Transformation of streptococci to streptomycin resistance by oral streptococcal DNA

Bacteriocin production and sensitivity among coaggregating and noncoaggregating oral streptococci

Twenty-one oral Streptococcus isolates of known interbacterial coaggregation groups were tested against one another (as both producers and indicators) to detect bacteriocin-like inhibitory activity. In agar-based antagonism tests, seven strains produced small inhibitory zones (< or = 3 mm diameter) but in liquid medium, only strain Streptococcus gordonii DL1 (Challis) produced a detectable antibacterial action (bacteriocin STH1). Five strains were sensitive to bacteriocin STH1, but neither the production of nor the sensitivity to any of the antagonistic agents correlated with coaggregation groupings. Four strains (C219, 903, 118 and Wicky) developed stable resistance in response to the bacteriocin, whereas one isolate (strain 34) remained sensitive following repeated bacteriocin exposure. With one exception (strain 903), bacteriocin STH1-sensitive strains were competent for genetic transformation, but not all competent strains were bacteriocin-sensitive. Bacteriocin-resistant derivatives of transformable strains exhibited decreased competence (80-90% reduction) compared with their parent strains.

Method and parameters for genetic transformation of Streptococcus sanguis Challis

A simple procedure for genetic transformation of Streptococcus sanguis Challis was developed and standardized. During the exponential phase of growth, cells became competent while growing as diplococci in broth containing 10% foetal calf serum. High levels of competence were maintained by the cultures for 60 min. Competent cells could be stored frozen without loss of competence for at least three years. Using total chromosomal DNA as donor, the dose-response curve for transformation of a point mutation (streptomycin resistance) showed one-hit kinetics, as the DNA concentration varied from 0.000001 to 10 micrograms/ml. At 10 micrograms/ml, more than 2.2% of the colony-forming units were transformed to streptomycin resistance, while transforming activity remained detectable with 1 pg of DNA/ml. Optimal time of exposure of competent cells to transforming DNA was 30 min. The transformation reaction was inhibited at 0 and 4 degrees C, whereas it occurred efficiently both at 25 and 37 degrees C.

Sorbitol-fermenting predominant cultivable flora of human dental plaque in relation to sorbitol adaptation and salivary secretion rate

The sorbitol-fermenting predominant flora of human dental plaque was studied in 12 people with low and 11 with normal salivary secretion rates before and after a period of frequent mouth rinses with sorbitol solution. A total of 277 sorbitol-fermenting isolates are described by their morphological and physiological characteristics. The flora was almost exclusively composed of gram-positive bacteria belonging to the genera Streptococcus, Lactobacillus and Actinomyces in people with low secretion rates and Streptococcus and Actinomyces in those with normal salivation. At the species level, Streptococcus mutans predominated in all. The frequent use of sorbitol resulted in an increase of the streptococcal species mainly and particularly of Streptococcus sanguis I in people with normal salivation. The counts of lactobacilli and Actinomyces remained unaffected. Almost all isolates appeared to belong to the resident plaque flora. Succession of new sorbitol-fermenting genera to the plaque community could not be observed as a result of the frequent exposure of the mouth to sorbitol.

Genetic relationships among the oral streptococci

Genetic relationships and species limits among the oral streptococci were determined by an analysis of electrophoretically demonstrable variation in 16 metabolic enzymes. Fifty isolates represented 40 electrophoretic types, among which the mean genetic diversity per locus was 0.857. Mannitol-1-phosphate dehydrogenase was not detected in isolates of the sanguis species complex, and glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase were absent in species of the mutans complex. Clustering from a matrix of Gower's coefficient of genetic similarity placed the 40 electrophoretic types in 10 well-defined groups corresponding to the Streptococcus species S. mutans, S. sobrinus, S. cricetus, S. rattus, S. ferus, S. oralis (mitior), two distinct assemblages of S. sanguis strains, and two subdivisions of "S. milleri." The assignments of isolates to these groups were the same as those indicated by DNA hybridization experiments, and the coefficient of correlation between genetic distance estimated by multilocus enzyme electrophoresis and genetic similarity indexed by DNA hybridization was -0.897 (P less than 0.001) for 50 pairwise combinations of isolates. S. ferus, which is widely believed to be a member of the mutans complex, was shown to be phylogenetically closer to species of the sanguis complex.

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.

Detection of streptococcal mutants presumed to be defective in sugar catabolism

The tetrazolium method for detection of bacterial mutants defective in sugar catabolism was modified for use with streptococci. The critical factors were (i) the concentration of tetrazolium, which must be titrated to determine the optimum concentration for each species or even strain, and (ii) anaerobic incubation of tetrazolium-containing agar plates. When used with standard mutagenesis protocols, this method yielded lactose-negative mutants of nine streptococcal strains representing six species. A collection of lactose-negative mutants of streptococcus, sanguis Challis was characterized and contained phospho-beta-galactosidase, lactose phosphotransferase, and general phosphotransferase mutants.

Prevalence of transformable Streptococcus mutans in human dental plaque

A total of 100 strains of Streptococcus mutans serotypes c/e/f and d/g, freshly isolated from dental plaque, were screened for their ability to undergo genetic transformation to streptomycin resistance. Of the serotype c/e/f strains, 28% were found to be transformable, whereas none of the serotype d/g strains could be transformed by donor DNA from streptomycin-resistant S. mutans strains of either serotype c or d/g. Two of the transformable serotype c/e/f strains were transformed by DNAs from a variety of oral streptococcal species commonly found in the microflora.

Implantation of transformant strains of the bacterium Streptococcus sanguis into adult human mouths

Streptococcus sanguis strains isolated from dental plaque of 12 subjects were screened for their ability to undergo genetic transformation using a streptomycin-resistance marker. All subjects harboured at least one transformable strain. Eight of the subjects were implanted with transformant strains originally isolated from their own mouth, whereas four subjects received bacteria from other donors. The Strep. sanguis transformants became successfully implanted; their oral levels remained virtually unchanged during the 3-month experimental period. Their oral establishment did not seem to be related to whether the subjects were implanted with their own transformant strains or not. The implanted Strep. sanguis could be recovered both from teeth, oral soft tissues and throat.

Colonization and cariogenic potential in hamsters of the bacterium Streptococcus sanguis isolated from human dental plaque

Strains of Strep. sanguis, freshly isolated from human dental plaque, were successfully implanted into albino hamsters. Transmission of the organisms from infected to uninfected animals occurred naturally. The transfer was as effective between unrelated hamsters as between dams and their offspring. Three of the strains tested did not cause caries in hamsters. Laboratory strains of Strep. sanguis did not colonize the hamsters. Two morphological variants of Strep. sanguis with different abilities to adhere to whale dentine in vitro, could infect hamsters; the more adhering phenotype was detected earlier and more frequently.

Influence of in vitro propagation on the adhesive qualities of Streptococcus mutans isolated from saliva

Streptococcus mutans from saliva of one test person were isolated on mitis/salivarius agar containing bacitracin and extra sucrose, and subcultured in vitro on trypticase soy broth with numerous transfers. Freshly isolated organisms and bacteria from selected subcultures were studied in adhesion tests using glass or dental zinc phosphate cement as substrate. Different bacterial isolates were used in separate experiments. Three different techniques were used; two based on radioactivity assays of adhering 3H-thymidine-labelled bacteria, and one based on visual counting. With all three techniques freshly isolated S. mutans adhered in significant numbers. It was consistently found that their ability to adhere decreased with the number of in vitro transfers. The results support the view that growth condition is an important factor governing the adhesive properties of oral bacteria.

Plasmids, drug resistance, and gene transfer in the genus Streptococcus

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Genetic transformation of Streptococcus mutans

Three strains of Streptococcus mutans belonging to serotypes a, c, and f were transformed to streptomycin resistance by deoxyribonucleic acids derived from homologous and heterologous streptomycin-resistant strains of S. mutans and Streptococcus sanguis strain Challis. Homologous transformation of S. mutans was less efficient than heterologous transformation by deoxyribonucleic acids from other strains of S. mutans.

In vitro development of chlorhexidine resistance in Streptococcus sanguis and its transmissibility by genetic transformation

Three chlorhexidine-sensitive strains of Streptococcus sanguis developed resistance to chlorhexidine when grown as continuous cultures in a fermenter containing medium with increasing concentrations of the drug. The MIC of the test strains increased by between two and three dilution steps and these high values were maintained after daily transfer in drug-free medium for at least 15 d. In addition, the resistance of one of the strains persisted for more than 1 month of continuous growth in drug-competent sensitive S. sanguis to increased chlorhexidine resistance, thus proving that resistance to this drug is an inheritable trait. Immunoelectrophoretic experiments showed that certain changes in the antigenic pattern of the Chxr variants had occurred as compared with their sensitive wild-type strains. Antigenic extracts of the Chxr variants and all Chxr recombinants tested contained consistently less protein than those of the sensitive wild-types.

Genetic transformation of Streptococcus sanguis (Challis) with cryptic plasmids from Streptococcus ferus

By using the basic methodology initially published by Kretschmer et al. (J. Bacteriol. 124:225-231, 1975), we have been able to introduce phenotypically cryptic plasmids from Streptococcus ferus (formerly Streptococcus mutans subsp. ferus) into Streptococcus sanguis by genetic transformation. In this system, the entry of the cryptic plasmids is selected indirectly. This is effected with transforming deoxyribonucleic acid mixtures in which the cryptic plasmid deoxyribonucleic acid is present in an approximate 10-fold molar excess with respect to a plasmid (pVA1) known to confer erythromycin resistance. Under such conditions, 5 to 10% of the pVA1-containing erythromycin-resistant transformants were cotransformed with cryptic plasmid deoxyribonucleic acid. pVA1 may be selectively eliminated by growth of its S. sanguis host strain at 42 degrees C, enabling the construction of isogenic strains with and without S. ferus cryptic plasmids. Comparative physiological studies of such strains have failed to reveal any plasmid-conferred phenotypes in S. sanguis. With this procedure, we have been able to physically separate two small cryptic plasmids (2.4 x 10(6) and 2.8 x 10(6) daltons) of S. ferus. Although these plasmids were found naturally to exist in a single S. ferus host, they were able to replicate independently of one another in S. sanguis. Restriction enzyme fingerprinting indicated that these plasmids did not share a common ancestry.

Biology, immunology, and cariogenicity of Streptococcus mutans

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Fluoride resistance and adherence of selected strains of Streptococcus mutans to smooth surfaces after exposure to fluoride

The fluoride resistance and smooth surface adherence characteristics of Streptococcus mutans were examined using tooth model and radioactive cell assays. Resistance to 600 ppmF by S. mutans isolated from the plaque of radiation-induced xerostomia patients receiving daily topical applications of a caries preventive 1% NaF gel was transient. Resistance induced in vitro in two strains of S. mutans by exposure to gradually increasing levels of NaF was apparently permanent. Smooth surface adherence by both fluoride-sensitive and -resistant strains of S. mutans 6715 in a tooth model system was slightly diminished by 1% NaF gel. Fluoride-resistant strains retained 89 to 93% of their adherence capability in 600 ppmF, as determined by the cell radiolabeling assay.

Effect of chlorhexidine on the relative proportions of Streptococcus mutans and Streptococcus sanguis in hamster plaque

The effect of chlorhexidine on the proportions of Streptococcus mutans and Streptococcus sanguis in plaque was studied in hamsters fed a diet containing 28% sucrose. In animals given chlorhexidine in their drinking water for 10 d a decrease in the population of S. mutans and an increase of S. sanguis occurred in the plaque. Following the removal of chlorhexidine the population of S. mutans increased again in the presence of sucrose and the number of S. sanguis returned to initial values. When animals were given a sucrose-free diet the low proportion of S. mutans observed following the short-term chlorhexidine period persisted. These data indicate that there is an inverse relationship between the number of S. sanguis and S. mutans in plaque and that the sensitivity in vivo of S. mutans to chlorheximide can be used to suppress the population of S. mutans with a concomitant rise in the proportion of S. sanguis.