Genetic transformation of Streptococcus mutans

Functional analysis of glucan binding protein B from Streptococcus mutans

Mutans streptococci are major etiological agents of dental caries, and several of their secreted products contribute to bacterial accumulation on teeth. Of these, Streptococcus mutans glucan binding protein B (GbpB) is a novel, immunologically dominant protein. Its biological function is unclear, although GbpB shares homology with a putative peptidoglycan hydrolase from S. agalactiae and S. pneumoniae, indicative of a role in murein biosynthesis. To determine the cellular function of GbpB, we used several approaches to inactivate the gene, analyze its expression, and identify interacting proteins. None of the transformants analyzed were true gbpB mutants, since they all contained both disrupted and wild-type gene copies, and expression of functional GbpB was always conserved. Thus, the inability to obtain viable gbpB null mutants supports the notion that gbpB is an essential gene. Northern blot and real-time PCR analyses suggested that induction of gbpB expression in response to stress was a strain-dependent phenomenon. Proteins that interacted with GbpB were identified in pull-down and coimmunoprecipitation assays, and these data suggest that GbpB interacts with ribosomal protein L7/L12, possibly as part of a protein complex involved in peptidoglycan synthesis and cell division.

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.

Regulation and physiologic significance of the agmatine deiminase system of Streptococcus mutans UA159

We previously demonstrated that Streptococcus mutans expresses a functional agmatine deiminase system (AgDS) encoded by the agmatine-inducible aguBDAC operon (A. R. Griswold, Y. Y. Chen, and R. A. Burne, J. Bacteriol. 186:1902-1904, 2004). The AgDS yields ammonia, CO2, and ATP while converting agmatine to putrescine and is proposed to augment the acid resistance properties and pathogenic potential of S. mutans. To initiate a study of agu gene regulation, the aguB transcription initiation site was identified by primer extension and a putative sigma70-like promoter was mapped 5' to aguB. Analysis of the genome database revealed an open reading frame (SMU.261c) encoding a putative transcriptional regulator located 239 bases upstream of aguB. Inactivation of SMU.261c decreased AgD activity by sevenfold and eliminated agmatine induction. AgD was also found to be induced by certain environmental stresses, including low pH and heat, implying that the AgDS may also be a part of a general stress response pathway of this organism. Interestingly, an AgDS-deficient strain was unable to grow in the presence of 20 mM agmatine, suggesting that the AgDS converts a growth-inhibitory substance into products that can enhance acid tolerance and contribute to the competitive fitness of the organism at low pH. The capacity to detoxify and catabolize agmatine is likely to have major ramifications on oral biofilm ecology.

Streptococcal viability and diminished stress tolerance in mutants lacking the signal recognition particle pathway or YidC2

The signal recognition particle (SRP)-translocation pathway is conserved in all three domains of life and delivers membrane and secretory proteins to the cytoplasmic membrane or endoplasmic reticulum. We determined the requirement in the cariogenic oral pathogen Streptocococcus mutans of the three universally conserved elements of the SRP pathway: Ffh/SRP54, scRNA, and FtsY/SRalpha. Previously, we reported that insertional interruption of S. mutans ffh was not lethal, but resulted in acid sensitivity. To test whether S. mutans could survive extensive disruption of the SRP pathway, single and double deletions of genes encoding Ffh, scRNA, and FtsY were generated. Without environmental stressors, all mutant strains were viable, but unlike the wild-type, none could initiate growth at pH 5.0 or in 3.5% NaCl. Survival of challenge with 0.3 mM H(2)O(2) was also diminished without ffh. Members of the YidC/Oxa1/Alb3 family are also ubiquitous, involved in the translocation and assembly of membrane proteins, and have been identified in prokaryotes/mitochondria/chloroplasts. Two genes encoding YidC homologs, YidC1 and YidC2, are present in streptococcal genomes with both expressed in S. mutans. Deletion of YidC1 demonstrated no obvious phenotype. Elimination of YidC2 resulted in a stress-sensitive phenotype similar to SRP pathway mutants. Mutants lacking both YidC2 and SRP components were severely impaired and barely able to grow, even in the absence of environmental stress. Here, we report the dispensability of the cotranslational SRP protein translocation system in a bacterium. In S. mutans, this pathway contributes to protection against rapid environmental challenge and may overlap functionally with YidC2.

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.

A hypothetical protein of Streptococcus mutans is critical for biofilm formation

Inactivation of the Smu0630 gene of Streptococcus mutans resulted in dramatic decreases in biofilm formation, regardless of the carbohydrate source. The Smu0630 protein contained numerous interesting features, including a possible signal sequence and two conserved regions of repeated sequences. Smu0630 may represent a potential target for novel therapeutics.

Vectors that facilitate the replacement of transcriptionallacZfusions inStreptococcus mutansandBacillus subtiliswith fusions togfporgusA

Plasmid vectors have been constructed for Streptococcus mutans and Bacillus subtilis that make possible rapid replacement of the widely used reporter gene lacZ (encoding beta-galactosidase) with either gfp (encoding green fluorescent protein) or gusA (encoding beta-glucuronidase). The lacZ-->gfp replacement vectors greatly facilitate the analysis of the spatial location of gene expression in biofilms of S. mutans and in sporulating B. subtilis. The lacZ-->gusA replacement vectors facilitate the comparison of two promoters within the same organism. A vector is also described that enables gusA to be replaced with gfp in B. subtilis.

Smx nuclease is the major, low-pH-inducible apurinic/apyrimidinic endonuclease in Streptococcus mutans

The causative agent of dental caries in humans, Streptococcus mutans, outcompetes other bacterial species in the oral cavity and causes disease by surviving acidic conditions in dental plaque. We have previously reported that the low-pH survival strategy of S. mutans includes the ability to induce a DNA repair system that appears to involve an enzyme with exonuclease functions (K. Hahn, R. C. Faustoferri, and R. G. Quivey, Jr., Mol. Microbiol 31:1489-1498, 1999). Here, we report overexpression of the S. mutans apurinic/apyrimidinic (AP) endonuclease, Smx, in Escherichia coli; initial characterization of its enzymatic activity; and analysis of an smx mutant strain of S. mutans. Insertional inactivation of the smx gene eliminates the low-pH-inducible exonuclease activity previously reported. In addition, loss of Smx activity renders the mutant strain sensitive to hydrogen peroxide treatment but relatively unaffected by acid-mediated damage or near-UV irradiation. The smx strain of S. mutans was highly sensitive to the combination of iron and hydrogen peroxide, indicating the likely production of hydroxyl radical by Fenton chemistry with concomitant formation of AP sites that are normally processed by the wild-type allele. Smx activity was sufficiently expressed in E. coli to protect an xth mutant strain from the effects of hydrogen peroxide treatment. The data indicate that S. mutans expresses an inducible, class II-like AP endonuclease, encoded by the smx gene, that exhibits exonucleolytic activity and is regulated as part of the acid-adaptive response of the organism. Smx is likely the primary, if not the sole, AP endonuclease induced during growth at low pH values.

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.

Effects of RelA on key virulence properties of planktonic and biofilm populations of Streptococcus mutans

Streptococcus mutans is a biofilm-forming bacterium that is adapted to tolerate rapid and dramatic fluctuations in nutrient availability, carbohydrate source, and pH in its natural environment, the human oral cavity. Dissecting the pathways used to form stable biofilms and to tolerate environmental stress is central to understanding the virulence of this organism. Here, we investigated the role of the S. mutans relA gene, which codes for a guanosine tetraphosphate and guanosine pentaphosphate [(p)ppGpp] synthetase/hydrolase, in biofilm formation and acid tolerance. Two mutants in which relA was insertionally inactivated or replaced by an antibiotic resistance determinant were constructed. Under normal growth and stress conditions, the mutants grew slower than the wild-type strain, although the final yields were similar. The mutants, which were still able to accumulate (p)ppGpp after the induction of a stringent response, showed significant reductions in biofilm formation on microtiter plates or hydroxylapatite disks. There was no difference in the sensitivities to acid killing of the parent and relA strains grown in planktonic cultures. However, when cells were grown in biofilms, the mutants became more acid resistant and could lower the pH through glycolysis faster and to a greater extent than the wild-type strain. Differences in acid resistance were not correlated with increases in F-ATPase activity, although bacterial sugar:phosphotransferase activity was elevated in the mutants. Expression of the luxS gene was increased as much as fivefold in the relA mutants, suggesting a link between AI-2 quorum sensing and the stringent response.

Roles of Streptococcus mutans dextranase anchored to the cell wall by sortase

In order to clarify the role that sortase (SrtA) plays in anchoring dextranase (Dex) to the cell wall of Streptococcus mutans, both Dex- and SrtA- mutants were constructed by insertional inactivation of the respective genes. Western blot analysis with a Dex antiserum showed that in the srtA mutant the Dex was not bound to the cell wall but was secreted into the culture supernatant. In contrast, in the wild type, Dex remained cell-wall-associated. Biological properties of the srtA mutant were examined in dextran fermentation, colony morphology and adherence to a smooth surface. The srtA mutant, as well as the wild type, retained the ability to ferment dextran. However, the colony morphology of the srtA mutant on Todd Hewitt agar containing sucrose was much larger than that of the wild type and showed a ring-like structure. In addition, the srtA mutant was more adhesive to a smooth surface than the wild type when sucrose was present. However, the adhesion of the srtA mutant remarkably decreased by addition of exogenous dextranase. These studies suggest that the SrtA mediates Dex-anchoring to the cell wall in S. mutans, and cell wall-anchored Dex plays a role in controlling both the adhesive properties of extracellular glucan and the ability to utilize extracellular glucan as a nutrient source. In contrast, extracellular Dex is only responsible for degrading extracellular glucan as a nutrient source.

Inactivation of srtA gene of Streptococcus mutans inhibits dextran-dependent aggregation by glucan-binding protein C

A sortase-deficient mutant of Streptococcus mutans was prepared by insertional inactivation of a sortase gene (srtA). The srtA mutant was defective in cell wall-anchoring of two surface proteins 200 and 75 kDa in size. A previous study has shown that the 200 kDa protein is a surface protein antigen PAc and that the sortase catalyzes cell wall-anchoring of PAc in S. mutans. In this study another surface protein 75 kDa in size was examined by immunologic and physiologic methods. Western blot analysis with a specific antiserum showed that the 75 kDa protein was a surface protein, glucan-binding protein C. The protein was overexpressed under a stress condition including a sublethal concentration of tetracycline. The srtA mutant cells also lost the ability of dextran-dependent aggregation. These results suggest that the S. mutans sortase mediates cell wall-anchoring of the glucan-binding protein C and dextran-dependent aggregation of this organism.

Characterization of a suppressor mutation complementing an acid-sensitive mutation inStreptococcus mutans

We isolated a spontaneous suppressor mutant complementing the acid-sensitive phenotype of Streptococcus mutans strain Tn-1, a mutant previously generated in this laboratory, defective in the activity of the dgk-encoded putative undecaprenol kinase. A relatively simple genetic method was developed to identify the suppressor mutation, based on selection for transformants containing two closely linked markers: a selectable allele of the unknown suppressor gene and an antibiotic resistance gene introduced on a suicide plasmid at random sites into the chromosome via homologous recombination. While we have not actually identified the original suppressor mutation, another mutated gene restoring acid resistance has been isolated, which suggests a possible mechanism of suppression.

Molecular genetic analysis of the virulence of oral bacterial pathogens: an historical perspective

This review will focus on the impact of molecular genetic approaches on elucidating the bacterial etiology of oral diseases from an historical perspective. Relevant results from the pre- and post-recombinant DNA periods will be highlighted, including the roles of gene cloning, mutagenesis, and nucleotide sequencing in this area of research. Finally, the impact of whole-genome sequencing on deciphering the virulence mechanisms of oral pathogens, along with new approaches to control these organisms, will be discussed.

The sortase of Streptococcus mutans mediates cell wall anchoring of a surface protein antigen

Sortase has been shown to be a protease that catalyzes the cell wall anchoring of surface proteins containing an LPXTG motif in gram-positive bacteria. In this study, we determined the complete nucleotide sequence of the sortase gene (srtA) of Streptococcus mutans and found a surface protein that was linked to the cell wall by the sortase. The results show that srtA gene of S. mutans consisted of 741 bp and encoded for a sortase protein of 246 amino acids with a molecular weight of 27 489. The deduced amino acid sequence of the S. mutans sortase was highly homologous (65-58%) to those of other Streptococcal species. In a S. mutans mutant lacking sortase, two surface proteins of 200 and 75 kDa were released to the culture supernatant. Western blot analysis with specific antiserum showed that the 200 kDa protein was a surface protein antigen designated PAc. These results suggest that the sortase catalyzes anchoring of the antigen PAc to the cell wall.

Characterization of Streptococcus mutans strains deficient in EIIAB Man of the sugar phosphotransferase system

The phosphoenolpyruvate:sugar phosphotransferase system (PTS) is the major sugar uptake system in oral streptococci. The role of EIIAB(Man) (encoded by manL) in gene regulation and sugar transport was investigated in Streptococcus mutans UA159. The manL knockout strain, JAM1, grew more slowly than the wild-type strain in glucose but grew faster in mannose and did not display diauxic growth, indicating that EIIAB(Man) is involved in sugar uptake and in carbohydrate catabolite repression. PTS assays of JAM1, and of strains lacking the inducible (fruI) and constitutive (fruCD) EII fructose, revealed that S. mutans EIIAB(Man) transported mannose and glucose and provided evidence that there was also a mannose-inducible or glucose-repressible mannose PTS. Additionally, there appears to be a fructose PTS that is different than FruI and FruCD. To determine whether EIIAB(Man) controlled expression of the known virulence genes, glucosyltransferases (gtfBC) and fructosyltransferase (ftf) promoter fusions of these genes were established in the wild-type and EIIAB(Man)-deficient strains. In the manL mutant, the level of chloramphenicol acetyltransferase activity expressed from the gtfBC promoter was up to threefold lower than that seen with the wild-type strain at pH 6 and 7, indicating that EIIAB(Man) is required for optimal expression of gtfBC. No significant differences were observed between the mutant and the wild-type background in ftf regulation, with the exception that under glucose-limiting conditions at pH 7, the mutant exhibited a 2.1-fold increase in ftf expression. Two-dimensional gel analysis of batch-grown cells of the EIIAB(Man)-deficient strain indicated that the expression of at least 38 proteins was altered compared to that seen with the wild-type strain, revealing that EIIAB(Man) has a pleiotropic effect on gene expression.

Regulation and Physiological Significance of ClpC and ClpP in Streptococcus mutans

Tolerance of environmental stress, especially low pH, by Streptococcus mutans is central to the virulence of this organism. The Clp ATPases are implicated in the tolerance of, and regulation of the response to, stresses by virtue of their protein reactivation and remodeling activities and their capacity to target misfolded proteins for degradation by the ClpP peptidase. The purpose of this study was to dissect the role of selected clp genes in the stress responses of S. mutans, with a particular focus on acid tolerance and adaptation. Homologues of the clpB, clpC, clpE, clpL, clpX, and clpP genes were identified in the S. mutans genome. The expression of clpC and clpP, which were chosen as the focus of this study, was induced at low pH and at growth above 40 degrees C. Inactivation of ctsR, the first of two genes in the clpC operon, demonstrated that CtsR acts as a repressor of clp and groES-EL gene expression. Strains lacking ClpP, but not strains lacking ClpC, were impaired in their ability to grow under stress-inducing conditions, formed long chains, aggregated in culture, had reduced genetic transformation efficiencies, and had a reduced capacity to form biofilms. Comparison of two-dimensional protein gels from wild-type cells and the ctsR and clpP mutants revealed many changes in the protein expression patterns. In particular, in the clpP mutant, there was an increased production of GroESL and DnaK, suggesting that cells were stressed, probably due to the accumulation of denatured proteins.

Functional variation of the antigen I/II surface protein in Streptococcus mutans and Streptococcus intermedius

Although Streptococcus intermedius and Streptococcus mutans are regarded as members of the commensal microflora of the body, S. intermedius is often associated with deep-seated purulent infections, whereas S. mutans is frequently associated with dental caries. In this study, we investigated the roles of the S. mutans and S. intermedius antigen I/II proteins in adhesion and modulation of cell surface characteristics. By using isogenic mutants, we show that the antigen I/II in S. mutans, but not in S. intermedius, was involved in adhesion to a salivary film under flowing conditions, as well as in binding to rat collagen type I. Binding to human fibronectin was a common function associated with the S. mutans and S. intermedius antigen I/II. Adhesion of S. mutans or S. intermedius to human collagen types I or IV was negligible. Hydrophobicity, as measured by water contact angles, and zeta potentials were unaltered in the S. intermedius mutant. The S. mutans isogenic mutants, on the other hand, exhibited more positive zeta potentials at physiological pH values than did the wild type. The results indicate common and species-specific roles for the antigen I/II in mediating the attachment of S. mutans and S. intermedius to host components and in determining cell surface properties.

A 60-kilodalton immunodominant glycoprotein is essential for cell wall integrity and the maintenance of cell shape in Streptococcus mutans

We have demonstrated previously by Western blotting that in naturally sensitized humans, the serum or salivary antibody response to Streptococcus mutans was directed predominantly to a protein antigen with a size of approximately 60-kDa. To identify this immunodominant antigen, specific serum antibodies were eluted from immunoblots and five positive clones with inserts ranging in length from 3 to 8 kb from identical chromosomal loci were obtained by screening a genomic expression library of Streptococcus mutans GS-5. Amino acid sequencing established the identity of this immunodominant antigen, a 60-kDa immunodominant glycoprotein (IDG-60), to be a cell wall-associated general stress protein GSP-781, which was originally predicted to have a molecular mass of approximately 45 kDa based on the derived nucleotide sequence. Discrepancy in the molecular mass was also observed in recombinant his-tagged IDG-60 (rIDG-60) expressed from Escherichia coli. Glycosylation, consisting of sialic acid, mannose galactose, and N-acetylgalactosamine, was detected by lectin binding to IDG-60 in cell wall extracts from S. mutans and rIDG-60 expressed in vivo or translated in vitro. Despite the presence of multiple Asn or Ser or Thr glycosylation sites, IDG-60 was resistant to the effect of N-glycosidase F and multiple O-glycosidase molecules but not to beta-galactosidase. Insertional inactivation of the gene encoding IDG-60, sagA, resulted in a retarded growth rate, destabilization of the cell wall, and pleiomorphic cell shape with multifold ingrowth of cell wall. In addition, distinct from the parental GS-5 strain, the isogenic mutant GS-51 was unable to survive the challenge of low pH and high osmotic pressure or high temperature. Expression of the wild-type gene in trans within GS-51 from plasmid pDL277 complemented the growth defect and restored normal cell shape. These results suggested that IDG-60 is essential for maintaining the integrity of the cell wall and the uniformity of cell shape, both of which are indispensable for bacteria survival under stress conditions.

Genetic competence and transformation in oral streptococci

The oral streptococci are normally non-pathogenic residents of the human microflora. There is substantial evidence that these bacteria can, however, act as "genetic reservoirs" and transfer genetic information to transient bacteria as they make their way through the mouth, the principal entry point for a wide variety of bacteria. Examples that are of particular concern include the transfer of antibiotic resistance from oral streptococci to Streptococcus pneumoniae. The mechanisms that are used by oral streptococci to exchange genetic information are not well-understood, although several species are known to enter a physiological state of genetic competence. This state permits them to become capable of natural genetic transformation, facilitating the acquisition of foreign DNA from the external environment. The oral streptococci share many similarities with two closely related Gram-positive bacteria, S. pneumoniae and Bacillus subtilis. In these bacteria, the mechanisms of quorum-sensing, the development of competence, and DNA uptake and integration are well-characterized. Using this knowledge and the data available in genome databases allowed us to identify putative genes involved in these processes in the oral organism Streptococcus mutans. Models of competence development and genetic transformation in the oral streptococci and strategies to confirm these models are discussed. Future studies of competence in oral biofilms, the natural environment of oral streptococci, will be discussed.

Genetic and physiologic analysis of the groE operon and role of the HrcA repressor in stress gene regulation and acid tolerance in Streptococcus mutans

Our working hypothesis is that the major molecular chaperones DnaK and GroE play central roles in the ability of oral bacteria to cope with the rapid and frequent stresses encountered in oral biofilms, such as acidification and nutrient limitation. Previously, our laboratory partially characterized the dnaK operon of Streptococcus mutans (hrcA-grpE-dnaK) and demonstrated that dnaK is up-regulated in response to acid shock and sustained acidification (G. C. Jayaraman, J. E. Penders, and R. A. Burne, Mol. Microbiol. 25:329-341, 1997). Here, we show that the groESL genes of S. mutans constitute an operon that is expressed from a stress-inducible sigma(A)-type promoter located immediately upstream of a CIRCE element. GroEL protein and mRNA levels were elevated in cells exposed to a variety of stresses, including acid shock. A nonpolar insertion into hrcA was created and used to demonstrate that HrcA negatively regulates the expression of the groEL and dnaK operons. The SM11 mutant, which had constitutively high levels of GroESL and roughly 50% of the DnaK protein found in the wild-type strain, was more sensitive to acid killing and could not lower the pH as effectively as the parent. The acid-sensitive phenotype of SM11 was, at least in part, attributable to lower F(1)F(0)-ATPase activity. A minimum of 10 proteins, in addition to GroES-EL, were found to be up-regulated in SM11. The data clearly indicate that HrcA plays a key role in the regulation of chaperone expression in S. mutans and that changes in the levels of the chaperones profoundly influence acid tolerance.

Characterization of the sat operon in Streptococcus mutans: evidence for a role of Ffh in acid tolerance

An essential protein translocation pathway in Escherichia coli and Bacillus subtilis involves the signal recognition particle (SRP), of which the 54-kDa homolog (Ffh) is an essential component. In a previous study, we found that a transposon insertion in the ylxM-ffh intergenic region of the designated secretion and acid tolerance (sat) operon of Streptococcus mutans resulted in an acid-sensitive phenotype. In the present study, we further characterized this genomic region in S. mutans after construction of bona fide sat operon mutants and confirmed the role of the SRP pathway in acid resistance. Northern blot and primer extension analyses identified an acid-inducible promoter upstream of ylxM that was responsible for upregulating the coordinate expression of all five genes of the sat operon when cells were grown at acid pH. Two constitutive promoters, one immediately upstream of satD and one just 3' to the acid-inducible promoter, were also identified. Except for Ffh, the functions of the sat operon gene products are unknown. SatC, SatD, and SatE have no homology to proteins with known functions, although YlxM may function as a transcriptional regulator linked to genes encoding SRP pathway proteins. Nonpolar mutations created in each of the five genes of the sat locus resulted in viable mutants. Most striking, however, was the finding that a mutation in ffh did not result in loss of cell viability, as is the case in all other microbial species in which this pathway has been described. This mutant also lacked immunologically detectable Ffh and was severely affected in resistance to acid. Complementation of the mutation resulted in restoration of acid tolerance and reappearance of cytoplasmic Ffh. These data provide evidence that the SRP pathway plays an important role in acid tolerance in S. mutans.

Identification of stress-responsive genes in Streptococcus mutans by differential display reverse transcription-PCR

Streptococcus mutans, which causes dental caries in the human oral cavity and occasionally causes infective endocarditis in the heart, withstands adverse environmental stress through diverse alterations in protein synthesis. Differential gene expression in response to environmental stress was analyzed by RNA fingerprinting using arbitrarily primed PCR with a panel of 11mer primers designed for differential display in Enterobacteriaceae. Dot and Northern blot hybridization confirmed that the transcription of several genes was up- or down-regulated following exposure to acid shock from pH 7.5 to 5.5. RNA of a gene designated AP-185 (acid-stress protein) was induced specifically by acid treatment, while RNA of GSP-781 (general-stress protein) was up-regulated significantly when bacteria were exposed to high osmolarity and temperature, as well as low pH. The deduced amino acid sequence of AP-185 shares homology (78% identity) with branched-chain amino acid aminotransferase. Cloning and sequence analysis of GSP-781 revealed a potential secreted protein of a molecular mass of about 43 kDa and with a pI predicted to be 5.5. Transcriptional levels of another gene, designated AR-186 (acid-repressed protein), which encodes putative aconitase, were repressed by acid treatment but were enhanced by plasma or serum components. Analogous results were identified in icd and citZ genes, and repression of these genes, along with AR-186, was also observed when they were exposed to high osmolarity and temperature. These results indicate that differential regulation of specific genes at the transcriptional level is triggered by different stress and that genes responsible for glutamate biosynthesis in the citrate pathway are coordinately regulated during the stress response of S. mutans.

Oral microbial communities: biofilms, interactions, and genetic systems

Oral microbial-plaque communities are biofilms composed of numerous genetically distinct types of bacteria that live in close juxtaposition on host surfaces. These bacteria communicate through physical interactions called coaggregation and coadhesion, as well as other physiological and metabolic interactions. Streptococci and actinomyces are the major initial colonizers of the tooth surface, and the interactions between them and their substrata help establish the early biofilm community. Fusobacteria play a central role as physical bridges that mediate coaggregation of cells and as physiological bridges that promote anaerobic microenvironments which protect coaggregating strict anaerobes in an aerobic atmosphere. New technologies for investigating bacterial populations with 16S rDNA probes have uncovered previously uncultured bacteria and have offered an approach to in situ examination of the spatial arrangement of the participant cells in oral-plaque biofilms. Flow cells with saliva-coated surfaces are particularly useful for studies of biofilm formation and observation. The predicted sequential nature of colonization of the tooth surface by members of different genera can be investigated by using these new technologies and imaging the cells in situ with confocal scanning laser microscopy. Members of at least seven genera now can be subjected to genetic studies owing to the discovery of gene-transfer systems in these genera. Identification of contact-inducible genes in streptococci offers an avenue to explore bacterial responses to their environment and leads the way toward understanding communication among inhabitants of a multispecies biofilm.

Natural genetic transformation of Streptococcus mutans growing in biofilms

Streptococcus mutans is a bacterium that has evolved to be dependent upon a biofilm "lifestyle" for survival and persistence in its natural ecosystem, dental plaque. We initiated this study to identify the genes involved in the development of genetic competence in S. mutans and to assay the natural genetic transformability of biofilm-grown cells. Using genomic analyses, we identified a quorum-sensing peptide pheromone signaling system similar to those previously found in other streptococci. The genetic locus of this system comprises three genes, comC, comD, and comE, that encode a precursor to the peptide competence factor, a histidine kinase, and a response regulator, respectively. We deduced the sequence of comC and its active pheromone product and chemically synthesized the corresponding 21-amino-acid competence-stimulating peptide (CSP). Addition of CSP to noncompetent cells facilitated increased transformation frequencies, with typically 1% of the total cell population transformed. To further confirm the roles of these genes in genetic competence, we inactivated them by insertion-duplication mutagenesis or allelic replacement followed by assays of transformation efficiency. We also demonstrated that biofilm-grown S. mutans cells were transformed at a rate 10- to 600-fold higher than planktonic S. mutans cells. Donor DNA included a suicide plasmid, S. mutans chromosomal DNA harboring a heterologous erythromycin resistance gene, and a replicative plasmid. The cells were optimally transformed during the formation of 8- to 16-h-old biofilms primarily consisting of microcolonies on solid surfaces. We also found that dead cells in the biofilms could act as donors of a chromosomally encoded antibiotic resistance determinant. This work demonstrated that a peptide pheromone system controls genetic competence in S. mutans and that the system functions optimally when the cells are living in actively growing biofilms.

Genetic transformation in Streptococcus mutans requires a peptide secretion-like apparatus

Competence for genetic transformation in Streptococcus pneumoniae and Streptococcus gordonii involves the ComAB secretion apparatus, which is thought to export the competence-stimulating peptide. Homologous secretory systems are also used for the export of certain bacteriocins and bacteriocin-like peptides. In this study, a similar secretory apparatus was found in the Streptococcus mutans genome, and its role in transformation was investigated. Gene inactivation resulted in a mutant deficient in transformability. We suggest that secretion of a peptide, possibly the competence-stimulating peptide itself, is involved in competence induction also in S. mutans.

Construction of region-specific partial duplication mutants (merodiploid mutants) to identify the regulatory gene for the glucan-binding protein C gene in vivo in Streptococcus mutans

The gbpC gene encoding the glucan-binding protein C which is involved in dextran (glucan)-dependent aggregation (ddag) of Streptococcus mutans has been identified by random mutagenesis. We analyzed ddag(-) mutants containing the intact gbpC gene and found that these mutants possessed a large and characteristic duplication of a region of the chromosome which was responsible for the phenotype. Based upon characterization of these duplications, we developed a strategy to introduce a duplication into any specific region of the chromosome of these organisms. The 690-bp gene responsible for the ddag(-) phenotype was identified within a 60-kb region by observing ddag (positive or negative) phenotypes of successively constructed specific duplication mutants.

Identification of a new gene responsible for the oxygen tolerance in aerobic life of Streptococcus mutans

Alkyl hydroperoxide reductase in Streptococcus mutans consists of two components, Nox-1 and AhpC. Deletion of nox-1 and ahpC in a double mutant as well as the wild-type of Streptococcus mutans can form colonies in the presence of air to the same extent. The evidence suggested the presence of some other antioxidant system(s) independent of the Nox-1/AhpC system in the bacterium. Here we identified a new antioxidant gene (dpr) and the gene product (Dpr) which complements the defect of peroxidase activity caused by the deletion of nox-1 and ahpC in S. mutans. The dpr-disruption mutant of S. mutans could form colonies anaerobically but not aerobically.

Construction and characterization of an effector strain of Streptococcus mutans for replacement therapy of dental caries

An effector strain has been constructed for use in the replacement therapy of dental caries. Recombinant DNA methods were used to make the Streptococcus mutans supercolonizing strain, JH1140, lactate dehydrogenase deficient by deleting virtually all of the ldh open reading frame (ORF). To compensate for the resulting metabolic imbalance, a supplemental alcohol dehydrogenase activity was introduced by substituting the adhB ORF from Zymomonas mobilis in place of the deleted ldh ORF. The resulting clone, BCS3-L1, was found to produce no detectable lactic acid during growth on a variety of carbon sources, and it produced significantly less total acid due to its increased production of ethanol and acetoin. BCS3-L1 was significantly less cariogenic than JH1140 in both gnotobiotic- and conventional-rodent models. It colonized the teeth of conventional rats as well as JH1140 in both aggressive-displacement and preemptive-colonization models. No gross or microscopic abnormalities of major organs were associated with oral colonization of rats with BCS3-L1 for 6 months. Acid-producing revertants of BCS3-L1 were not observed in samples taken from infected animals (reversion frequency, <10(-3)) or by screening cultures grown in vitro, where no revertants were observed among 10(5) colonies examined on pH indicator medium. The reduced pathogenic potential of BCS3-L1, its strong colonization potential, and its genetic stability suggest that this strain is well suited to serve as an effector strain in the replacement therapy of dental caries in humans.

Functions of two types of NADH oxidases in energy metabolism and oxidative stress of Streptococcus mutans

We have previously identified two distinct NADH oxidases corresponding to H(2)O(2)-forming oxidase (Nox-1) and H(2)O-forming oxidase (Nox-2) induced in Streptococcus mutans. Sequence analyses indicated a strong similarity between Nox-1 and AhpF, the flavoprotein component of Salmonella typhimurium alkyl hydroperoxide reductase; an open reading frame upstream of nox-1 also showed homology to AhpC, the direct peroxide-reducing component of S. typhimurium alkyl hydroperoxide reductase. To determine their physiological functions in S. mutans, we constructed knockout mutants of Nox-1, Nox-2, and/or the AhpC homologue; we verified that Nox-2 plays an important role in energy metabolism through the regeneration of NAD(+) but Nox-1 contributes negligibly. The Nox-2 mutant exhibited greatly reduced aerobic growth on mannitol, whereas there was no significant effect of aerobiosis on the growth on mannitol of the other strains or growth on glucose of any of the strains. Although the Nox-2 mutants grew well on glucose aerobically, the end products of glucose fermentation by the Nox-2 mutant were substantially shifted to higher ratios of lactic acid to acetic acid compared with wild-type cells. The resistance to cumene hydroperoxide of Escherichia coli TA4315 (ahpCF-defective mutant) transformed with pAN119 containing both nox-1 and ahpC genes was not only restored but enhanced relative to that of E. coli K-12 (parent strain), indicating a clear function for Nox-1 as part of an alkyl hydroperoxide reductase system in vivo in combination with AhpC. Surprisingly, the Nox-1 and/or AhpC deficiency had no effect on the sensitivity of S. mutans to cumene hydroperoxide and H(2)O(2), implying that the existence of some other antioxidant system(s) independent of Nox-1 in S. mutans compensates for the deficiency.

Identification of a new regulator in Streptococcus pneumoniae linking quorum sensing to competence for genetic transformation

Competence for genetic transformation in Streptococcus pneumoniae is regulated by a quorum-sensing system encoded by two genetic loci, comCDE and comAB. Additional competence-specific operons, cilA, cilB, cilC, cilD, cilE, cinA-recA, coiA, and cfl, involved in the DNA uptake process and recombination, share an unusual consensus sequence at -10 and -25 in the promoter, which is absent from the promoters of comAB and comCDE. This pattern suggests that a factor regulating transcription of these transformation machinery genes but not involved with comCDE and comAB expression might be an alternative sigma factor. A search for such a global transcriptional regulator was begun by purifying pneumococcal RNA polymerase holoenzyme. In preparations from competent pneumococcal cultures a protein which seemed to be responsible for cilA transcription in vitro was identified. The corresponding gene was identified and found to be present in two copies, designated comX1 and comX2, located adjacent to two of the repeated rRNA operons. Expression of transformation machinery operons, such as cilA, cilD, cilE, and cfl, but not that of the quorum-sensing operons comAB and comCDE, was shown to depend on comX, while comX expression depended on ComE but not on ComX itself. We conclude that the factor is a competence-specific global transcription modulator which links quorum-sensing information transduced to ComE to competence and propose that it acts as an alternate sigma factor. We also report that comAB and comCDE are not sufficient for shutoff of competence-stimulating peptide-induced gene expression nor for the subsequent refractory period, suggesting that these phenomena depend on one or more ComX-dependent genes.

Physical and genetic map of Streptococcus mutans GS-5 and localization of five rRNA operons

The physical map of the 2.1 megabase chromosome of Streptococcus mutans GS-5 has been refined by including all ApaI and SmaI fragments of 5 kbp or greater, and by positioning the fragments generated by the endonuclease I-CeuI. Sixty-three new genetic loci have been added to the map, so that it now contains 90 loci. The new loci include those for 35 cloned streptococcal genes of established function and for 23 S. mutans genes of putative function. In addition, five rrn operons were identified and placed on the map of the chromosome. The presence of a SmaI site in each of the rrn operons allowed the direction of transcription of each operon to be deduced. The orientation of the rrn loci indicates that their transcription is directed away from a small region of the chromosome, identifying a possible region for the initiation of chromosome replication.

Virulence of a spaP mutant of Streptococcus mutans in a gnotobiotic rat model

Streptococcus mutans, the principal etiologic agent of dental caries in humans, possesses a variety of virulence traits that enable it to establish itself in the oral cavity and initiate disease. A 185-kDa cell surface-localized protein known variously as antigen I/II, antigen B, PAc, and P1 has been postulated to be a virulence factor in S. mutans. We showed previously that P1 expression is necessary for in vitro adherence of S. mutans to salivary agglutinin-coated hydroxyapatite as well as for fluid-phase aggregation. Since adherence of the organism is a necessary first step toward colonization of the tooth surface, we sought to determine what effect deletion of the gene for P1, spaP, has on the colonization and subsequent cariogenicity of this organism in vivo. Germ-free Fischer rats fed a diet containing 5% sucrose were infected with either S. mutans NG8 or an NG8-derived spaP mutant strain, PC3370, which had been constructed by allelic exchange mutagenesis. At 1-week intervals for 6 weeks after infection, total organisms recovered from mandibles were enumerated. At week 6, caries lesions also were scored. A significantly lower number of enamel and dentinal carious lesions was observed for the mutant-infected rats, although there was no difference between parent and mutant in the number of organisms recovered from teeth through 6 weeks postinfection. Coinfection of animals with both parent and mutant strains resulted in an increasing predominance of the mutant strain being recovered over time, suggesting that P1 is not a necessary prerequisite for colonization. These data do, however, suggest a role for P1 in the virulence of S. mutans, as reflected by a decrease in the cariogenicity of bacteria lacking this surface protein.

Action of agents on glucosyltransferases from Streptococcus mutans in solution and adsorbed to experimental pellicle

Glucosyltransferase (Gtf) activity mediates sucrose-dependent adherence of mutans streptococci to the tooth surface, is essential for the cariogenicity of these micro-organisms, and contributes significantly to the exopolysaccharide component of the dental-plaque matrix. Clearly, agents that inhibit Gtfs could have therapeutic benefit. Here the effects of agents that inhibit Gtfs in solution and adsorbed to a surface were explored. Various classes of chemical reagents were tested for their ability to inhibit the enzymes responsible for insoluble-glucan synthesis (GtfB), insoluble/soluble glucan synthesis (GtfC), and soluble-glucan (GtfD) from Streptococcus mutans. Standard inhibition assays were done with Gtf enzyme in solution or with Gtf adsorbed to parotid saliva-coated hydroxylapatite (surface phase). Reagents tested included the metallic cations Li+, Zn2+, Cu2+, Fe2+ and Fe3+; the oxidizing compounds hypochlorite, Rose Bengal, perborate, and sodium-meta-periodate; and a panel of sugars and sugar analogues including sorbitol, xylitol, 1',4',6' trideoxy-trichloro-galactosucrose (TGS), and 1-deoxynojirimycin (dNJ). In solution, Gtf activity was inhibited significantly, at the highest concentrations tested: by the metal ions Zn2+, Cu2+, Fe2+ and Fe3+ (approx. 40-80% inhibition); by Rose Bengal and hypochlorite (approx. 80-90% inhibition); and by TGS and dNJ (approx. 50-80%). However, surface-adsorbed Gtfs displayed increased resistance to inhibition by the same metal cations and oxidizing compounds that inhibited them in solution. In contrast, both TGS and dNJ possessed similar inhibition profiles for both surface-bound Gtf and enzyme in solution. These data indicate that the nature of the inhibitor is important, and also whether the Gtf enzyme is in solution or adsorbed to saliva-coated hydroxylapatite.

A medium-copy-number plasmid for insertional mutagenesis of Streptococcus mutans

We have constructed a plasmid useful for insertional mutagenesis in Streptococcus mutans. The molecule, pSU20Erm, is based on a derivative of pACYC184 known as pSU20. The plasmid described here is approximately 3.7 kb in size and has the following properties: it replicates in Escherichia coli, does not replicate in S. mutans, contains an erythromycin-resistance marker which can be selected in E. coli or the streptococci, contains a multiple cloning site with few restriction sites in the remainder of the molecule, and can be screened on X-Gal-containing medium for the presence of insertions into the multiple cloning site. We have used the plasmid to construct a library of S. mutans DNA in E. coli and show that the clones can be reintegrated into the S. mutans chromosome via homologous recombination, thereby interrupting native genes. The plasmid has been used to clone part of a homologue of the E. coli drpA gene, encoding a global regulatory element for RNA synthesis. Further, we have identified an element closely linked to drpA in S. mutans with high homology to IS861.

Genetic and biochemical analysis of mutacin 1140, a lantibiotic from Streptococcus mutans

Streptococcus mutans JH1000 and its derivatives were previously shown (J. D. Hillman, K. P. Johnson, and B. I. Yaphe, Infect. Immun. 44:141-144, 1984) to produce a low-molecular-weight, broad-spectrum bacteriocin-like inhibitory substance (BLIS). The thermosensitive vector pTV1-OK harboring Tn917 was used to isolate a BLIS-deficient mutant, DM25, and the mutated gene was recovered by shotgun cloning in Escherichia coli. Sequence analysis of insert DNA adjacent to Tn917 led to the identification of four open reading frames including two (lanA and lanB) which have substantial homology to the Staphylococcus epidermidis structural gene (epiA) and a modifying enzyme gene (epiB) for biosynthesis of the lantibiotic epidermin, respectively. Although the BLIS activity could not be recovered from broth cultures, high yields were obtained from a solid medium consisting of Todd-Hewitt broth containing 0.5% agarose that was stab inoculated with JH1140 (a spontaneous mutant of JH1000 that produces threefold-elevated amounts of activity). Agar could not substitute for agarose. Chloroform extraction of the spent medium produced a fraction which yielded two major bands on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The faster-migrating band was absent in chloroform extracts of the mutant, DM25. The amino acid sequence of this band was determined by Edman sequencing and mass spectroscopy. The results showed that it is a lantibiotic, which we have named mutacin 1140, and that the sequence corresponded to that deduced from the lanA sequence. We observed a number of similarities of mutacin 1140 to epidermin and an S. mutans lantibiotic, B-Ny266, but it appears to have significant differences in the positions of its thioether bridges. It also has other unique features with regard to its leader sequence and posttranslational modification. A proposed structure for mutacin 1140 is presented.

Natural competence in the genus Streptococcus: evidence that streptococci can change pherotype by interspecies recombinational exchanges

To map the incidence of natural competence in the genus Streptococcus, we used PCR to screen a number of streptococcal strains for the presence of the recently identified competence regulation operon, containing the comC, -D, and -E genes. This approach established that the operon is present in strains belonging to the S. mitis and S. anginosus groups, but it was not detected in the other strains examined. Competence is induced in S. pneumoniae and S. gordonii by strain-specific peptide pheromones, competence-stimulating peptides (CSPs). With its unique primary structure, each CSP represents a separate pheromone type (pherotype), which is recognized by the signalling domain of the downstream histidine kinase, ComD. Thus, all bacteria induced to competence by a particular CSP belong to the same pherotype. In this study, we identified a number of new pherotypes by sequencing the genes encoding the CSP and its receptor from different streptococcal species. We found that in several cases, these genes have a mosaic structure which must have arisen as the result of recombination between two distinct allelic variants. The observed mosaic blocks encompass the region encoding the CSP and the CSP-binding domain of the histidine kinase. Consequently, the recombination events have led to switches in pherotype for the strains involved. This suggests a novel mechanism for the adaptation of naturally competent streptococci to new environmental conditions.

Construction and characterization of a recombinant ureolytic Streptococcus mutans and its use to demonstrate the relationship of urease activity to pH modulating capacity

To begin to understand the contribution of oral microbial ureolysis to the inhibition of dental caries, we sought to construct a recombinant, ureolytic mutans streptococcus and correlate the ureolytic capacity of plaque bacteria with pH moderating ability. Streptococcus mutans GS-5 was transformed with a plasmid containing the urease genes from Streptococcus salivarius 57.I. The recombinant strain, S. mutans AC04, stably maintained the urease genes. High levels of urease activity were detected, with a maximum specific activity of 0.9 mumol of urea hydrolyzed/min/mg cell dry weight when the growth medium was supplemented with 50 microM exogenous NiCl2. Harboring the recombinant plasmid, or growth in NiCl2, did not markedly affect the glycolytic capacity of S. mutans. In vitro pH drop analysis of S. mutans AC04, metabolizing glucose and physiologically relevant concentrations of urea simultaneously, demonstrated that increasing the urease activity of plaque bacteria resulted in a corresponding reduction in the depth and the duration of the glycolytic pH fall. The results demonstrate the feasibility of engineering urease producing S. mutans and suggest that enhancing the ureolytic capacity of dental plaque, particularly cariogenic plaque, may help to offset the progression of the caries process.

Genetic and physiologic analysis of a formyl-tetrahydrofolate synthetase mutant of Streptococcus mutans

Previously we reported that transposon Tn917 mutagenesis of Streptococcus mutans JH1005 yielded an isolate detective in its normal ability to produce a mutacin (P. J. Crowley, J. D. Hillman, and A. S. Bleiweis, abstr. D55, p. 258 in Abstracts of the 95th General Meeting of the American Society for Microbiology 1995, 1995). In this report we describe the recovery of the mutated gene by shotgun cloning. Sequence analysis of insert DNA adjacent to Tn917 revealed homology to the gene encoding formyl-tetrahydrofolate synthetase (Fhs) from both prokaryotic and eukaryotic sources. In many bacteria, Fhs catalyzes the formation of 10-formyl-tetrahydrofolate, which is used directly in purine biosynthesis and formylation of Met-tRNA and indirectly in the biosynthesis of methionine, serine, glycine, and thymine. Analysis of the fhs mutant grown anaerobically in a minimal medium demonstrated that the mutant had an absolute dependency only for adenine, although addition of methionine was necessary for normal growth. Coincidently it was discovered that the mutant was sensitive to acidic pH; it grew more slowly than the parent strain on complex medium at pH 5. Complementation of the mutant with an integration vector harboring a copy of fhs restored its ability to grow in minimal medium and at acidic pH as well as to produce mutacin. This represents the first characterization of Fhs in Streptococcus.

Genetic and physiological analysis of the lethal effect of L-(+)-lactate dehydrogenase deficiency in Streptococcus mutans: complementation by alcohol dehydrogenase from Zymomonas mobilis

CH4ts is a previously isolated recombinant mutant of Streptococcus mutans NG8 which produces a thermolabile L-(+)-lactate dehydrogenase (LDH) activity. It does not grow at 42 degrees C under a variety of cultivation conditions. In this study, we show that a batch culture of CH4ts shifted from 30 to 42 degrees C underwent rapid cessation of growth and accelerated cell death. The mutant grew at 42 degrees C in continuous culture under glucose-limiting conditions. Under these conditions, lactate production was replaced by production of ethanol and, to a smaller extent, acetoin. The cloned Zymomonas mobilis gene for alcohol dehydrogenase II, placed under the control of the S. mutans spaP regulatory signals, complemented LDH deficiency. The alcohol dehydrogenase-complemented mutant grew as well or better than NG8 on a variety of carbon sources at 42 degrees C and produced significant amounts of ethanol in place of lactic acid. These results are in accord with other approaches indicating that S. mutans has other enzymatic activities, including pyruvate formate-lyase and pyruvate dehydrogenase, for pyruvate metabolism. However, at high glucose concentrations, the levels of activity of these enzymes are apparently insufficient to compensate for the absence of LDH.

Characterization of a P1-deficient strain of Streptococcus mutans that expresses the SpaA protein of Streptococcus sobrinus

The Streptococcus sobrinus SpaA protein and the Streptococcus mutans P1 protein share 66% sequence homology at the amino acid level. To determine if the SpaA protein can be expressed in S. mutans and functionally replace the P1 protein, the spaA gene of S. sobrinus 6715 was isolated from plasmid pX1303 and inserted into the Escherichia coli-Streptococcus shuttle vector pVA838. The resulting plasmid pX1600 was transformed into the P1-deficient strain S. mutans 834 that has defects in saliva-mediated aggregation and in the ability to adhere to saliva-coated hydroxyapatite surfaces. Western blot (immunoblot) analysis of cellular protein fractions of S. mutans 834 (pX1600) detected in mutanolysin-solubilized cell walls a major protein of 210 kDa with an electrophoretic mobility similar to that of S. sobrinus SpaA protein and a minor 210-kDa protein and a major 64-kDa protein in the extracellular protein fraction. Analysis of virulence traits showed that expression of SpaA protein by S. mutans 834(pX1600) cells had restored the ability of the S. mutans 834 cells to aggregate in the presence of saliva or salivary agglutinin but not to adhere to saliva-coated hydroxyapatite. This cell aggregation was inhibited specifically by antisera to S. sobrinus SpaA protein. These results indicate that SpaA plays a role in the virulence of S. sobrinus by specifically interacting with fluid-phase salivary agglutinin to mediate cell aggregation.

Insertional mutagenesis and recovery of interrupted genes of Streptococcus mutans by using transposon Tn917: preliminary characterization of mutants displaying acid sensitivity and nutritional requirements

New vectors were constructed for efficient transposon Tn917-mediated mutagenesis of poorly transformable strains of Streptococcus mutans(pTV1-OK) and subsequent recovery of interrupted genes in Escherichia coli (pT21delta2TetM). In this report, we demonstrate the utility of Tn917 mutagenesis of a poorly transformable strain of S. mutans (JH1005) by showing (i) the conditional replication of pTV1-OK, a repA(Ts) derivative of the broad-host-range plasmid pWVO1 harboring Tn9l7, in JH1005 at the permissive temperature (30 degrees C) versus that at the nonpermissive temperature (45 degrees C); (ii) transposition frequencies similar to those reported for Bacillus subtilis (10(-5) to 10(-4)) with efficient plasmid curing in 90 to 97% of the erythromycin-resistant survivors following a temperature shift to 42 to 45 degrees C; and (iii) the apparent randomness of Tn917 insertion as determined by Southern hybridization analysis and the ability to isolate nutritional mutants, mutants in acid tolerance, and mutants in bacteriocin production, at frequencies ranging from 0.1 to 0.7%. Recovery of transposon-interrupted genes was achieved by two methods: (i) marker rescue in E. coli with the recovery vector pTV21delta2TetM, a tetracycline-resistant and ampicillin-sensitive Tn9l7-pBR322 hybrid, and (ii) "shotgun" cloning of genomic libraries of Tn917 mutants into pUC19. Sequence analyses revealed insertions at five different genetic loci in sequences displaying homologies to Clostridium spp.fhs (66% identity), E. coli dfp (43% identity), and B. subtilis ylxM-ffh (58% identity), icd (citC [69% identity]), and argD (61% identity). Insertions in icd and argD caused nutritional requirements; the one in ylxM-ffh caused acid sensitivity, while those in fhs and dfp caused both acid sensitivity and nutritional requirements. This paper describes the construction of pTV1-OK and demonstrates that it can be efficiently employed to deliver Tn917 into S. mutans for genetic analyses with some degree of randomness and that insertions in the chromosome can be easily recovered for subsequent characterization. This represents the first published report of successful Tn9l7 mutagenesis in the genus Streptococcus.

Role of the C terminus in antigen P1 surface localization in Streptococcus mutans and two related cocci

The C terminus of the major surface protein P1 from Streptococcus mutans is composed of a hydrophilic domain, an LPNTGV motif, a hydrophobic domain, and a charged tail. These features are shared by surface proteins from many gram-positive coccal bacteria. To investigate the role of the C-terminal domains in antigen P1 surface localization, full-length and truncated P1 gene constructs, which were expressed on the shuttle vector pDL276, were transformed into the P1-negative mutant S. mutans SM3352, Streptococcus gordonii DL-1, and Enterococcus faecalis UV202. Transformants were tested for expression of P1 by enzyme-linked immunosorbent assaying and Western blotting. The results showed that full-length P1 was expressed by transformants of all three bacteria and was localized on the cell surface. A fusion protein composed of the Staphylococcus aureus fibronectin binding protein C terminus and the P1 protein N terminus was found to surface localize in S. mutans. Deletion of the entire C-terminal domains resulted in P1 being expressed in the culture supernatant. A P1 truncation, which carried only the hydrophilic domain at its C terminus, was found partially associated with the cell surface. This truncated P1 was readily removed from the isolated cell wall by hot sodium dodecyl sulfate-mercaptoethanol extraction. In contrast, the full-length P1 remained associated with the isolated cell wall after similar treatment, suggesting covalent linkages between the full-length P1 and the cell wall. The results described above showed that antigen P1 was anchored to the cell wall by its C-terminal domains probably via covalent linkages with the cell wall. The results also support a universal mechanism involving the C-terminal domains for protein surface localization among this group of gram-positive bacteria.

Localization on the cell surface of streptococcal protein antigen A

The cellular localization of the major surface protein SpaA of Streptococcus sobrinus 6715 was examined by immunoelectron microscopy with rabbit polyclonal antibodies directed against purified SpaA protein. Immunoelectron microscopic analysis of thin sections of S. sobrinus cells revealed that the SpaA protein is associated with the fibrillar fuzzy coat of S. sobrinus cells and appears to be distributed over the entire surface of S. sobrinus cells.

Role of the Streptococcus mutans gtf genes in caries induction in the specific-pathogen-free rat model

The role of each of the Streptococcus mutans gtf genes coding for glucan synthesis in cariogenesis was evaluated by using strain UA130 in the specific-pathogen-free (SPF) rat model system. Mutants defective in either or both of the genes required for insoluble glucan synthesis, the gtfB and gtfC genes, exhibited markedly reduced levels of smooth-surface carious lesions relative to that of the parental organism. Likewise, the mutant defective in the gtfD gene coding for the glucosyltransferase-S enzyme synthesizing water-soluble glucans also produced significantly fewer smooth-surface lesions than strain UA130. None of these mutations markedly altered the rate of sulcal caries induction relative to that of the parental organism. In addition, a mutant of strain UA130 defective in the gtfA gene was reexamined in the SPF rat model. In contrast to previous results from a gnotobiotic rat system, these mutants also induced significantly fewer smooth-surface carious lesions compared with that by strain UA130. These results suggest that all four genes are important for smooth-surface caries formation. Furthermore, these results are discussed relative to the differences in the diets utilized in the SPF and gnotobiotic rat model systems for assessing the virulence factors of S. mutans.

Transformation efficiency of EMS-induced mutants of Streptococcus mutans of altered cell shape

Some Streptococcus mutans strains change shape from bacillary to coccal or ellipsoid form in response to the ratio of bicarbonate to potassium or of borate to potassium in growth media. So that insight into determinants of shape of these streptococci could be gained, and future genetic studies facilitated, the shapes of a series of transformable and nontransformable strains of S. mutans were studied and attempts made to isolate a mutant of augmented transformability. Several strains were mutagenized by ethylmethane sulfonate and mutants with altered colonial and cellular morphologies isolated. Cell shapes were studied by Gram stain and Nomarski interference microscopy, and by scanning and transmission electron microscopy. Diverse shape-altered mutants were isolated from seven transformable and two nontransformable strains of S. mutans. Among these, length-to-width ratios ranged from > 10 to about 0.25. Regulation of timing of cell division, septum formation, or septum completion events may have been altered in these mutants. While most mutants substantially or completely lost transformability, mutant LT11 had transformation efficiency of 1.3 x 10(-4) to 2.3 x 10(-3), more than two to three orders of magnitude greater than its parental UA159 and the well-known transformable strain GS5(HK), respectively. There was no evidence of production of competence factor by LT11. Competence of LT11 was maintained for at least six months upon storage at -70 degrees C, facilitating its use for genetic studies. While the morphologies of several shape-altered mutants were no longer responsive to changes of the bicarbonate/potassium, unlike those of their parentals, the morphology of LT11 persisted in its response to this condition.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification of several rod loci and cloning of the rodD locus of Streptococcus mutans

Previous work has shown that Streptococcus mutans is normally a short rod or a sphere, depending on its environment. This paper describes two distinct genetic approaches used to identify multiple loci and isolate one locus, rodD, controlling S. mutans rod shape. The first method involved isolation of a group of rod- mutants caused by transposon Tn916 insertion, and analysis of the inactivated genes by Southern hybridization. The second method involved mutagenesis via a shotgun insertion-duplication technique, isolation of a rod- mutant, and cloning the intact rod locus, employing an integration shuttle plasmid, pVA891. These approaches have led to the identification of multiple rod loci involved in determining the rod shape of S. mutans, and also cloning of one rod locus, rodD. The cloning strategy may also be useful for cloning other streptococcal genes which cannot be detected by their expression in Escherichia coli.