results showed that LytA amidase, LytC lysozyme, LytB glucosaminidase, CbpA adhesin, PcpA putative adhesin, and PspA (pneumococcal surface protein A) mutants had a decreased capacity to form biofilms, whereas no such reduction was observed in Pce phosphocholinesterase or CbpD putative amidase mutants. Moreover, encapsulated, clinical pneumococcal isolates were impaired in their capacity to form biofilms. In addition, a role for extracellular DNA and proteins in the establishment of S. pneumoniae biofilms was demonstrated. Taken together, these observations provide information on conditions that favor the sessile mode of growth by S. pneumoniae. The experimental approach described here should facilitate the study of bacterial genes that are required for biofilm formation. Those results, in turn, may provide insight into strategies to prevent pneumococcal colonization of its human host.

Pheromone-induced expression of recombinant proteins in Streptococcus thermophilus

A locus encoding proteins with high homology to the pneumococcal BlpABCHR quorum-sensing system was identified in Streptococcus thermophilus LMG 18311. The BlpABCHR system regulates bacteriocin production in Streptococcus pneumoniae by monitoring the extracellular concentration of a peptide-pheromone encoded by blpC. The homologous system in S. thermophilus, termed StbABCHR, contains a corresponding gene (stbC) encoding a possible peptide-pheromone (STP) that presumably controls bacteriocin production in S. thermophilus. We synthesized this peptide and found that it activates transcription of a gusA reporter gene placed behind the promoter of the bacteriocin-like gene stbD. Furthermore, deletion mapping and mutational analysis of the stbD promoter region were used to identify a degenerated direct repeat motif required for STP induced GusA expression. Our findings provide strong evidence that STP regulates bacteriocin production in S. thermophilus LMG 18311, and show that the StbABCHR quorum-sensing system can be exploited for inducible expression of recombinant proteins in this bacterial species.

Biofilms: a role in recurrent mastitis infections?

Mastitis remains one the most important diseases in dairy cattle despite the progress made in improving general udder health in recent years. Epidemiological studies have revealed that following treatment with antimicrobials, bacteriological cure rates vary between 0% and 80% but with no evidence of a significant loss of activity of the major classes of antibiotics licensed for the treatment of bovine mastitis. Recurrent infections are often attributable to biofilm growth of bacteria and this review provides an overview of those mechanisms related to bacterial biofilm growth in mastitis. Biofilm formation is accompanied by significant genetic and subsequent physiological changes in the microorganisms resulting, inter alia, in a loss of sensitivity to virtually all classes of antibiotics.

Multilevel control of competence development and stress tolerance in Streptococcus mutans UA159

Genetic competence appears to be important in establishment of biofilms and tolerance of environmental insults. We report here that the development of competence is controlled at multiple levels in a complex network that includes two signal-transducing two-component systems (TCS). Using Streptococcus mutans strain UA159, we demonstrate that the histidine kinase CiaH, but not the response regulator CiaR, causes a dramatic decrease in biofilm formation and in transformation efficiency. Inactivation of comE or comD had no effect on stress tolerance, but transformability of the mutants was poor and was not restored by addition of competence-stimulating peptide (CSP). Horse serum (HS) or bovine serum albumin (BSA) had no impact on transformability of any strains. Interestingly, though, the presence of HS or BSA in combination with CSP was required for efficient induction of comD, comX, and comYA, and induction was dependent on ComDE and CiaH, but not CiaR. Inactivation of comC, encoding CSP, had no impact on transformation, and CiaH was shown to be required for optimal comC expression. This study reveals that S. mutans integrates multiple environmental signals through CiaHR and ComDE to coordinate induction of com genes and that CiaH can exert its influence through CiaR and as-yet-unidentified regulators. The results highlight critical differences in the role and regulation of CiaRH and com genes in different S. mutans isolates and between S. mutans and Streptococcus pneumoniae, indicating that substantial divergence in the role and regulation of TCS and competence genes has occurred in streptococci.

Different roles of EIIABMan and EIIGlc in regulation of energy metabolism, biofilm development, and competence in Streptococcus mutans

The phosphoenolpyruvate:sugar phosphotransferase system (PTS) is the major carbohydrate transport system in oral streptococci. The mannose-PTS of Streptococcus mutans, which transports mannose and glucose, is involved in carbon catabolite repression (CCR) and regulates the expression of known virulence genes. In this study, we investigated the role of EII(Glc) and EIIAB(Man) in sugar metabolism, gene regulation, biofilm formation, and competence. The results demonstrate that the inactivation of ptsG, encoding a putative EII(Glc), did not lead to major changes in sugar metabolism or affect the phenotypes of interest. However, the loss of EII(Glc) was shown to have a significant impact on the proteome and to affect the expression of a known virulence factor, fructan hydrolase (fruA). JAM1, a mutant strain lacking EIIAB(Man), had an impaired capacity to form biofilms in the presence of glucose and displayed a decreased ability to be transformed with exogenous DNA. Also, the lactose- and cellobiose-PTSs were positively and negatively regulated by EIIAB(Man), respectively. Microarrays were used to investigate the profound phenotypic changes displayed by JAM1, revealing that EIIAB(Man) of S. mutans has a key regulatory role in energy metabolism, possibly by sensing the energy levels of the cells or the carbohydrate availability and, in response, regulating the activity of transcription factors and carbohydrate transporters.

Communication in bacteria: an ecological and evolutionary perspective

Individual bacteria can alter their behaviour through chemical interactions between organisms in microbial communities - this is generally referred to as quorum sensing. Frequently, these interactions are interpreted in terms of communication to mediate coordinated, multicellular behaviour. We show that the nature of interactions through quorum-sensing chemicals does not simply involve cooperative signals, but entails other interactions such as cues and chemical manipulations. These signals might have a role in conflicts within and between species. The nature of the chemical interaction is important to take into account when studying why and how bacteria react to the chemical substances that are produced by other bacteria.

In vitro activity of xanthorrhizol against Streptococcus mutans biofilms

AIMS

We determined the effect of xanthorrhizol (XTZ) purified from the rhizome of Curcuma xanthorrhiza Roxb. on the Streptococcus mutans biofilms in vitro.

METHODS AND RESULTS

The biofilms of S. mutans at different phases of growth were exposed to XTZ at different concentrations (5, 10 and 50 micromol l(-1)) and for different time exposures (1, 10, 30 and 60 min). The results demonstrated that the activity of XTZ in removing S. mutans biofilm was dependent on the concentration, exposure time and the phase growth of biofilm. A concentration of 5 micromol l(-1) of XTZ completely inhibited biofilm formation by S. mutans at adherent phases of growth, whereas 50 micromol l(-1) of XTZ removed 76% of biofilm at plateau accumulated phase when exposed to S. mutans biofilm for 60 min.

CONCLUSIONS

Xanthorrhizol isolated from an edible plant (C. xanthorrhiza Roxb.) shows promise as an antibacterial agent for inhibiting and removing S. mutans biofilms in vitro.

SIGNIFICANCE AND IMPACT OF THE STUDY

XTZ could be used as a potential antibacterial agent against biofilm formation by S. mutans.

Differences between single- and dual-species biofilms of Streptococcus mutans and Veillonella parvula in growth, acidogenicity and susceptibility to chlorhexidine

Streptococcus mutans, considered a primary pathogen in dental caries, thrives in dental plaque, which is a multispecies biofilm. Metabolic interactions between S. mutans and Veillonella parvula have been suggested. In this study we developed a biofilm model to quantify single-species (S. mutans or V. parvula) and dual-species (S. mutans and V. parvula) biofilm formation, and we identified the differences between the respective biofilms in terms of growth, acid formation, and response to chlorhexidine. Polystyrene 96-well microtiter plates were used for biofilm formation. These biofilms were exposed to various chlorhexidine concentrations (0.025-0.4 mg ml(-1)) and treatment conditions. Growth of the biofilms and the effects of chlorhexidine were evaluated by viable counts. Viability of the two species in all biofilm types was similar ( approximately 10(8) colony-forming units per well) after 72 h. Lactic acid accumulation of dual-species biofilms was significantly lower at 48 and 72 h than single-species biofilms of S. mutans. Dual-species biofilms were less susceptible to chlorhexidine than single-species biofilms when a neutralization step was included. These results indicate that bacteria in dual-species biofilms have different properties from bacteria in single-species biofilms.

Quorum sensing and DNA release in bacterial biofilms

The multicellular behavior of bacteria has been the subject of much recent interest. This behavior includes coordinated control of virulence, luminescence, competence and biofilm formation; each of these appears to be regulated or influenced by quorum sensing. An understanding of what biofilms constitute, and how they develop, is emerging. It is clear that biofilm formation is a carefully orchestrated process that is dependent on quorum sensing. Somewhat surprisingly, several independent observations have noted an important role for DNA in the structure of biofilms. Recent studies describe a mechanism for linking DNA release to quorum sensing, providing a possible mechanism for the coordinated release of DNA, and its integration into a biofilm. A review of the literature reveals that similar observations have been made for biofilms of both Gram-positive and Gram-negative organisms. Further study will determine whether this is a general trend, however.

Influence of BrpA on critical virulence attributes of Streptococcus mutans

Streptococcus mutans, the primary etiological agent of human dental caries, has developed multiple mechanisms to colonize and form biofilms on the tooth surface. The brpA gene codes for a predicted surface-associated protein with apparent roles in biofilm formation, autolysis, and cell division. In this study, we used two models to further characterize the biofilm-forming characteristics of a BrpA-deficient mutant, strain TW14. Compared to those of the parent strain, UA159, TW14 formed long chains and sparse microcolonies on hydroxylapatite disks but failed to accumulate and form three-dimensional biofilms when grown on glucose as the carbohydrate source. The biofilm formation defect was also readily apparent by confocal laser scanning microscopy when flow cells were used to grow biofilms. When subjected to acid killing at pH 2.8 for 45 min, the survival rate of strain TW14 was more than 1 log lower than that of the wild-type strain. TW14 was at least 3 logs more susceptible to killing by 0.2% hydrogen peroxide than was UA159. The expression of more than 200 genes was found by microarray analysis to be altered in cells lacking BrpA (P < 0.01). These results suggest that the loss of BrpA can dramatically influence the transcriptome and significantly affects the regulation of acid and oxidative stress tolerance and biofilm formation in S. mutans, which are key virulence attributes of the organism.

A eukaryotic-type serine/threonine protein kinase is required for biofilm formation, genetic competence, and acid resistance in Streptococcus mutans

We report an operon encoding a eukaryotic-type serine/threonine protein kinase (STPK) and its cognate phosphatase (STPP) in Streptococcus mutans. Mutation of the gene encoding the STPK produced defects in biofilm formation, genetic competence, and acid resistance, determinants important in caries pathogenesis.

Peptide pheromone induced cell death of Streptococcus mutans

Streptococcus mutans is considered one of the main causative agents of human dental caries. Cell-cell communication through two-component signal transduction systems (TCSTS) plays an important role in the pathogenesis of S. mutans. One of the S. mutans TCSTS, ComDE, controls both competence development and biofilm formation. In this study, we showed that addition of exogenous competence-stimulating peptide (CSP) beyond the levels necessary for competence inhibited the growth of S. mutans in a ComDE-dependent manner. We also demonstrated that further increases of CSP stopped S. mutans cell division leading to cell death. Use of CSP as a possible therapeutic agent is discussed.

Co-ordinated bacteriocin production and competence development: a possible mechanism for taking up DNA from neighbouring species

It is important to ensure DNA availability when bacterial cells develop competence. Previous studies in Streptococcus pneumoniae demonstrated that the competence-stimulating peptide (CSP) induced autolysin production and cell lysis of its own non-competent cells, suggesting a possible active mechanism to secure a homologous DNA pool for uptake and recombination. In this study, we found that in Streptococcus mutans CSP induced co-ordinated expression of competence and mutacin production genes. This mutacin (mutacin IV) is a non-lantibiotic bacteriocin which kills closely related Streptococcal species such as S. gordonii. In mixed cultures of S. mutans and S. gordonii harbouring a shuttle plasmid, plasmid DNA transfer from S. gordonii to S. mutans was observed in a CSP and mutacin IV-dependent manner. Further analysis demonstrated an increased DNA release from S. gordonii upon addition of the partially purified mutacin IV extract. On the basis of these findings, we propose that Streptococcus mutans, which resides in a multispecies oral biofilm, may utilize the competence-induced bacteriocin production to acquire transforming DNA from other species living in the same ecological niche. This hypothesis is also consistent with a well-known phenomenon that a large genomic diversity exists among different S. mutans strains. This diversity may have resulted from extensive horizontal gene transfer.

LuxS controls bacteriocin production in Streptococcus mutans through a novel regulatory component

The oral pathogen Streptococcus mutans employs a variety of mechanisms to maintain a competitive advantage over many other oral bacteria which occupy the same ecological niche. Production of the bacteriocin, mutacin I, is one such mechanism. However, little is known about the regulatory mechanisms associated with mutacin I production. Previous work has demonstrated that the production of mutacin I greatly increased with cell density. In this study, we found that high cell density also triggered high level mutacin I gene transcription. However, this response was abolished upon deletion of luxS. Further analysis using real-time reverse transcription polymerase chain reaction (RT-PCR) demonstrated that in the luxS mutant transcription of both the mutacin I structural gene mutA and the mutacin I transcriptional activator mutR was impaired. Through microarray analysis, a putative transcription repressor annotated as Smu1274 in the Los Alamos National Laboratory Oral Pathogens Sequence Database was identified, which was strongly induced in the luxS mutant. Characterization of Smu1274, which we referred to as irvA, suggested that it may act as an inducible repressor to suppress mutacin I gene expression. A luxS and irvA double mutant regained the ability to produce mutacin I; whereas a constitutive irvA-producing strain was impaired in mutacin I production. These findings reveal a novel regulatory pathway for mutacin I gene expression, which may provide clues to the regulatory mechanisms of other cellular functions regulated by luxS in S. mutans.

Up-regulation of competence- but not stress-responsive proteins accompanies an altered metabolic phenotype in Streptococcus mutans biofilms

Mature biofilm and planktonic cells of Streptococcus mutans cultured in a neutral pH environment were subjected to comparative proteome analysis. Of the 242 protein spots identified, 48 were significantly altered in their level of expression (P<0.050) or were unique to planktonic or biofilm-grown cells. Among these were four hypothetical proteins as well as proteins known to be associated with the maintenance of competence or found to possess a cin-box-like element upstream of their coding gene. Most notable among the non-responsive genes were those encoding the molecular chaperones DnaK, GroEL and GroES, which are considered to be up-regulated by sessile growth. Analysis of the rest of the proteome indicated that a number of cellular functions associated with carbon uptake and cell division were down-regulated. The data obtained were consistent with the hypothesis that a reduction in the general growth rate of mature biofilms of S. mutans in a neutral pH environment is associated with the maintenance of transformation without the concomitant stress response observed during the transient state of competence in bacterial batch cultures.

Quorum-sensing mutations affect attachment and stability of Burkholderia cenocepacia biofilms

Biofilm formation in Burkholderia cenocepacia has been shown to rely in part on acylhomoserine lactone-based quorum sensing. For many other bacterial species, it appears that both the initial adherence and the later stages of biofilm maturation are affected when quorum sensing pathways are inhibited. In this study, we examined the effects of mutations in the cepIR and cciIR quorum-sensing systems of Burkholderia cenocepacia K56-2 with respect to biofilm attachment and antibiotic resistance. We also examined the role of the cepIR system in biofilm stability and structural development. Using the high-throughput MBEC assay system to produce multiple equivalent biofilms, the biomasses of both the cepI and cepR mutant biofilms, measured by crystal violet staining, were less than half of the value observed for the wild-type strain. Attachment was partially restored upon providing functional gene copies via multicopy expression vectors. Surprisingly, neither the cciI mutant nor the double cciI cepI mutant was deficient in attachment, and restoration of the cciI gene resulted in less attachment than for the mutants. Meanwhile, the cciR mutant did show a significant reduction in attachment, as did the cciR cepIR mutant. While there was no change in antibiotic susceptibility with the individual cepIR and cciIR mutants, the cepI cciI mutant biofilms were more sensitive to ciprofloxacin. A significant increase in sensitivity to removal by sodium dodecyl sulfate was seen for the cepI and cepR mutants. Flow cell analysis of the individual cepIR mutant biofilms indicated that they were both structurally and temporally impaired in attachment and development. These results suggest that biofilm structural defects might be present in quorum-sensing mutants of B. cenocepacia that affect the stability and resistance of the adherent cell mass, providing a basis for future studies to design preventative measures against biofilm formation in this species, an important lung pathogen of cystic fibrosis patients.

Dental plaque: biological significance of a biofilm and community life-style

BACKGROUND

Most microorganisms in nature attach to surfaces and form matrix-embedded biofilms. Biofilms are highly structured and spatially organized, and are often composed of consortia of interacting microorganisms, termed microbial communities, the properties of which are more than the sum of the component species. Microbial gene expression alters markedly in biofilms; organisms communicate by gene transfer and by secretion of diffusible signalling molecules. Cells in biofilms are less susceptible to antimicrobial agents.

AIM AND MATERIALS & METHODS

To comprehensively review the literature to determine whether dental plaque displays properties consistent with those of a typical biofilm and microbial community.

RESULTS

Novel microscopic and molecular techniques have demonstrated that plaque has a structured architecture with an extracellular matrix, and a diverse composition (around 50% of cells are unculturable). The constituent species communicate by gene transfer, by secreted peptides (gram-positive bacteria) and autoinducer-2 (gram-positive and gram-negative bacteria). These organisms are functionally organized for increased metabolic efficiency, greater resistance to stress and for enhanced virulence. Plaque formation has direct and indirect effects on gene expression.

CONCLUSION

Dental plaque displays properties that are typical of biofilms and microbial communities in general, a clinical consequence of which is a reduced susceptibility to antimicrobial agents as well as pathogenic synergism.

Tansmission, diversity and virulence factors of Sreptococcus mutans genotypes

Dental caries is an infectious and transmissible disease, in which many genetic, environmental and behavioral risk factors interact. The mutans streptococci (MS), mainly Streptococcus mutans and Streptococcus sobrinus are the microorganisms most strongly associated with this disease. The main virulence factors associated with MS cariogenicity include adhesion, acidogenicity and acid tolerance. These properties work together to modify the physico-chemical properties of the biofilm, resulting in ecological changes in the form of increased proportions of S. mutans and other acidogenic and aciduric species. In addition, reports of higher numbers of S. mutans genotypes with increased virulence in caries-active subjects suggest the importance of microenvironmental factors in increasing the risk of caries. This review focuses on the transmission and establishment of different genotypes of S. mutans and the role they play in the development of dental caries.

DNA binding-uptake system: a link between cell-to-cell communication and biofilm formation

DNA has recently been described as a major structural component of the extracellular matrix in biofilms. In streptococci, the competence-stimulating peptide (CSP) cell-to-cell signal is involved in competence for genetic transformation, biofilm formation, and autolysis. Among the genes regulated in response to the CSP are those involved in binding and uptake of extracellular DNA. We show in this study that a functional DNA binding-uptake system is involved in biofilm formation. A comGB mutant of Streptococcus mutans deficient in DNA binding and uptake, but unaffected in signaling, showed reduced biofilm formation. During growth in the presence of DNase I, biofilm was reduced in the wild type to levels similar to those found with the comGB mutant, suggesting that DNA plays an important role in the wild-type biofilm formation. We also showed that growth in the presence of synthetic CSP promoted significant release of DNA, with similar levels in the wild type and in the comGB mutant. The importance of the DNA binding-uptake system in biofilm formation points to possible novel targets to fight infections.

Quorum sensing in streptococcal biofilm formation

Bacteria in their natural ecosystems preferentially grow as polysaccharide-encased biofilms attached to surfaces. Although quorum-sensing (QS) systems directing the 'biofilm phenotype' have been extensively described in Gram-negative bacteria, there is little understanding of the importance of these systems in Gram-positive biofilm formation. Streptococci are a diverse group of Gram-positive bacteria that colonize epithelial, mucosal and tooth surfaces of humans. In several streptococci, competence-stimulating peptide (CSP)-mediated QS has been connected with competence development for genetic transformation. Recent work, especially with bacteria that inhabit the biofilm of dental plaque, has linked CSP stimuli to other cell-density adaptations, such as biofilm formation.

A VicRK signal transduction system in Streptococcus mutans affects gtfBCD, gbpB, and ftf expression, biofilm formation, and genetic competence development

Bacteria exposed to transient host environments can elicit adaptive responses by triggering the differential expression of genes via two-component signal transduction systems. This study describes the vicRK signal transduction system in Streptococcus mutans. A vicK (putative histidine kinase) deletion mutant (SmuvicK) was isolated. However, a vicR (putative response regulator) null mutation was apparently lethal, since the only transformants isolated after attempted mutagenesis overexpressed all three genes in the vicRKX operon (Smuvic+). Compared with the wild-type UA159 strain, both mutants formed aberrant biofilms. Moreover, the vicK mutant biofilm formed in sucrose-supplemented medium was easily detachable relative to that of the parent. The rate of total dextran formation by this mutant was remarkably reduced compared to the wild type, whereas it was increased in Smuvic+. Based on real-time PCR, Smuvic+ showed increased gtfBCD, gbpB, and ftf expression, while a recombinant VicR fusion protein was shown to bind the promoter regions of the gtfB, gtfC, and ftf genes. Also, transformation efficiency in the presence or absence of the S. mutans competence-stimulating peptide was altered for the vic mutants. In vivo studies conducted using SmuvicK in a specific-pathogen-free rat model resulted in significantly increased smooth-surface dental plaque (Pearson-Filon statistic [PF], <0.001). While the absence of vicK did not alter the incidence of caries, a significant reduction in SmuvicK CFU counts was observed in plaque samples relative to that of the parent (PF, <0.001). Taken together, these findings support involvement of the vicRK signal transduction system in regulating several important physiological processes in S. mutans.

Regulation of bacteriocin production in Streptococcus mutans by the quorum-sensing system required for development of genetic competence

In Streptococcus mutans, competence for genetic transformation and biofilm formation are dependent on the two-component signal transduction system ComDE together with the inducer peptide pheromone competence-stimulating peptide (CSP) (encoded by comC). Here, it is shown that the same system is also required for expression of the nlmAB genes, which encode a two-peptide nonlantibiotic bacteriocin. Expression from a transcriptional nlmAB'-lacZ fusion was highest at high cell density and was increased up to 60-fold following addition of CSP, but it was abolished when the comDE genes were interrupted. Two more genes, encoding another putative bacteriocin and a putative bacteriocin immunity protein, were also regulated by this system. The regions upstream of these genes and of two further putative bacteriocin-encoding genes and a gene encoding a putative bacteriocin immunity protein contained a conserved 9-bp repeat element just upstream of the transcription start, which suggests that expression of these genes is also dependent on the ComCDE regulatory system. Mutations in the repeat element of the nlmAB promoter region led to a decrease in CSP-dependent expression of nlmAB'-lacZ. In agreement with these results, a comDE mutant and mutants unable to synthesize or export CSP did not produce bacteriocins. It is speculated that, at high cell density, bacteriocin production is induced to liberate DNA from competing streptococci.

Etiology and pathogenesis of periodontal diseases

The two most prevalent and most investigated periodontal diseases are dental plaque-induced gingivitis and chronic periodontitis. The last 10 to 15 years have seen the emergence of several important new findings and concepts regarding the etiopathogenesis of periodontal diseases. These findings include the recognition of dental bacterial plaque as a biofilm, identification and characterization of genetic defects that predispose individuals to periodontitis, host-defense mechanisms implicated in periodontal tissue destruction, and the interaction of risk factors with host defenses and bacterial plaque. This article reviews current aspects of the etiology and pathogenesis of periodontal diseases.

Involvement of sortase anchoring of cell wall proteins in biofilm formation by Streptococcus mutans

Streptococcus mutans is one of the best-known biofilm-forming organisms associated with humans. We investigated the role of the sortase gene (srtA) in monospecies biofilm formation and observed that inactivation of srtA caused a decrease in biofilm formation. Genes encoding three putative sortase-dependent proteins were also found to be up-regulated in biofilms versus planktonic cells and mutations in these genes resulted in reduced biofilm biomass.

Identification and characterization of an autolysin-encoding gene of Streptococcus mutans

We identified a gene (atlA) encoding autolytic activity from Streptococcus mutans Xc. The AtlA protein predicted to be encoded by atlA is composed of 979 amino acids with a molecular weight of 107,279 and has a conserved beta-1,4-N-acetylmuramidase (lysozyme) domain in the C-terminal portion. Sodium dodecyl sulfate extracts of strain Xc showed two major bacteriolytic bands with molecular masses of 107 and 79 kDa, both of which were absent from a mutant with inactivated atlA. Western blot analysis revealed that the 79-kDa band was derived from the 107-kDa peptide by cleavage of its N-terminal portion. The inactivation of atlA resulted in a marked decrease in autolysis and the formation of very long chains of cells compared to the case for the parent strain. Although both the parent and mutant strains formed biofilms in the presence of sucrose, the biofilms formed by the mutant had a sponge-like architecture with large gaps and contained 30% less biomass than those formed by the parent strain. Furthermore, strain Xc formed glucose-dependent, loose biofilms in the absence of sucrose, but the mutant lost this ability. These results suggest that AtlA may play an important role in biofilm formation by S. mutans. The antibody produced against the C-terminal peptide containing the beta-1,4-N-acetylmuramidase domain drastically inhibited the autolytic activity of strain Xc. This inhibition was specific among the oral streptococci to S. mutans. These results indicate that the catalytic domain of AtlA is located at the C terminus, suggesting that further characterization of this domain may provide a means to control cariogenic dental plaque formation.

LuxS-based signaling affects Streptococcus mutans biofilm formation

Streptococcus mutans is implicated as a major etiological agent in human dental caries, and one of the important virulence properties of this organism is its ability to form biofilms (dental plaque) on tooth surfaces. We examined the role of autoinducer-2 (AI-2) on S. mutans biofilm formation by constructing a GS-5 luxS-null mutant. Biofilm formation by the luxS mutant in 0.5% sucrose defined medium was found to be markedly attenuated compared to the wild type. Scanning electron microscopy also revealed that biofilms of the luxS mutant formed larger clumps in sucrose medium compared to the parental strain. Therefore, the expression of glucosyltransferase genes was examined and the gtfB and gtfC genes, but not the gtfD gene, in the luxS mutant were upregulated in the mid-log growth phase. Furthermore, we developed a novel two-compartment system to monitor AI-2 production by oral streptococci and periodontopathic bacteria. The biofilm defect of the luxS mutant was complemented by strains of S. gordonii, S. sobrinus, and S. anginosus; however, it was not complemented by S. oralis, S. salivarius, or S. sanguinis. Biofilm formation by the luxS mutant was also complemented by Porphyromonas gingivalis 381 and Actinobacillus actinomycetemcomitans Y4 but not by a P. gingivalis luxS mutant. These results suggest that the regulation of the glucosyltransferase genes required for sucrose-dependent biofilm formation is regulated by AI-2. Furthermore, these results provide further confirmation of previous proposals that quorum sensing via AI-2 may play a significant role in oral biofilm formation.

The putative autolysin regulator LytR in Streptococcus mutans plays a role in cell division and is growth-phase regulated

Streptococcus mutans is the primary odontopathogen present in supragingival plaque and causes the oral disease known as dental caries. Colonization of the oral cavity by S. mutans requires the bacteria to adhere to the tooth surface and occurs by both sucrose-dependent and -independent mechanisms. Sucrose-independent adhesion of S. mutans in vitro has been shown to involve an ORF (ORF0317) encoding a homologue (39 %) to LytR, a regulator of autolysin activity in Bacillus subtilis. The protein encoded by ORF0317, LytR, belongs to the LytR/CpsA/Psr protein family. This family has a putative role in cell-wall structural maintenance, possibly through autolysin regulation. Autolysins have also been shown to be important in surface adhesion in Lactococcus lactis and in the pathogenic properties of Streptococcus pneumoniae. To investigate the role of autolysins in the adhesion and pathogenesis of S. mutans, a LytR mutant was constructed. The mutant grows in long chains, which may indicate a defect in cell division. Further experiments with the mutant strain show increased autolytic activity, indicating that LytR attenuates S. mutans autolytic activity, possibly through regulation of the expression of autolytic enzymes. No defect in cell-to-surface adherence or biofilm growth was seen in the LytR mutant. However, a connection between cell growth phase and transcription of lytR was found.

Genetic analysis of a unique bacteriocin, Smb, produced by Streptococcus mutans GS5

A dipeptide lantibiotic, named Smb, in Streptococcus mutans GS5 was characterized by molecular genetic approaches. The Smb biosynthesis gene locus is encoded by a 9.5-kb region of chromosomal DNA and consists of seven genes in the order smbM1, -T, -F, -M2, -G, -A, -B. This operon is not present in some other strains of S. mutans, including strain UA159. The genes encoding Smb were identified as smbA and smbB. Inactivation of smbM1, smbA, or smbB attenuated the inhibition of the growth of the indicator strain RP66, confirming an essential role for these genes in Smb expression. Mature Smb likely consists of the 30-amino-acid SmbA together with the 32-amino-acid SmbB. SmbA exhibited similarity with the mature lantibiotic lacticinA2 from Lactococcus lactis, while SmbB was similar to the mersacidin-like peptides from Bacillus halodurans and L. lactis. We also demonstrated that Smb expression is induced by the competence-stimulating peptide (CSP) and that a com box-like sequence is located in the smb promoter region. These results suggest that Smb belongs to the class I bacteriocin family, and its expression is dependent on CSP-induced quorum sensing.

A unique nine-gene comY operon in Streptococcus mutans

Many Gram-positive and Gram-negative bacteria possess natural competence mechanisms for DNA capture and internalization. In Bacillus subtilis, natural competence is absolutely dependent upon the presence of a seven-gene operon known as the comG operon (comGA-G). In species of Streptococcus, this function has been described for a four-gene operon (comYA-D in Streptococcus gordonii and cglA-D in Streptococcus pneumoniae). In this study, a nine-orf operon (named comYA-I) required for natural competence in Streptococcus mutans was identified and characterized. Orf analysis of this operon indicates that the first four Orfs (ComYA-D) share strong homology with ComYA-D of S. gordonii and CglA-D of S. pneumoniae, the fifth to seventh Orfs (ComYE-G) match conserved hypothetical proteins from various species of Streptococcus with ComYF possessing a predicted ComGF domain, the eighth Orf (ComYH) shows a strong homology to numerous DNA methyltransferases from restriction/modification systems, and the ninth Orf (ComYI) is homologous to acetate kinase (AckA). RT-PCR analysis of the orf junctions confirmed that all nine orfs were present in a single transcript, while real-time RT-PCR analysis demonstrated that these orfs were expressed at a level very similar to that of the first orf in the operon. Mutations were constructed in all nine putative orfs. The first seven genes (comYA-G) were found to be essential for natural competence, while comYH and comYI had reduced and normal natural competence ability, respectively. Analyses of S. mutans comY-luciferase reporter fusions indicated that comY expression is growth-phase dependent, with maximal expression at an OD(600) of about 0.2, while mutations in ciaH, comC and luxS reduced the level of comY expression. In addition, comY operon expression appears to be correlated with natural competence ability.

Interactions between oral bacteria: inhibition of Streptococcus mutans bacteriocin production by Streptococcus gordonii

Streptococcus mutans has been recognized as an important etiological agent in human dental caries. Some strains of S. mutans also produce bacteriocins. In this study, we sought to demonstrate that bacteriocin production by S. mutans strains GS5 and BM71 was mediated by quorum sensing, which is dependent on a competence-stimulating peptide (CSP) signaling system encoded by the com genes. We also demonstrated that interactions with some other oral streptococci interfered with S. mutans bacteriocin production both in broth and in biofilms. The inhibition of S. mutans bacteriocin production by oral bacteria was stronger in biofilms than in broth. Using transposon Tn916 mutagenesis, we identified a gene (sgc; named for Streptococcus gordonii challisin) responsible for the inhibition of S. mutans bacteriocin production by S. gordonii Challis. Interruption of the sgc gene in S. gordonii Challis resulted in attenuated inhibition of S. mutans bacteriocin production. The supernatant fluids from the sgc mutant did not inactivate the exogenous S. mutans CSP as did those from the parent strain Challis. S. gordonii Challis did not inactivate bacteriocin produced by S. mutans GS5. Because S. mutans uses quorum sensing to regulate virulence, strategies designed to interfere with these signaling systems may have broad applicability for biological control of this caries-causing organism.

Can laboratory reference strains mirror ‘real-world’ pathogenesis?

The extraordinary plasticity of bacterial genomes raises concerns about the adequacy of laboratory-adapted reference strains for the study of "real-world" pathogenesis. Some laboratory strains have been sub-cultured for decades since their first isolation and might have lost important pathophysiological characteristics. Evidence is presented that bacteria rapidly adapt to in vitro conditions. Genomic differences between laboratory reference strains and corresponding low-passage clinical isolates are reviewed. It appears that no bacterial strain can truly represent its species. For DNA microarray and proteomic studies, this limitation might be overcome by the summation of individual genomes to produce a species-specific virtual supragenome.

Role of a nosX homolog in Streptococcus gordonii in aerobic growth and biofilm formation

Oral streptococci such as Streptococcus gordonii are facultative anaerobes that initiate biofilm formation on tooth surfaces. An isolated S. gordonii::Tn917-lac biofilm-defective mutant contained a transposon insertion in an open reading frame (ORF) encoding a homolog of NosX of Ralstonia eutropha, a putative maturation factor of nitrous oxide reductase. Located downstream are two genes, qor1 and qor2, predicted to encode two putative NADPH quinone oxidoreductases. These three genes are cotranscribed, forming a putative oxidative stress response (osr) operon in S. gordonii. Inactivation of nosX, qor1, or qor2 resulted in biofilm-defective phenotypes. Expression of nosX, measured by the beta-galactosidase activity of the nosX::Tn917-lac mutant, was growth-phase dependent and enhanced when grown under aerobic conditions or in the presence of paraquat. Real-time reverse transcription-PCR revealed that nosX-specific mRNA levels were increased approximately 8.4 and 3.5 fold in biofilm-derived cells grown on plastic and glass, respectively, when compared to planktonic cells. Expression of nosX increased 19.9 fold in cells grown under aerated aerobic conditions and 4.7 fold in cells grown under static aerobic conditions. Two ORFs immediately adjacent to the osr operon encode a putative NADH oxidase (Nox) and a putative thiol-specific antioxidant enzyme (AhpC, for alkyl hydroperoxide peroxidase C). Expression of nox and ahpC was also significantly increased in cells grown under aerated and static aerobic conditions when compared to anaerobic conditions. In addition, nox expression was increased in biofilm cells compared to planktonic cells. These genes may be part of an island that deals with oxidoreductive response, some of which may be important in S. gordonii biofilm formation.