Enhanced transformation of Streptococcus mutans by modifications in culture conditions

The transcription regulator BrsR serves as a network hub of natural competence protein-protein interactions in Streptococcus mutans

Genome evolution is an essential and stringently regulated aspect of biological fitness. For bacteria, natural competence is one of the principal mechanisms of genome evolution and is frequently subject to multiple layers of regulation derived from a plethora of environmental and physiological stimuli. Here, we present a regulatory mechanism that illustrates how such disparate stimuli can be integrated into the Streptococcus mutans natural competence phenotype. S. mutans possesses an intriguing, but poorly understood ability to coordinately control its independently regulated natural competence and bacteriocin genetic pathways as a means to acquire DNA released from closely related, bacteriocin-susceptible streptococci. Our results reveal how the bacteriocin-specific transcription activator BrsR directly mediates this coordination by serving as an anti-adaptor protein responsible for antagonizing the proteolysis of the inherently unstable, natural competence-specific alternative sigma factor ComX. This BrsR ability functions entirely independent of its transcription regulator function and directly modulates the timing and severity of the natural competence phenotype. Additionally, many of the DNA uptake proteins produced by the competence system were surprisingly found to possess adaptor abilities, which are employed to terminate the BrsR regulatory circuit via negative feedback. BrsR-competence protein heteromeric complexes directly inhibit nascent brsR transcription as well as stimulate the Clp-dependent proteolysis of extant BrsR proteins. This study illustrates how critical genetic regulatory abilities can evolve in a potentially limitless variety of proteins without disrupting their conserved ancestral functions. These unrecognized regulatory abilities are likely fundamental for transducing information through complex genetic networks.

Identification of Streptococcus gallolyticus subsp. gallolyticus (Biotype I) Competence-Stimulating Peptide Pheromone

Streptococcus gallolyticus subsp. gallolyticus, a member of the group D streptococci, is normally found in the bovine rumen and human gut. It is an opportunistic pathogen that was recently determined to be a bacterial driver of colorectal cancer, in addition to causing other diseases, such as infective endocarditis, bacteremia, neonatal meningitis, and septicemia. As an emerging pathogen, not much is known about this bacterium, its virulence mechanisms, or its virulence regulatory pathways. Previous studies suggest that S. gallolyticus subsp. gallolyticus uses a ComRS pathway, one of many Streptococcus quorum-sensing circuitries, for competence. However, thus far, the ubiquitous ComABCDE pathway has not been studied, nor has its regulatory role in S. gallolyticus subsp. gallolyticus We therefore sought to study the S. gallolyticus subsp. gallolyticus ComABCDE quorum-sensing pathway and have identified its peptide pheromone, which is termed the competence-stimulating peptide (CSP). We further determined that this peptide regulates the production of bacteriocin-like inhibitory substances (BLISs), a phenotype that has been linked with the ComABCDE pathway in both Streptococcus pneumoniae and Streptococcus mutans Our data show that S. gallolyticus subsp. gallolyticus TX20005 produces a 21-mer CSP signal, which differs from CSP signals of other Streptococcus species in that its active form begins three residues after the double-glycine leader signal of the ComC precursor peptide. Additionally, our data suggest that this peptide might not be related to competence induction, as opposed to CSP signaling peptides in other Streptococcus species. This study provides the first evidence that S. gallolyticus subsp. gallolyticus utilizes quorum sensing to eliminate competitors, presenting a potential pathway to target this emerging human pathogen.IMPORTANCEStreptococcus gallolyticus subsp. gallolyticus is an emerging human pathogen known as a causative agent of infective endocarditis, and recently, of colorectal cancer. In this work, we revealed a functional quorum-sensing circuitry in S. gallolyticus subsp. gallolyticus, including the identification of the central signaling peptide pheromone, competence-stimulating peptide (CSP), and the regulatory role of this circuitry in the production of bacteriocin-like inhibitory substances (BLISs). This work uncovered a mechanism by which this bacterium outcompetes other bacterial species and thus provides a potential tool to study this opportunistic pathogen.

Investigation of simulated microgravity effects on Streptococcus mutans physiology and global gene expression

Astronauts have been previously shown to exhibit decreased salivary lysozyme and increased dental calculus and gingival inflammation in response to space flight, host factors that could contribute to oral diseases such as caries and periodontitis. However, the specific physiological response of caries-causing bacteria such as Streptococcus mutans to space flight and/or ground-based simulated microgravity has not been extensively investigated. In this study, high aspect ratio vessel S. mutans simulated microgravity and normal gravity cultures were assessed for changes in metabolite and transcriptome profiles, H₂O₂ resistance, and competence in sucrose-containing biofilm media. Stationary phase S. mutans simulated microgravity cultures displayed increased killing by H₂O₂ compared to normal gravity control cultures, but competence was not affected. RNA-seq analysis revealed that expression of 153 genes was up-regulated ≥2-fold and 94 genes down-regulated ≥2-fold during simulated microgravity high aspect ratio vessel growth. These included a number of genes located on extrachromosomal elements, as well as genes involved in carbohydrate metabolism, translation, and stress responses. Collectively, these results suggest that growth under microgravity analog conditions promotes changes in S. mutans gene expression and physiology that may translate to an altered cariogenic potential of this organism during space flight missions.

The gene expression patterns, metabolism and physiology of tooth cavities-causing microbes change in a space-like gravity environment. These findings could help explain why astronauts are at a greater risk for dental diseases when in space. Kelly Rice and colleagues from the University of Florida, Gainesville, USA, cultured Streptococcus mutans bacteria under simulated microgravity and normal gravity conditions. The bacteria grown in microgravity were more susceptible to killing with hydrogen peroxide, tended to aggregate in more compact cellular structures, showed changes in their metabolite profile and expressed around 250 genes at levels that were either much higher or lower than normal gravity control cultures. These genes included many involved in carbohydrate metabolism, protein production and stress responses. The observed changes collectively suggest that space flight and microgravity could alter the cavities-causing potential of S. mutans.

Genome editing by natural genetic transformation in Streptococcus mutans

Classical mutagenesis strategies using selective markers linked to designed mutations are powerful and widely applicable tools for targeted mutagenesis via natural genetic transformation in bacteria and archaea. However, the markers that confer power are also potentially problematic as they can be cumbersome, risk phenotypic effects of the inserted genes, and accumulate as unwanted genes during successive mutagenesis cycles. Alternative mutagenesis strategies use temporary plasmid or cassette insertions and can in principle achieve equally flexible mutation designs, but design of suitable counter-selected markers can be complex. All these drawbacks are eased by use of direct genome editing. Here we describe a strategy for directly editing the genome of S. mutans, which is applied to the widely studied reference strain UA159 (ATCC 700610) and has the advantage of extreme simplicity, requiring construction of only one synthetic donor amplicon and a single transformation step, followed by a simple PCR screen among a few dozen clones to identify the desired mutant. The donor amplicon carries the mutant sequence and extensive flanking segments of homology, which ensure efficient and precise integration by the recombination machinery specific to competent cells. The recipients are highly competent cells, in a state achieved by treatment with a synthetic competence pheromone.

Cryptic Streptococcus mutans 5.6-kb plasmids encode a toxin-antitoxin system for plasmid stabilization

BACKGROUND

In all Streptococcus mutans strains, 5-13% carry a 5.6-kb plasmid. Despite its frequency, little is known about its mediated functions with most of the information coming from a single study focussing on plasmid pUA140.

OBJECTIVE

Here, we describe the sequence and genetic organization of two S. mutans 5.6-kb plasmids, pDC09 and pNC101.

RESULTS

Based on PicoGreen dsDNA quantification and Real-Time quantitative PCR (RTQ-PCR), the plasmid copy number was found to range between 10 and 74, depending on the strain tested. In contrast to literature, we identified six instead of five open reading frames (ORFs). While the putative gene products of ORF1 (as a Rep-protein) and ORF2 (as a Mob-protein) could be confirmed as being identical to those from pUA140, the functions of ORF3 (unknown) and ORF 4 (possibly AtpE homologue) could not be further revealed. However, the product of ORF5 showed a fairly high identity (38-50%) and structural similarity (58-74%) to RelE of Streptococcus pneumoniae, Streptococcus equi, and Streptococcus downei. In addition, we identified a functionally corresponding ORF6 encoding a protein with 61-68% identity (81-86% similarity) to the S. equi and S. downei antitoxin of the RelB family. RelE and RelB together form a plasmid-encoded toxin-antitoxin (TA) system, RelBE(plas). Despite its rather limited sequence similarity with chromosomal TA systems in S. mutans (RelBE(chro), MazEF, HicBA), we found similar tertiary structures applying I-Tasser protein prediction analysis.

CONCLUSION

Type II-toxins, as the plasmid-encoded RelE, are RNA endonucleases. Depending on their mRNA cleavage activity, they might 1) kill every plasmid-free progeny, thereby stabilizing plasmid transfer at the expense of the host and/or 2) help S. mutans enter a dormant state and survive unfavourable environmental conditions. Whilst a function in plasmid stabilization has been confirmed, a function in persistence under nutritional stress, tested here by inducing amino acid starvation, could not be demonstrated so far.

Development of competence for genetic transformation of Streptococcus mutans in a chemically defined medium

Streptococcus mutans develops competence for genetic transformation in response to regulatory circuits that sense at least two peptide pheromones. One peptide, known as CSP, is sensed by a two-component signal transduction system through a membrane receptor, ComD. The other, derived from the primary translation product ComS, is thought to be sensed by an intracellular receptor, ComR, after uptake by oligopeptide permease. To allow study of this process in a medium that does not itself contain peptides, development of competence was examined in the chemically defined medium (CDM) described by van de Rijn and Kessler (Infect. Immun. 27:444, 1980). We confirmed a previous report that in this medium comS mutants of strain UA159 respond to a synthetic peptide comprising the seven C-terminal residues of ComS (ComS(11-17)) by increasing expression of the alternative sigma factor SigX, which in turn allows expression of competence effector genes. This response provided the basis for a bioassay for the ComS pheromone in the 100 to 1,000 nM range. It was further observed that comS(+) (but not comS mutant) cultures developed a high level of competence in the late log and transition phases of growth in this CDM without the introduction of any synthetic stimulatory peptide. This endogenous competence development was accompanied by extracellular release of one or more signals that complemented a comS mutation at levels equivalent to 1 μM synthetic ComS(11-17).

Role of Streptococcus mutans eukaryotic-type serine/threonine protein kinase in interspecies interactions with Streptococcus sanguinis

OBJECTIVE

Interspecies interactions of oral streptococci involve the production and excretion of antimicrobial compounds to compete successfully during colonization. Bacteriocin production by Streptococcus mutans and hydrogen peroxide (H2O2) production by Streptococcus sanguinis have been demonstrated as crucial for the clinical relevant antagonism between both species. A potential target of H2O2 is the cell-envelop of S. mutans. In the present study, the role of cell-envelop associated eukaryotic serine/threonine protein kinase (STPK) in S. mutans during interspecies competition has been investigated.

DESIGN

Allelic replacement via homologous recombination of the STPK encoding gene with a kanamycin resistant determinant has been constructed. The mutant has been screened for the susceptibility towards cell-envelope stress. A previously developed spotting assay was used to simulate interspecies competition.

RESULTS

The STPK(-) mutant showed an increased susceptibility towards envelop stress caused by H2O2 and was significantly more inhibited during interspecies competition assays.

CONCLUSIONS

S. mutans is able to sense antimicrobial compounds excreted by competing species and can potentially adjust the cell-envelop towards an increased resistance.

Natural transformation of oral streptococci

Natural transformation is found in most groups of oral streptococci, including the mitis, the anginosus, and the mutans groups. This ability has been applied as a powerful tool to explore streptococcal gene functions and regulatory pathways, particularly in Streptococcus mutans and Streptococcus gordonii. The range of strains and species amenable to transformation has expanded in recent years with the identification of several competence-stimulating peptide signals (CSPs). In this chapter we present protocols for natural transformation in strains found in the three groups of transformable oral streptococci, with focus on methods using synthetic CSPs. We also include suggestions on how to optimize competence conditions for individual species or strains.

Genetic characterization of the hdrRM operon: a novel high-cell-density-responsive regulator in Streptococcus mutans

Many species of bacteria can adhere to surfaces and grow as sessile communities. The continued accumulation of bacteria can eventually lead to the extremely high-cell-density environment characteristic of many biofilms or cell colonies. This is the normal habitat of the cariogenic species Streptococcus mutans, which normally resides in the high-cell-density, multispecies community commonly referred to as dental plaque. Previous work has demonstrated that the transcription of two separate bacteriocins can be activated by the high-cell-density conditions created through the centrifugation and incubation of cell pellets. In this study, we identified an uncharacterized two-gene operon that was induced >10-fold by conditions of high cell density. The genes of the operon encode a putative transcription regulator and a membrane protein, which were renamed as hdrR and hdrM, respectively. A transcription fusion to the hdrRM operon confirmed its induction by high cell density. Mutation of hdrM abolished bacteriocin production, greatly increased natural competence, reduced the growth rate, and severely affected biofilm formation. Interestingly, no obvious phenotypes were observed from a non-polar mutation of hdrR or mutations affecting the entire operon. These data suggest that the hdrRM operon may constitute a novel regulatory system responsible for mediating a cellular response to a high-cell-density environment.

Atomic force microscopy study of the structure-function relationships of the biofilm-forming bacterium Streptococcus mutans

Atomic force microscopy (AFM) has garnered much interest in recent years for its ability to probe the structure, function and cellular nanomechanics inherent to specific biological cells. In particular, we have used AFM to probe the important structure-function relationships of the bacterium Streptococcus mutans. S. mutans is the primary aetiological agent in human dental caries (tooth decay), and is of medical importance due to the virulence properties of these cells in biofilm initiation and formation, leading to increased tolerance to antibiotics. We have used AFM to characterize the unique surface structures of distinct mutants of S. mutans. These mutations are located in specific genes that encode surface proteins, thus using AFM we have resolved characteristic surface features for mutant strains compared to the wild type. Ultimately, our characterization of surface morphology has shown distinct differences in the local properties displayed by various S. mutans strains on the nanoscale, which is imperative for understanding the collective properties of these cells in biofilm formation.

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.

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.

Inactivation of the ciaH Gene in Streptococcus mutans diminishes mutacin production and competence development, alters sucrose-dependent biofilm formation, and reduces stress tolerance

Many clinical isolates of Streptococcus mutans produce peptide antibiotics called mutacins. Mutacin production may play an important role in the ecology of S. mutans in dental plaque. In this study, inactivation of a histidine kinase gene, ciaH, abolished mutacin production. Surprisingly, the same mutation also diminished competence development, stress tolerance, and sucrose-dependent biofilm formation.

Transcriptional analysis of mutacin I (mutA) gene expression in planktonic and biofilm cells of Streptococcus mutans using fluorescent protein and glucuronidase reporters

Streptococcus mutans is implicated as the primary pathogen involved in the development of dental caries. The production of specific bacteriocins (called mutacins) by S. mutans is one of the major virulence factors which facilitate the dominance of the bacterium within dental plaque. While much has been revealed about the biochemical structures of mutacins, little is known about the expression and regulation of mutacin genes, largely due to the lack of proper methods to monitor mutacin gene expression, especially under biofilm conditions. In this study, a set of reporter systems with the green fluorescent protein (gfp), the monomeric red fluorescent protein (mrfp1), and the glucuronidase (gusA) are introduced to S. mutans to study the transcriptional activities of the mutacin I gene (mutA). Although the mutA-reporter fusions are in single copy on the chromosome, these reporter systems display strong signals that allow us to effectively monitor mutA gene expression in S. mutans. Using these reporter systems, we show that mutA is expressed in both planktonic and biofilm cells, even though mutacin activities are normally detected only in biofilm cells. Furthermore, we confirm that mutR, the gene upstream of the mutacin operon, is required for mutacin I gene expression. The success of this study validates the feasibility of using these reporter systems to study gene expression and regulation in S. mutans.

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.

Complete nucleotide sequence and characterization of pUA140, a cryptic plasmid from Streptococcus mutans

Approximately 5% of strains of Streptococcus mutans contain plasmid DNA. Strain UA140 harbors a 5.6-kb cryptic plasmid, pUA140, with an overall G+C content of 32.7%. Five open reading frames (ORF), encoding peptides of larger than 100 amino acid residues, were initially designated as ORF1 to ORF5. These five ORFs were located on the same strand of pUA140. ORF1 (258 amino acids) resembled a replication protein, Rep. Upstream of the putative Rep gene, a double-stranded origin for plasmid replication that showed strong similarity to those of a number of plasmids in the pT181 family was identified. Further upstream was a region constituting the single-stranded origin of replication. A single-stranded DNA intermediate was detected during plasmid replication. Taken together, these results suggest that pUA140 replicated by the rolling circle replication mechanism but exhibited several characteristics that differ from those of other members of the pT181 plasmid family.

Effect of amino acid substitutions in conserved residues in the leader peptide on biosynthesis of the lantibiotic mutacin II

The lantibiotic mutacin II, produced by Streptococcus mutans T8, is a ribosomally synthesized peptide antibiotic that contains thioether amino acids such as lanthionine and methyllanthionine as a result of post-translational modifications. The mutacin II leader peptide sequence shares a number of identical amino acid residues with class AII lantibiotic leader peptides. To study the role of these conservative residues in the production of active antimicrobial mutacin, 15 mutations were generated by site-directed mutagenesis. The effects of these substitutions vary from no effect to complete block-out. Mutations G-1A, G-2A, I-4D, and L-7K completely blocked the production of mature mutacin. Other mutations (I-4V, L-7M, E-8D, S-11T/A, V-12I/A, and E-13D) had no detectable effect on mutacin production. The changes of Glu-8 to Lys, Val-12 to Leu, Glu-13 to Lys reduced the mutacin production level to about 75%, 50%, and 10% of the wild-type, respectively. Thus, our data indicated that some of these conserved residues are essential for the mutacin biosynthesis, whereas others are important for optimal biosynthesis rates.

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.

Biochemical structural analysis of the lantibiotic mutacin II

Mutacin II is a post-translationally modified lantibiotic peptide secreted by Streptococcus mutans T8, which inhibits the energy metabolism of sensitive cells. The deduced amino acid sequence of promutacin II is NRWWQGVVPTVSYECRMNSWQHVFTCC, which is capable of forming three thioether bridges. It was not obvious, however, how the three thioether bridges are organized. To examine the bridging, the cyanogen bromide cleavage products of mutacin II and its variants generated by protein engineering, C15A, C26A, and C15A/C26A, were analyzed by mass spectrometry. Analysis of the wild type molecule and the C15A variant excluded several possibilities and also indicated a high fidelity of formation of the thioether bridges. This allowed us to further resolve the structure by analysis (mass spectrometry and tandem mass spectrometry) of the cyanogen bromide cleavage fragments of the C26A and C15A/C26A mutants. Nuclear magnetic resonance analysis established the presence of one and two dehydrobutyrine residues in mutacin II and the C15A variant, respectively, thus yielding the final structure. The results of this investigation showed that the C-terminal part contains three thioether bridges connecting Cys residues 15, 26, and 27 to Ser/Thr residues 10, 12 and 19, respectively, with Thr(25) being modified to dehydrobutyrine.

Heterologous inducible expression of Enterococcus faecalis pCF10 aggregation substance asc10 in Lactococcus lactis and Streptococcus gordonii contributes to cell hydrophobicity and adhesion to fibrin

Aggregation substance proteins encoded by the sex pheromone plasmid family of Enterococcus faecalis have been shown previously to contribute to the formation of a stable mating complex between donor and recipient cells and have been implicated in the virulence of this increasingly important nosocomial pathogen. In an effort to characterize the protein further, prgB, the gene encoding the aggregation substance Asc10 on pCF10, was cloned in a vector containing the nisin-inducible nisA promoter and its two-component regulatory system. Expression of aggregation substance after nisin addition to cultures of E. faecalis and the heterologous bacteria Lactococcus lactis and Streptococcus gordonii was demonstrated. Electron microscopy revealed that Asc10 was presented on the cell surfaces of E. faecalis and L. lactis but not on that of S. gordonii. The protein was also found in the cell culture supernatants of all three species. Characterization of Asc10 on the cell surfaces of E. faecalis and L. lactis revealed a significant increase in cell surface hydrophobicity upon expression of the protein. Heterologous expression of Asc10 on L. lactis also allowed the recognition of its binding ligand (EBS) on the enterococcal cell surface, as indicated by increased transfer of a conjugative transposon. We also found that adhesion of Asc10-expressing bacterial cells to fibrin was elevated, consistent with a role for the protein in the pathogenesis of enterococcal endocarditis. The data demonstrate that Asc10 expressed under the control of the nisA promoter in heterologous species will be an useful tool in the detailed characterization of this important enterococcal conjugation protein and virulence factor.

Natural genetic transformation in the rumen bacterium Streptococcus bovis JB1

Natural transformation of Streptococcus bovis JB1 was demonstrated after development of competence in normal culture medium. Transformation efficiencies were not significantly increased when heat-inactivated horse serum was added to the medium before growth. This is the first time that a resident rumen bacterial species has been shown to be naturally transformable. Transformation allowed the acquisition of plasmids or integration of sequences into the chromosome. No transformation was observed in the presence of undiluted autoclaved or filter-sterilised ovine rumen fluid or filter-sterilised ovine saliva, suggesting that transformation in the ruminant digestive tract is a rare event, although transformation was observed in the presence of 1% and 0.5% filter-sterilised rumen fluid. The use of natural transformation of S. bovis should facilitate further molecular biological studies on this species.

Purification of mutacin III from group III Streptococcus mutans UA787 and genetic analyses of mutacin III biosynthesis genes

Previously, members of our group reported the isolation and characterization of mutacin II from Streptococcus mutans T8 and the genetic analyses of the mutacin II biosynthesis genes (J. Novak, P. W. Caufield, and E. J. Miller, J. Bacteriol. 176:4316-4320, 1994; F. Qi, P. Chen, and P. W. Caufield, Appl. Environ. Microbiol. 65:652-658, 1999; P. Chen, F. Qi, J. Novak, and P. W. Caufield, Appl. Environ. Microbiol. 65:1356-1360, 1999). In this study, we cloned and sequenced the mutacin III biosynthesis gene locus from a group III strain of S. mutans, UA787. DNA sequence analysis revealed eight open reading frames, which we designated mutR, -A, -A', -B, -C, -D, -P, and -T. MutR bears strong homology with MutR of mutacin II, while MutA, -B, -C, -D, -P, and -T are counterparts of proteins in the lantibiotic epidermin group. MutA' has 60% amino acid identity with MutA and therefore appears to be a duplicate of MutA. Insertional inactivation demonstrated that mutA is an essential gene for mutacin III production, while mutA' is not required. Mutacin III was purified to homogeneity by using reverse-phase high-pressure liquid chromatography. N-terminal peptide sequencing of the purified mutacin III determined mutA to be the structural gene for prepromutacin III. The molecular mass of the purified peptide was measured by laser disorption mass spectrophotometry and found to be 2,266.43 Da, consistent with our supposition that mutacin III has posttranslational modifications similar to those of the lantibiotic epidermin.

The specific genes for lantibiotic mutacin II biosynthesis in Streptococcus mutans T8 are clustered and can be transferred en bloc

Mutacin II is a ribosomally synthesized peptide lantibiotic produced by group II Streptococcus mutans. DNA sequencing has revealed that the mutacin II biosynthetic gene cluster consists of seven specific open reading frames: a regulator (mutR), the prepromutacin structural gene (mutA), a modifying protein (mutM), an ABC transporter (mutT), and an immunity cluster (mutFEG). Transformations of a non-mutacin-producing strain, S. mutans UA159, and a mutacin I-producing strain, S. mutans UA140, with chromosomal DNA from S. mutans T8 with an aphIII marker inserted upstream of the mutacin II structural gene yielded transformants producing mutacin II and mutacins I and II, respectively.

Functional analyses of the promoters in the lantibiotic mutacin II biosynthetic locus in Streptococcus mutans

The lantibiotic bacteriocin mutacin II is produced by the group II Streptococcus mutans. The mutacin II biosynthetic locus consists of seven genes, mutR, -A, -M, -T, -F, -E, and -G, organized as two operons. The mutAMTFEG operon is transcribed from the mutA promoter 55 bp upstream of the translation start codon for MutA, while the mutR promoter is 76 bp upstream of the mutR structural gene. Expression of the mutA promoter is regulated by the components of the growth medium, while the mutR promoter activity does not seem to be affected by these conditions. Inactivation of mutR abolishes transcription of the mutA operon but does not affect its own promoter activity. The expressions of both mutA and mutR promoters are independent of the growth stage, while the production of mutacin II is only elevated at the early stationary phase. Taken together, these results suggest that expression of the mutacin operon is regulated by a complex system involving transcriptional and posttranscriptional or posttranslational controls.

Fate of free DNA and transformation of the oral bacterium Streptococcus gordonii DL1 by plasmid DNA in human saliva

Competitive PCR was used to monitor the survival of a 520-bp DNA target sequence from a recombinant plasmid, pVACMC1, after admixture of the plasmid with freshly sampled human saliva. The fraction of the target remaining amplifiable ranged from 40 to 65% after 10 min of exposure to saliva samples from five subjects and from 6 to 25% after 60 min of exposure. pVACMC1 plasmid DNA that had been exposed to degradation by fresh saliva was capable of transforming naturally competent Streptococcus gordonii DL1 to erythromycin resistance, although transforming activity decreased rapidly, with a half-life of approximately 50 s. S. gordonii DL1 transformants were obtained in the presence of filter-sterilized saliva and a 1-microg/ml final concentration of pVACMC1 DNA. Addition of filter-sterilized saliva instead of heat-inactivated horse serum to S. gordonii DL1 cells induced competence, although with slightly lower efficiency. These findings indicate that DNA released from bacteria or food sources within the mouth has the potential to transform naturally competent oral bacteria. However, further investigations are needed to establish whether transformation of oral bacteria can occur at significant frequencies in vivo.

Structure-activity study of the lantibiotic mutacin II from Streptococcus mutans T8 by a gene replacement strategy

Mutacin II, elaborated by group II Streptococcus mutans, is a ribosomally synthesized and posttranslationally modified polypeptide antibiotic containing unusual thioether and didehydro amino acids. To ascertain the role of specific amino acid residues in mutacin II antimicrobial activity, we developed a streptococcal expression system that facilitates the replacement of the mutA gene with a single copy of a mutated variant gene. As a result, variants of mutacin II can be designed and expressed. The system was tested by constructing the following mutant peptides: delta N1, V7A, P9A, T10A, T10S, C15A, C26A, and C27A. All of these mutacin II variants except delta N1 and T10A, which were not secreted, were isolated, and their identities were verified by mass spectrometry. Variants P9A, C15A, C26A, and C27A failed to exert antimicrobial activity. Because the P9A and T10A variants comprise the "hinge" region of mutacin II, these observations suggest that in addition to the thioether and didehydro amino acids, the hinge region is essential for biological activity and biosynthesis or export of the peptide. Tandem mass spectrometry of the N-terminal part of the wild-type molecule and its C15A variant confirmed that the threonine at position 10 is dehydrated and present as a didehydrobutyrine residue. This analysis of the active T10S variant further suggested that a didehydro amino acid at this position is specific for antimicrobial activity and that the biosynthetic machinery does not discriminate between threonine and serine. In contrast, the lack of production of mutacin variants with alanine substituted for threonine at position 10, as well as the deletion of asparagine at the N terminus (delta N1), indicates that specific residues in the propeptide may be crucial for certain steps in the biosynthetic pathway of this lantibiotic.

Sequence analysis of mutA and mutM genes involved in the biosynthesis of the lantibiotic mutacin II in Streptococcus mutans

Streptococcus mutans, along with many other gram-positive bacteria produce small antibacterial peptides called bacteriocins. Bacteriocins elaborated by S. mutans, termed mutacins, may provide a selective force necessary for initial or sustained colonization in dental plaque by this major dental pathogen. Previously, we purified and characterized mutacin II, the first lantibiotic found in S. mutans. Specific oligonucleotides designed according to the N-terminal amino acid sequence permitted amplification of 0.7 kb upstream and 2.1 kb downstream of the N-terminus, using single-specific-primer PCR (SSP-PCR). The gene encoding the mutacin II prepeptide, mutA, was subsequently cloned and sequenced. The complete prepeptide consists of 53 amino acids, including the 26 amino acid amphipathic leader peptide with the Gly(-2)-Gly(-1) sequence at the processing site. The prepeptide showed similarity to the lantibiotics lacticin 481, variacin, salivaricin and streptococcin A-FF22. A 3 kb open reading frame immediately downstream of mutA, denoted mutM, showed sequence similarities to LCNDR2 from Lactococcus lactis. By analogy, mutM is probably involved in post-translational modification of the mutacin prepeptide. Gene disruption with an insertional vector pVA891 showed that intact copies of mutA and mutM are required for production of mutacin II.

Diacylglycerol kinase is involved in regulation of expression of the lantibiotic mutacin II of Streptococcus mutans

Genetic characterization of a Tn916 transposon mutant, Streptococcus mutans T8-1, defective in mutacin II production, revealed that the transposon was inserted into the 3' region of a diacylglycerol kinase (dgk) gene. The insertion occurred in the same region as described for another S. mutans mutant, GS5Tn1, which was altered in its ability to respond to environmental stress (Y. Yamashita, T. Takehara, and H. K. Kuramitsu, J. Bacteriol. 175:6220-6228, 1993). Quantitative primer extension from the mutacin structural gene mutA showed a decreased level (about eightfold) of mutA transcription for mutant T8-1. Mutacin production was restored by transforming mutant T8-1 with integration vector pVA891 containing an intact dgk gene. These data indicated that the full-length dgk gene product along with the mutacin biosynthetic operon are required for the production of the mutacin II lantibiotic.

Bacterial gene transfer by natural genetic transformation in the environment

Natural genetic transformation is the active uptake of free DNA by bacterial cells and the heritable incorporation of its genetic information. Since the famous discovery of transformation in Streptococcus pneumoniae by Griffith in 1928 and the demonstration of DNA as the transforming principle by Avery and coworkers in 1944, cellular processes involved in transformation have been studied extensively by in vitro experimentation with a few transformable species. Only more recently has it been considered that transformation may be a powerful mechanism of horizontal gene transfer in natural bacterial populations. In this review the current understanding of the biology of transformation is summarized to provide the platform on which aspects of bacterial transformation in water, soil, and sediments and the habitat of pathogens are discussed. Direct and indirect evidence for gene transfer routes by transformation within species and between different species will be presented, along with data suggesting that plasmids as well as chromosomal DNA are subject to genetic exchange via transformation. Experiments exploring the prerequisites for transformation in the environment, including the production and persistence of free DNA and factors important for the uptake of DNA by cells, will be compiled, as well as possible natural barriers to transformation. The efficiency of gene transfer by transformation in bacterial habitats is possibly genetically adjusted to submaximal levels. The fact that natural transformation has been detected among bacteria from all trophic and taxonomic groups including archaebacteria suggests that transformability evolved early in phylogeny. Probable functions of DNA uptake other than gene acquisition will be discussed. The body of information presently available suggests that transformation has a great impact on bacterial population dynamics as well as on bacterial evolution and speciation.