Helper plasmid cloning in Streptococcus sanguis: cloning of a tetracycline resistance determinant from the Streptococcus mutans chromosome

Cnm of Streptococcus mutans is important for cell surface structure and membrane permeability

Streptococcus mutans, a Gram-positive facultative anaerobic bacterium, is a major pathogen of dental caries. The protein Cnm of S. mutans is involved in collagen binding, but its other biological functions are unknown. In this study, a Cnm-deficient isogenic mutant and a complementation strain were generated from a Cnm-positive S. mutans strain to help determine the properties of Cnm. Initially, comparison of the cell surface structure was performed by electron microscopy, which demonstrated that Cnm appears to be localized on the cell surface and associated with a protruding cell surface structure. Deep RNA sequencing of the strains revealed that the defect in Cnm caused upregulated expression of many genes related to ABC transporters and cell-surface proteins, while a few genes were downregulated. The amount of biofilm formed by the Cnm-defective strain increased compared with the parental and complemented strains, but the biofilm structure was thinner because of elevated expression of genes encoding glucan synthesis enzymes, leading to increased production of extracellular polysaccharides. Particular antibiotics, including bacitracin and chloramphenicol, had a lower minimum inhibitory concentration for the Cnm-defective strain than particular antibiotics, including bacitracin and chloramphenicol, compared with the parental and complemented strains. Our results suggest that S. mutans Cnm is located on the cell surface, gives rise to the observed protruding cell surface, and is associated with several biological properties related to membrane permeability.

Oral Delivery of a Novel Recombinant Streptococcus mitis Vector Elicits Robust Vaccine Antigen-Specific Oral Mucosal and Systemic Antibody Responses and T Cell Tolerance

The pioneer human oral commensal bacterium Streptococcus mitis has unique biologic features that make it an attractive mucosal vaccine or therapeutic delivery vector. S. mitis is safe as a natural persistent colonizer of the mouth, throat and nasopharynx and the oral commensal bacterium is capable of inducing mucosal antibody responses. A recombinant S. mitis (rS. mitis) that stably expresses HIV envelope protein was generated and tested in the germ-free mouse model to evaluate the potential usefulness of this vector as a mucosal vaccine against HIV. Oral vaccination led to the efficient and persistent bacterial colonization of the mouth and the induction of both salivary and systemic antibody responses. Interestingly, persistently colonized animals developed antigen-specific systemic T cell tolerance. Based on these findings we propose the use of rS. mitis vaccine vector for the induction of mucosal antibodies that will prevent the penetration of the mucosa by pathogens such as HIV. Moreover, the first demonstration of rS. mitis having the ability to elicit T cell tolerance suggest the potential use of rS. mitis as an immunotherapeutic vector to treat inflammatory, allergic and autoimmune diseases.

Contribution of glucan-binding protein A to firm and stable biofilm formation by Streptococcus mutans

Glucan-binding proteins (Gbps) of Streptococcus mutans, a major pathogen of dental caries, mediate the binding of glucans synthesized from sucrose by the action of glucosyltransferases (GTFs) encoded by gtfB, gtfC, and gtfD. Several stress proteins, including DnaK and GroEL encoded by dnaK and groEL, are related to environmental stress tolerance. The contribution of Gbp expression to biofilm formation was analyzed by focusing on the expression levels of genes encoding GTFs and stress proteins. Biofilm-forming assays were performed using GbpA-, GbpB-, and GbpC-deficient mutant strains and the parental strain MT8148. The expression levels of gtfB, gtfC, gtfD, dnaK, and groEL were evaluated by reverse transcription-quantitative polymerase chain reaction (RT-qPCR). Furthermore, the structure of biofilms formed by these Gbp-deficient mutant strains was observed using confocal laser scanning microscopy (CLSM). Biofilm-forming assay findings demonstrated that the amount formed by the GbpA-deficient mutant strain (AD1) was nearly the same as that by the parental strain, while the GbpB- and GbpC-deficient mutant strains produced lower amounts than MT8148. Furthermore, RT-qPCR assay results showed that the expressions of gtfB, dnaK, and groEL in AD1 were elevated compared with MT8148. CLSM also revealed that the structure of biofilm formed by AD1 was prominently different compared with that formed by the parental strain. These results suggest that a defect in GbpA influences the expression of genes controlling biofilm formation, indicating its importance as a protein for firm and stable biofilm formation.

Comparison of glucan-binding proteins in cariogenicity of Streptococcus mutans

Streptococcus mutans has been implicated as a primary causative agent of dental caries in humans. Bacterial components associated with the adhesion phase of S. mutans include cell-associated and cell-free glucosyltransferases (GTFs), as well as protein antigen c and proteins that bind glucan. At least four types of S. mutans glucan-binding protein (Gbp) have been identified; GbpA, GbpB, GbpC and GbpD. In the present study, GbpA-, GbpB- and GbpC-deficient mutants (AD1, BD1 and CD1, respectively) were constructed, and their cariogenic properties were evaluated by comparing them to those of their parent strain MT8148. All of the Gbp mutants showed lower levels of dextran binding, while the sucrose-dependent adhesion levels of AD1 and CD1 were lower than in the parental strain. The expression of each GTF was detected in the Gbp mutants, however, they had lower levels of cell-free-GTF activity than the parental strain. On the other hand, in acid tolerance assays, BD1 was the most sensitive among all of the tested strains. These results suggest that GbpA and GbpC in S. mutans have strong relationships with cariogenicity, while GbpB may have another biological function.

Multiple antibiotic resistances of Enterococcus isolates from raw or sand-filtered sewage

Fifty antibiotic-resistant Enterococcus strains were isolated from raw sewage of a wastewater treatment plant and from the same sewage after trickling through a 25-cm sand column, which retained >99% of the initial population. All 50 Enterococcus isolates were resistant against triple sulfa and trimethoprim/sulfamethoxazole and none were resistant against vancomycin. Most of the isolates from raw sewage were resistant to more antibiotics than the isolates from sand column effluent. One Enterococcus isolate from raw sewage (no. 61) and one Enterococcus isolate from sand column effluent (no. 95) had ten antibiotic resistances each. Isolate no. 95 maintained its resistances in the absence of antibiotics during the whole study. It was compared with isolate no. 70, which was one of the isolates, being resistant only against the two sulfonamides. Phenotypically and biochemically, the two organisms were strains of Enterococcus faecalis. Sequence analysis of partical 16S rDNA allowed alignment of isolate no. 95 as a strain of Enterococcus faecium and of isolate no. 70 as a strain of E. faecalis. E. faecium strain no. 95 carried at least six different plasmids, whereas for E. faecalis strain no. 70, no discrete plasmid band was seen on the gels.

Contribution of cell surface protein antigen PAc of Streptococcus mutans to bacteremia

Streptococcus mutans, a major cariogenic bacterium, is occasionally isolated from the blood of patients with bacteremia and infective endocarditis. Mutant strains of S. mutans MT8148, defective in the major surface proteins glucosyltransferase (GTF) B-, C-, and D-, and protein antigen c (PAc), were constructed by insertional inactivation of each respective gene with an antibiotic resistant cassette. Susceptibility to phagocytosis was determined by analyses of interactions of the bacteria with human polymorphonuclear leukocytes, and the PAc-defective mutant strain (PD) showed the lowest rate of phagocytosis. Further, when PD and MT8148 were separately injected into the jugular veins of Sprague-Dawley rats, PD was recovered in significantly larger numbers and for a longer duration, and caused more severe systemic inflammation than MT8148, indicating that S. mutans PAc is associated with its systemic virulence in blood. Next, 100 S. mutans clinical isolates from 100 Japanese children and adolescents were analyzed by Western blotting using antisera raised against recombinant PAc, generated based on the pac sequence of MT8148. Four of the 100 strains showed no positive band and each exhibited a significantly lower phagocytosis rate than that of 25 randomly selected clinical strains (P < 0.01). In addition, three of the 100 strains possessed a lower molecular weight PAc and a significantly lower rate of phagocytosis than the 25 reference strains (P < 0.05). These results suggest that S. mutans PAc may be associated with phagocytosis susceptibility to human polymorphonuclear leukocytes, with approximately 7% of S. mutans clinical isolates possible high-risk strains for the development of bacteremia.

Binding of glucan-binding protein C to GTFD-synthesized soluble glucan in sucrose-dependent adhesion of Streptococcus mutans

Streptococcus mutans produces glucan-binding proteins (Gbp proteins) which promote the adhesion of the organism to teeth. Three Gbp proteins, GbpA protein, GbpB protein, and GbpC protein have been identified; however, the mechanism of adhesion between glucans and bacterial cell surfaces is unknown. We used glucosyltransferase (GTF)- and/or Gbp-deficient mutants to examine the role of GbpC protein in the sucrose-dependent cellular adhesion of S. mutans to glass surfaces. The wild-type strain MT8148 and a GbpA-deficient mutant strain displayed increased sucrose-dependent adhesion following the addition of rGTFD. However, a GbpC-deficient mutant strain demonstrated no changes in the level of sucrose-dependent adhesion in spite of the addition of rGTFD. Further, the binding of rGbpC protein to the glucan synthesized by rGTFD was significantly higher than that to the glucan synthesized by either rGTFB or rGTFC. These results suggest that GbpC protein may play an important role in sucrose-dependent adhesion by binding to the soluble glucan synthesized by GTFD.

Molecular analysis of the genes involved in the biosynthesis of serotype specific polysaccharide in the novel serotype k strains of Streptococcus mutans

We previously reported the new serotype k of Streptococcus mutans, which, compared to serotypes c, e, and f, features a drastic reduction in the length of the glucose side chain linked to the rhamnose backbone of the serotype specific polysaccharide. The 5' region of the rgpF gene of serotype k strains contains a distinctive nucleotide sequence, which suggests that an alteration of the rgpF gene in serotype k strains may explain the shortened glucose side chain. However, in the present study, expression of the rgpF gene of MT8148 (serotype c) in serotype k isolates was not found to lead to serotype conversion. Furthermore, mRNA expression of rgpE, known to be associated with glucose side chain formation, was not detected in any of the tested serotype k isolates with an RT-PCR method. The nucleotide alignment of all genes known to be involved in the biosynthesis of serotype specific polysaccharide in serotype k strains was shown to be quite similar to that of serotype c strains, as compared to serotype e and f strains, especially in the region downstream of rgpF. Our results indicate that the common characteristics of serotype k isolates may be caused by a lack of expression of the gene involved in glucose side chain formation.

Contribution of biofilm regulatory protein A of Streptococcus mutans, to systemic virulence

Streptococcus mutans is occasionally isolated from the blood of patients with bacteremia and infective endocarditis (IE), and the possibility that it could be pathogenic for those diseases has been discussed. The initial important step for the involvement of bacterial pathogens in the virulence of IE is thought to be survival in blood for an extended period. Recently, the brpA gene encoding biofilm regulatory protein A (BrpA) of S. mutans was cloned and sequenced, after which it was shown that inactivation of brpA in an isogenic mutant strain resulted in longer chain formation than in the parental strain. In the present study, a BrpA-defective isogenic mutant strain (MT8148BRD) was constructed from strain MT8148. In an analysis of its susceptibility to phagocytosis by human polymorphonuclear leukocytes (PMNs), the phagocytosis rate of MT8148BRD was shown to be significantly lower than that of MT8148 (P < 0.01). Next, strains with various chain lengths were produced by culturing MT8148 in media with various initial pH levels, which revealed that there was a statistically negative correlation between phagocytosis susceptibility and chain length (P < 0.01). Further, MT8148BRD was found to possess higher platelet aggregation properties than MT8148 (P < 0.05). In addition, injection of MT8148BRD into the jugular vein of specific pathogen-free Sprague-Dawley rats resulted in a longer duration of bacteremia, which prolonged systemic inflammation for a longer period than in those infected with MT8148. These results indicate that S. mutans BrpA is associated with virulence in blood, due to its correlation to phagocytosis susceptibility and platelet aggregation properties.

Demonstration of Streptococcus mutans with a cell wall polysaccharide specific to a new serotype, k, in the human oral cavity

Streptococcus mutans organisms are occasionally isolated from the blood of patients with infective endocarditis, though the mechanisms of invasion and survival remain to be elucidated. Two of four blood isolates from patients with bacteremia or infective endocarditis (strains TW295 and TW871) were serologically untypeable by immunodiffusion testing, which was due to a lack of the glucose side chain of the serotype-specific polysaccharide antigen of S. mutans. Immunodiffusion analyses using antisera against these strains demonstrated that 2 of 100 isolates from 100 subjects showed a positive reaction, while further analysis of 2500 isolates from 50 subjects revealed that all 50 isolates from a single subject were not reactive with anti-c, -e, and -f antisera, though they were reactive with anti-TW295 and -TW871 antisera. The oral isolates showed biological properties similar to those of the reference S. mutans strain MT8148, including high levels of sucrose-dependent adhesion and cellular hydrophobicity, along with expression of glucosyltransferases and a protein antigen, PA. We designated these organisms serotype k. A glucose side chain-defective mutant strain was then constructed by insertional inactivation of the gluA gene of strain MT8148, which showed biological properties similar to those of serotype k of S. mutans. Serotype k oral isolates were less susceptible to phagocytosis, as were the gluA-inactivated mutant of strain MT8148 and blood isolates. These results indicate that S. mutans serotype k strains are present in the oral cavity in humans and may be able to survive longer in blood owing to their low susceptibility to phagocytosis.

Attenuation of glucan-binding protein C reduces the cariogenicity of Streptococcus mutans: analysis of strains isolated from human blood

A blood isolate of Streptococcus mutans strain TW871 shows relatively low homology with MT8148, a reference oral isolate strain, and lacks the serotype-specific polysaccharide antigen, suggesting that other cell-surface structures correlate with cariogenicity. We compared cariogenicity of TW871 with MT8148 (serotype c) and blood isolate TW964 (serotype f) in rats. Strain TW871 showed significantly lower cariogenicity than MT8148 or TW964 and expressed significantly lower sucrose-independent cellular adhesion to saliva-coated hydroxyapatite and dextran-binding activity than strain MT8148. Strains TW871 and TW964 showed a defect in the gbpA gene by Southern hybridization analysis, while sequencing analysis revealed gbpC variation in TW871. These results suggest that variation in GbpC may alter cellular adherence properties and can be correlated with the cariogenicity of S. mutans in this strain.

Molecular, genetic, and functional analysis of the basic replicon of pVA380-1, a plasmid of oral streptococcal origin

The 4.2-kb cryptic plasmid pVA380-1 has been used as a vector for the cloning of antibiotic resistance genes directly in streptococci, and in the construction of Escherichia coli/Streptococcus shuttle vectors. The results of subcloning experiments located the basic replicon of pVA380-1 within a 2.5-kb region. The nucleotide base sequence of this region was determined and contained a single complete open reading frame (ORF) encoding a 237-amino-acid peptide with a predicted size of 29 kDa. This peptide and a region of the DNA molecule 5' to the ORF encoding it shared homology with the replication protein and plus origin, respectively, of the Staphylococcus aureus plasmid pUB110. Data from Tn5 mutagenesis and complementation studies indicated that the protein product of the ORF was required for pVA380-1 replication in streptococci. Deletion of a region of the basic replicon distal to the plus origin and ORF produced an unstable derivative, and resulted in the accumulation of single-stranded replicative intermediates, consistent with the loss of a minus origin. All of these results suggest that pVA380-1 replicates by a rolling circle mode, and is most closely related to the pC194 family of single-stranded DNA plasmids.

Genetic approaches to the study of oral microflora: a review

As the study of oral microorganisms intensified almost 2 decades ago, the application of genetic techniques resulted in important contributions to the understanding of this clinically and ecologically important group of bacteria. The isolation and characterization of mutants of cariogenic streptococci helped to focus attention on traits that were important in colonization and virulence. Such classic genetic approaches gave way to molecular genetic techniques, including recombinant DNA methodology in the late 1970s. Gene cloning systems and methods to move DNA into cells have been developed for oral streptococci. Many streptococcal genes thought to be important in colonization and virulence have since been cloned and their nucleotide sequence determined. Mutant strains have been constructed using defective copies of cloned genes in order to create specific genetic lesions on the bacterial chromosome. By testing such mutants in animal models, a picture of the cellular and molecular basis of dental caries is beginning to emerge. These modern genetic methodologies also are being employed to develop novel and efficacious cell-free or whole cell vaccines against this infection. Genetic approaches and analyses are now being used to dissect microorganisms important in periodontal disease as well. Such systems should be able to exploit advances made in genetically manipulating related anaerobes, such as the intestinal Bacteroides. Gene cloning techniques in oral anaerobes, Actinomyces and Actinobacillus, are already beginning to pay dividends in helping understand gene structure and expression. Additional effort is needed to develop facile systems for genetic manipulation of these important groups of microorganisms.

Transformation of Acholeplasma laidlawii with streptococcal plasmids pVA868 and pVA920

One limitation with studying mycoplasma genetics is the lack of cloning vectors. Studies were undertaken to determine whether streptococcal plasmids could replicate in Acholeplasma laidlawii, for the purpose of identifying potential vectors. Plasmids pVA868 and pVA920 contain the same origin of replication and tetracycline resistance determinant. pVA920 also contains an erythromycin resistance determinant not present in pVA868. A. laidlawii was transformed with plasmids pVA868 (13.7 kb) and pVA920 (12.2 kb), and isolated from the transformants were deletion derivatives of the parent plasmids having sizes of 3.7 and 10.3 kb, respectively. The tetracycline and erythromycin resistance markers functioned in A. laidlawii, and the deletion derivatives may be useful for development of mycoplasma vectors. However, difficulties may arise due to plasmid instability.

Tn916 insertional inactivation of multiple genes on the chromosome of Streptococcus mutans GS-5

Streptococcus mutans GS-5 was transformed with the Escherichia coli plasmid pAM150 containing the cloned streptococcal transposon Tn916. Southern blot analyses with the tetracycline-resistant determinant of Tn916 showed that Tn916 was inserted into the chromosome of S. mutans at a variety of different sites. Tn916 insertions resulted in the inactivation of genes that code for various steps in the biosynthesis of several different amino acids. Two auxotrophs which contained a single copy of Tn916 were shown to revert to prototrophy at frequencies of about 10(-8). All of the revertant prototrophs were susceptible to tetracycline, indicating regeneration of the functional gene by excision of Tn916.

Nucleotide sequence analysis of tetracycline resistance gene tetO from Streptococcus mutans DL5

Streptococcus mutans DL5, isolated from the dental plaque of a pig, was resistant to high levels of streptomycin (Sm, 20 mg/ml), erythromycin (Em, 1 mg/ml), and tetracycline (Tc, greater than 100 micrograms/ml), but contained no detectable plasmid DNA. The Smr and Emr determinants were cloned from cellular DNA on the self-replicating 5-kilobase-pair (kbp) EcoRI fragment of pAM beta 1 and the 4.2-kbp cryptic plasmid pVA380-1, respectively, by transformation of Streptococcus sanguis Challis. Helper plasmid cloning, with a Challis host containing pVA380-1, was required to clone the Tcr determinant of strain DL5 on this vector. A single-colony isolate of the original Tcr clone contained a hybrid plasmid, pDL421, composed of 2.6 kbp of vector DNA and 11.4 kbp of S. mutans DNA. Plasmid pDL421 did not hybridize to plasmids containing the streptococcal Tcr determinants tetL, tetM, and tetN. A shortened derivative of this hybrid plasmid, pDL422, missing a 4.9-kbp HincII fragment from the S. mutans DNA but still encoding Tcr, was obtained by subcloning in S. sanguis Challis. The Tcr gene was located in a 1,917-base-pair open reading frame (ORF) corresponding to a 72-kilodalton protein. The ORF exhibited 99.4% sequence identity with the 1,917-base-pair tetO gene from a strain of Campylobacter coli (W. Sougakoff, B. Papadopoulou, P. Nordmann, and P. Courvalin, FEMS Microbiol. Lett. 44:153-160, 1987). A 1.67-kbp NdeI fragment, internal to the ORF from strain DL5, as well as pDL421 hybridized under stringent conditions to DNA from 10 of 10 Tcr strains of C. coli and Campylobacter jejuni from human and animal sources, but not to DNA from Tcs isolates of these two species.

Cloning and characterization of the tetracycline resistance determinant of and several promoters from within the conjugative transposon Tn919

Tn919 is a 15- to 16-kilobase (kb) tetracycline resistance conjugative transposon that was originally isolated from Streptococcus sanguis FC1. The tetracycline resistance determinant (tet) was found on a 4.2-kb HindII fragment by in vitro deletion analysis. This fragment was subcloned to a pWV01 origin capable of directing replication in Escherichia coli, Bacillus subtilis, and Streptococcus lactis, and expression was observed in all three genera. In all cases, expression was weaker when only the 4.2-kb cloned fragment rather than the full transposon was present. The resistance gene is of the streptococcal tetM class and codes for a protein of approximately 70 kilodaltons. The restriction map resembles that of the tetM gene of Tn1545 (P. Martin, P. Trieu-Cuot, and P. Courvalin, Nucleic Acids Res. 14:7047-7058, 1986), which codes for a protein of 72.5 kilodaltons. A number of transposon-derived promoter-bearing fragments were also cloned and sequenced. These closely resemble the consensus sequence of E. coli and B. subtilis promoters. Fusion experiments with a truncated lacZ gene indicate the possibility of an open reading frame for one of the promoters.

Intergeneric and intrageneric conjugal transfer of plasmid-encoded antibiotic resistance determinants in Leuconostoc spp

Transfer of the broad-host-range resistance plasmids pIP501 and pAM beta 1 from Streptococcus faecalis to Leuconostoc dextranicum and Leuconostoc cremoris occurred between cells that were immobilized on nitrocellulose filters in the presence of DNase. Transfer of pIP501 to Leuconostoc spp. also occurred when Streptococcus sanguis and Streptococcus lactis were used as donors. In addition, transfer of pIP501 and pAM beta 1 was observed from L. cremoris and L. dextranicum transconjugants to S. sanguis and S. faecalis. Expression of the pAM beta 1 erythromycin and pIP501 erythromycin and chloramphenicol resistance determinants was essentially equivalent in donors and transconjugants. Frequencies of transfer generally ranged from 10(-4) to 10(-7) transconjugants per input donor cell. Intrageneric transfer of pIP501 and pAM beta 1 occurred between L. cremoris and L. dextranicum strains in the same approximate range. These data further extend the host range of pIP501 and pAM beta 1 and demonstrate another example of gene transfer in the genus Leuconostoc.

Characterization of genetic transformation in Streptococcus mutans by using a novel high-efficiency plasmid marker rescue system

We developed a marker rescue system for study of competence development and genetic transformation in Streptococcus mutans. The system involved the recombinational rescue of a tetracycline resistance (Tcr) determinant by a homologous, inactive locus (Tcs because of a small deletion). Streptococcal cells harboring this in vitro-prepared Tcs construct (pVA1208) were restored to Tcr when plasmid (pVA981) DNA was used as donor material. pVA981 contained the intact streptococcal Tcr locus and was unable to autonomously replicate in streptococci. Marker rescue with this system followed first-order kinetics and occurred at a frequency 8- or 160-fold higher than did transformation with homologous chromosomal or plasmid DNA, respectively. By using the rescue system, we were able to confirm that competence of S. mutans appeared to be inducible. This was indicated by a sequential increase and then decrease in Tcr transformation frequencies during growth in complex medium. Also, donor DNA binding was not sequence specific, since the recovery of Tcr transformants was reduced by increasing the concentrations of heterologous DNA. We investigated the fate of donor DNA and the kinetics of plasmid establishment in the transformation of S. mutans with plasmid DNA. Monomeric plasmid molecules transformed S. mutans as a second-order process, whereas multimeric plasmid DNA and chromosomal markers were recovered as a first-order process. Approximately 50% of the initially bound donor plasmid DNA was found to remain in a trichloroacetic acid-insoluble form. Our results suggested that molecular cloning in S. mutans would be conducted most efficiently by using helper plasmid systems or shuttle vectors and that gene transfer by transformation of S. mutans occurred in a manner similar to that observed in Streptococcus sanguis.

Conjugal transfer of plasmid-mediated antibiotic resistance from streptococci to Clostridium acetobutylicum

Three broad host-range MLS plasmids, pAM beta 1 (Em), pIP501 (Em, Cm) and pJH4 (Em, Sm, Km), were transferred by a conjugation-like process from Streptococcus faecalis to Clostridium acetobutylicum. The plasmids were stably maintained and antibiotic resistances were fully expressed in C. acetobutylicum, except for chloramphenicol resistance, which was poorly expressed, if at all. The C. acetobutylicum strains harbouring every plasmid could be used as donors in intraspecific matings with frequencies of approximately 1 X 10(-5) to 1 X 10(-6).

A conjugative transposon (Tn919) in Streptococcus sanguis

Streptococcus sanguis FC1, originally isolated from dental plaque, was found to be naturally resistant to tetracycline. Although no plasmid DNA could be detected, tetracycline resistance was transferable in filter matings to Streptococcus faecalis FA2-2. Again, no plasmid DNA was detectable in transconjugants, and the latter could donate tetracycline resistance to S. faecalis, S. sanguis, and Streptococcus lactis. The tetracycline resistance element was able to transpose to several sites on the S. faecalis hemolysin plasmid pAD1 and in each case resulted in a 15-kilobase insert. DNA filter blot hybridization studies showed that the element bears significant homology with the conjugative transposon Tn916. Designated Tn919, it was cloned into an Escherichia coli plasmid vector (pGL101) and, as has been shown for Tn916, excised readily in the absence of selective pressure.

Characterization and expression of a cloned tetracycline resistance determinant from the chromosome of Streptococcus mutans

A chromosomal tetracycline resistance (Tcr) determinant previously cloned from Streptococcus mutans into Streptococcus sanguis (Tobian and Macrina, J. Bacteriol. 152:215-222, 1982) was characterized by using restriction endonuclease mapping, deletion analysis, and Southern blot hybridization. Deletion analysis allowed localization of the Tcr determinant to a 2.8-kilobase region of the originally cloned 10.4-kilobase sequence. This cloned determinant hybridized to a representative of the tetM class of streptococcal Tcr determinants but not to representatives of the tetL and tetN classes and, like other tetM determinants, mediated high-level resistance to tetracycline and low-level resistance to minocycline. A portion (approximately 3 kilobases) of the isolated streptococcal fragment was subcloned into Escherichia coli, where it conferred resistance to tetracycline and minocycline. Two proteins with apparent molecular weights of 33,000 and 35,000, encoded by the S. mutans DNA, were synthesized in E. coli minicells. Insertion of DNA into a unique SstI site of the cloned S. mutans fragment resulted in inactivation of Tcr expression in E. coli and S. sanguis, as well as loss of production of both the 33,000- and 35,000-dalton proteins in E. coli minicells. Incubation of minicells in subinhibitory concentrations of tetracycline did not result in changes in the levels of synthesis of either protein. Our data suggest that at least one of these proteins is involved in the expression of Tcr.

Disseminated tetracycline resistance in oral streptococci: implication of a conjugative transposon

A DNA sequence specifying tetracycline resistance (Tcr) has been previously cloned from a clinical isolate of Streptococcus mutans designated U202 (J. A. Tobian and F. L. Macrina, J. Bacteriol. 152:215-222, 1982). We used this sequence as a molecular probe in studying the dissemination of Tcr among oral streptococcal species isolated from patients treated with tetracycline. Eleven strains (including S. sanguis I, S. sanguis II, S. mitis, and S. salivarius) from seven patients were examined by Southern blot analysis. Seven strains showed strong hybridization to the Tcr probe, two showed weak hybridization, and two did not display detectable hybridization. Based on previous characterization of the cloned sequence, our data suggest the dissemination of the tetM class of resistance determinants among these oral streptococci. One of the clinical S. sanguis I isolates studied was able to transfer its Tcr phenotype to other oral streptococci and to enteric streptococci in the absence of plasmid DNA. This transfer appeared to be conjugation-like on the basis of its insensitivity to DNase and its dependence on intimate cell-to-cell contact. Using the cloned Tcr sequence, we were able to study the progeny of the matings. Our data suggest that this resistance transfer element occupies a chromosomal location in streptococcal cells and that it strongly resembles the conjugative transposon Tn916 in its behavior.

Novel shuttle plasmid vehicles for Escherichia-Streptococcus transgeneric cloning

A novel plasmid vector that is able to replicate both in Escherichia coli and in Streptococcus sanguis is described. This 9.2-kb plasmid, designated pVA856, carries Cmr, Tcr, and Emr determinants that are expressed in E. coli. Only the Emr determinant is expressed in S. sanguis. Both the Cmr and the Tcr of pVA856 may be insertionally inactivated. This plasmid affords several different cleavage-ligation strategies for cloning in E. coli followed by subsequent introduction of chimeras into S. sanguis. In addition, we have modified a previously described E. coli-S. sanguis shuttle plasmid [pVA838; Macrina et al., Gene 19 (1982) 345-353], so that it is unable to replicate in S. sanguis. The utility of such a plasmid for cloning and selecting sequences enabling autonomous replication in S. sanguis is demonstrated.

Streptococcal R plasmid pIP501: endonuclease site map, resistance determinant location, and construction of novel derivatives

The streptococcal resistance plasmid pIP501 (30 kilobase pairs [kb]) encodes resistance to chloramphenicol (Cmr) and erythromycin (Emr) and is capable of conjugative transfer among numerous streptococcal species. By using a streptococcal host-vector recombinant DNA system, the Cmr and Emr determinants of pIP501 were localized to 6.3-kb HindIII and 2.1-kb HindIII-AvaI fragments, respectively. pIP501 was lost at a frequency of 22% in Streptococcus sanguis cells grown at 42 degrees C but was stable in cells grown at 37 degrees C (less than 1% frequency of loss). Sequences from a cryptic multicopy plasmid, pVA380-1, were substituted for the pIP501 Emr determinant in vitro, and the resulting recombinant plasmid, designated pVA797, was recovered in transformed S. sanguis cells. The replication of pVA797 was governed by the pVA380-1 sequences based on temperature-stable replication and incompatibility with pVA380-1-derived replicons. The self-ligation of partially cleaved HindIII pIP501 DNA fragments allowed the localization of a pIP501 region involved in autonomous plasmid replication. A small pIP501 derivative (pVA798) obtained from this experiment had a greatly increased copy number but was unstably inherited. Our data indicate that the sequences encoding the resistance determinants and some of the plasmid replication machinery are relatively clustered on the pIP501 molecule. The properties of pVA797 and pVA798 indicate that these molecules will enhance current streptococcal genetic systems from the standpoint of conjugative mobilization (pVA797) and gene amplification (pVA798).

A cloning vector able to replicate in Escherichia coli and Streptococcus sanguis

A plasmid that is able to replicate in both Escherichia coli and Streptococcus sanguis has been constructed by the in vitro joining of the pACYC184 (Cmr Tcr) and pVA749 (Emr) replicons. This plasmid, designated pVA838, is 9.2 kb in size and expresses Emr in both E. coli and S. sanguis. Its Cmr marker is expressed only in E. coli and may be inactivated by addition of DNA inserts at its internal EcoRI or PvuII sites. The pVA838 molecule also contains unique SalI, SphI, BamHI, NruI and XbaI cleavage sites suitable for molecular cloning. pVA838 may be amplified in E. coli but not in S. sanguis. We have used the pVA838 plasmid as a shuttle vector to clone streptococcal plasmid fragments in E. coli. Such chimeras isolated from E. coli were readily introduced into S. sanguis by transformation.