INTRASPECIFIC AND INTERSPECIFIC TRANSFORMATION IN STREPTOCOCCI

Lysis of grouped and ungrouped streptococci by lysozyme

Thirty strains of streptococci were tested for lysis with lysozyme, and 29 of these could be lysed by the following method: (i) suspension of the cells to a Klett reading of 200 units (no. 42 filter) in 0.01 m tris(hydroxymethyl)aminomethane buffer, pH 8.2, after washing twice with the buffer; (ii) addition of lysozyme to a final concentration of 250 mug/ml with incubation for 60 min at 37 C; (iii) addition of sodium lauryl sulfate (SLS) to a final concentration of 0.2% and incubation up to an additional 15 min at 37 C. Significant lysis was obtained only after the addition of SLS. (Strains of groups A, E, and G were treated with trypsin at a concentration of 200 mug/ml for 2 hr at 37 C before exposure to lysozyme.) These parameters for optimal lysis of streptococci by lysozyme were established by testing the group D Streptococcus faecalis strain 31 which lyses readily with lysozyme and the group H strain Challis which is less susceptible to the action of the enzyme. Viability of S. faecalis decreased 96% after 3 min of exposure to 250 mug of lysozyme per ml, whereas the more resistant strain Challis retained 27% of the initial viability after the same period. After 60 min, there was almost total loss of viability in each case. Variations of three methods of lysing streptococci with lysozyme were compared with respect to the decrease in turbidity and the release of protein and deoxyribonucleic acid (DNA) effected by each variation. The method presented in this paper allowed the greatest release of these cytoplasmic constituents from S. faecalis and strain Challis. Transformation experiments using DNA obtained from strain Challis (streptomycinresistant) by this method showed that the DNA released is biologically active.

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.

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.

Transformation of Streptococcus mutans with chromosomal and shuttle plasmid (pYA629) DNAs

Transformation (i.e., DNase-sensitive genetic transfer) of strains of Streptococcus mutans representing serotypes c and e was accomplished by using chromosomal DNA from a Rifr Strr Spcr isolate of strain GS5 (UAB525) and a chimeric plasmid, pYA629. Shuttle plasmid pYA629 comprises the S. mutans plasmid pVA318, an inducible erythromycin resistance determinant originally isolated from a group A streptococcal strain, the tetracycline resistance gene and replication region of the Escherichia coli plasmid pBR322, and the promoter region of the S. mutans gene for aspartate beta-semialdehyde dehydrogenase. The strains examined for recipient ability included those known to lack a cryptic plasmid (GS5, UA130, UA159, and MT8148) and those known to contain a widely disseminated 5.8-kilobase cryptic plasmid (LM7, V318, UA101, UA174, and 3098791). The transformation frequencies in GS5 for GS5 chromosomal antibiotic resistance markers were comparable to those reported by others, but UA101, UA130, UA159 and UA174 were transformed with both chromosomal and plasmid markers at much higher efficiencies. In a larger strain survey, strains containing the 5.8-kilobase cryptic plasmid were more frequently transformable with both chromosomal and pYA629 DNAs than were strains lacking this cryptic plasmid. All plasmid-containing strains except LM7 lost their resident cryptic plasmids when transformed with pYA629. LM7 transformed with pYA629 retained pLM7. There are therefore at least two incompatibility groups among S. mutans cryptic plasmids. yPA629 DNA isolated from either E. coli or S. mutans transformed S. mutans with equal efficiency. pYA629 DNA isolated from S. mutans transformed both restriction-deficient and restriction-proficient E. coli recipients. Therefore, the strains of S. mutans used lack a restriction-modification system for pYA629 DNA sequences. S. mutans strains that are readily transformable, display maximal cariogenicity in gnotobiotic rats, and give high scores for in vitro measures of important virulence attributes have been identified to facilitate studies on the genetic basis and control of virulence.

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.

Transformation of Streptococcus sanguis Challis by plasmid deoxyribonucleic acid from Streptococcus faecalis

Plasmid deoxyribonucleic acid (DNA) from Streptococcus faecalis, strain DS5, was transferred to the Challis strain of Streptococcus sanguis by transformation. Two antibiotic resistance markers carried by the beta plasmid from strain DS5, erythromycin and lincomycin, were transferred to S. sanguis at a maximum frequency of 1.8 x 10-5/colony-forming unit. Approximately 70% of the covalently closed circular DNA isolated from transformant cultures by dye buoyant density gradients was shown to be hybridizable to beta plasmid DNA. Two major differences were observed between the beta plasmid from S. faecalis and the plasmid isolated from transformed S. sanguis: (i) the beta plasmid from strain DS5 sedimented in velocity gradients at 43S, whereas the covalently closed circular DNA from transformed Challis sedimented at 41S, suggesting a 1.5-Mdal deletion from the beta plasmid occurred; (ii) although the 43S beta plasmid remained in the supercoiled configuration for several weeks after isolation, the 41S plasmid was rapidly converted to a linear double-stranded molecule. Attempts to transform S. sanguis with the alpha plasmid from S. faecalis, strain DS5, were unsuccessful.

Determination of the rate of transformation from growth curves of transformed streptococci

A rapid method of determining the rate of transformation of group H streptococci to streptomycin resistance has been developed. The new technique, which involves analysis of the growth response of transformed streptococci in liquid medium containing streptomycin, is independent of chain length fluctuations and is demonstrably more accurate than the standard plating method. The relatively short generation time of streptococci under these conditions permits transformation rates to be estimated in 9 to 14 hr depending on the number of transformants in the inocula as compared to 50 hr by the agar plate procedure.

Binding of deoxyribonucleic acid by cell walls of transformable and nontransformable streptococci

Cell walls isolated from competent streptococci (group H strain Challis) were shown to bind more homologous and heterologous deoxyribonucleic acid (DNA) than noncompetent walls. Heat- and alkali-denatured DNA was not bound by either wall preparation. Pretreatment of cell walls with cetyltrimethylammonium bromide sharply increased the binding of DNA but did not increase transformation of whole cells. Pretreatment of the walls with either sodium dodecylsulfate, deoxyribonuclease and ribonuclease, or with crude competence-provoking factor did not affect the binding of DNA. Antiserum prepared against whole competent cells completely blocked transformation and also inhibited DNA binding to competent cell walls. Adsorption of this antiserum with competent Challis cells removed its blocking action for both binding and transformation. Pretreatment of walls with trypsin and Pronase destroyed their ability to bind DNA. Trypsin treatment also blocked transformation in whole cells. The transforming activity of DNA bound to cell walls was found to be protected from deoxyribonuclease action. Significant differences were observed in the arginine, proline, and phenylalanine content of competent and noncompetent walls. With few exceptions, the amino acids released from competent cell walls by trypsin were several-fold greater than from noncompetent walls. The results indicate that (i) two binding sites exist, one in competent cells only and essential for subsequent transformation, and a second, present in all cells, which is not involved in transformation; (ii) both sites are protein in nature; (iii) the transformation site is blocked by antibody; and (iv) the competent cell wall possesses tryptic-sensitive protein not present in the noncompetent wall.

Induction of hemolysin synthesis by tranformation in Staphylococcus aureus

A nonhemolytic strain of Staphylococcus aureus was transformed with deoxyribonucleic acid extracted from two hemolytic strains of S. aureus. In each case the hemolysin pattern after transformation was identical to that of the donor strain. However, bacteriophage type, serotypes, and other biological properties of the recipient strain remained unaffected.

Transformation of type polysaccharide antigen synthesis and hemolysin synthesis in streptococci

Transformation of the ability to synthesize type polysaccharide antigen and beta-hemolysin has been obtained in group F streptococci. Colonies possessing cells transformed to antigen synthesis were detected on the agar surface with fluorescein-labeled anti-type serum. This selection method, in contrast to those with antibiotics, allowed both transformed and nontransformed cells to grow, resulting in sectored colonies. These colonies could be subcultured to further establish the synthesis of antigen. Group F, group A, and group-like z deoxyribonucleic acid (DNA) labeled with type II antigen and hemolysin, and streptomycin resistance transferred each marker to a group F strain lacking a type antigen. DNA from group F and z3 strains labeled with type III antigen, and streptomycin resistance transferred both markers to group F and z3 strains lacking type antigen. A second F strain without type antigen was not transformed with any of these markers. A group H strain was transformed to streptomycin resistance only by the same types of DNA. Transformation to type II antigen synthesis always resulted in the formation of beta-hemolysin. All strains isolated from natural sources contained both markers. A mutant, obtained by nitrosoguanidine treatment of an FII(sr) strain, did not synthesize either the hemolysin or the antigen. This mutant still possessed the group antigen and streptomycin resistance. A close linkage of type II antigen and beta-hemolysin is indicated. The fluorescent-antibody staining of cells containing both group and type antigens showed a more intense ultraviolet adsorption for type than group antigen. A surface location (microcapsular) for the type antigen appeared likely. These results are of interest for studies on antigen biosynthesis, genetics, and classification of the streptococci.

Genetic differentiation by nucleic acid homology. IV. Relationships among Lancefield groups and serotypes of streptococci

Weissman, Sherman M. (National Cancer Institute, Bethesda, Md.), Paul R. Reich, Norman L. Somerson, and Roger M. Cole. Genetic differentiation by nucleic acid homology. IV. Relationships among Lancefield groups and serotypes of streptococci. J. Bacteriol. 92:1372-1377. 1966.-A deoxyribonucleic acid-ribonucleic acid homology technique was used to measure the genetic relatedness among Lancefield groups and serotypes of streptococci. Organisms in different Lancefield groups were related, but were differentiated from each other. Serotypes within group A could be distinguished. Two isolates serologically classified in group A type 1 were indistinguishable. Streptococcus faecalis was only remotely related to streptococci of Lancefield groups A and H, whereas Diplococcus pneumoniae was more closely related to these organisms. The serological data were in good agreement with the results obtained with the homology method.

Effect of filtrates from transformable and nontransformable streptococci on the transformation of streptococci

Perry, Dennis (Northwestern University Medical School, Chicago, Ill.), and Hutton D. Slade. Effects of filtrates from transformable and nontransformable streptococci on the transformation of streptococci. J. Bacteriol. 91:2216-2222. 1966.-The nature of the transformation competence factor from a group H streptococcus was investigated. The activity of competence factor reached a maximum at the time that optimal competence was attained, the maxima of both occurring in the early log phase of growth. The decrease in competence factor was much more gradual than the decrease in number of competent cells. No inhibitor, however, was detected as being responsible for the decrease in either competent cells or competence factor activity. Efforts to induce transformation in other serological groups of streptococci with the use of group H competence factor were unsuccessful. The development of competence in group H when grown in the presence of nontransformable group A strains resulted in a significant increase in the number of transformants. Culture filtrates from early log phase group A cells also caused an increase in the number of transformants from the group H strain. The addition of 10(-4)m ethylenediaminetetraacetic acid to group A (or group H) culture filtrates caused significant increases in the number of transformants. These results thus indicate that group A streptococci, although nontransformable, produce low levels of "competence factor." Late culture filtrates from the group H streptococcus and several strains of group A streptococci possessed deoxyribonuclease-like activity which inhibited the transformation of the group H strain. This activity in the A filtrates, however, was not prevented by group A anti-deoxyribonuclease sera. Instead, these sera also inhibited transformation. Evidence indicates that the lack of transformation of group A streptococci is due to factors other than the production of deoxyribonuclease.

Transformation of auxotrophic mutants of group H streptococci

Stable auxotrophic mutants of a group H streptococcus (strain Challis) were isolated on a modified Mickelson defined medium after exposure to N-methyl- N-nitro-N-nitrosoguanidine. Such mutants were transformed to both prototrophy and streptomycin-resistance and hence may be used as nutritional markers in the study of streptococcal genetics.