Horizontal gene transfer refers to the transfer of genes from one organism to another through a method other than reproduction. Genetic transformation, a form of horizontal gene transfer, involves the altering of a cell through the uptake of naked DNA. Naked DNA refers to DNA which has been released from lysed or disrupted cells and is taken up by a recipient cell. If a cell is able to take up naked DNA, they are referred to as competent. This finding is accredited to Frederick Griffith, a bacteriologist who conducted an experiment in which a nonpathogenic strain of Streptococcus pneumoniae was exposed to a heat-killed pathogenic strain of the same bacteria. ...view middle of the document...
If a cell did not lyse, it was heat-killed by the water during incubation. While the sample was being incubated, a TSY agar plate was divided into 5 sectors and labeled according to the solution each sector would contain. In the first sector, the lysed mixture of resistant cells (the source of DNA) was carefully transferred to the TSY agar plate using an inoculated loop. Using the same technique, both the R and S-strains of Acinetobacter were placed into their appropriate sectors. The preceding technique was used for the next sector, in which an S-strain of Acinetobacter was placed on the TSY agar plate and carefully topped with a loopful of DNA. In the fifth and final sector, an S-strain of Acinetobacter was placed on the plate followed by DNase, an enzyme that breaks down DNA, and finally DNA which was placed on top. For the next 48 hours, the plate was incubated at 37o C to allow time for growth to occur.
In the following session, the TSY agar plate was observed and the results were noted after they had been properly incubated (Figure 1). In order to test if gene transformation occurred, a TSY agar plate containing streptomycin, an antibiotic, was divided into four sectors and labeled similar to the aforementioned TSY agar plate with the exception of the DNA sector, which was omitted because it showed no growth. After the samples were transferred to the TSY + streptomycin plate, it was incubated for 48 hours at room temperature. Figure 2 illustrates the appearance of the TSY + streptomycin plate prior to incubation.
In the third and final session, the results from the incubated TSY + streptomycin plate were observed and recorded (Figure 3). If the sector containing S + DNA was able to grow on the plate, it would confirm that transformation had occurred because it was able to grow in the presence of an antibiotic.
When the TSY agar plate was observed after 48 hours at room temperature, growth was found on all sectors apart from the DNA sector (Figure 1). When the TSY + streptomycin plate was observed, growth was found on all sectors apart from the sector containing the S-strain.
The experiment was successful in showing gene transformation. Visible growth of the S + DNA sector on the TSY + streptomycin plate confirmed the hypothesis that when mixed with DNA from lysed pathogenic Acinetobacter cells, susceptible or sensitive cells transformed to resistant cells which could withstand the presence of streptomycin. These...