Microbiology: Bacterial Transduction

Transduction

• Transduction is the transfer of DNA from one bacterium to another via a bacteriophage, a virus that specifically infects bacterial cells.
• Bacteriophages that can transfer bacterial DNA include P22, which infects Salmonella typhimurium, and P1, which infects Escherichia coli.
• The process of transduction involves a donor cell with a his+ gene on a chromosome fragment being packaged into a phage particle, which then infects a recipient cell, making it recombinant and his+.

Transformation

• Transformation is the process by which a bacterium takes up extracellular DNA released by a dead bacterium.
• It was discovered by Frederick Griffith in 1928 while working with strains of Streptococcus pneumoniae.
• There are two types of transformation: natural and artificial.
• Natural transformation occurs naturally in a wide variety of bacteria, while artificial transformation occurs with the help of experimental techniques.
• Competent cells are able to take up DNA and have genes that encode proteins called competence factors, which facilitate the binding, uptake, and incorporation of DNA into the bacterial chromosome.

Steps of Transformation

• Extracellular DNA is broken down into smaller fragments, and one strand is degraded and transported into the competent bacterial cell.
• Homologous recombination can occur, leading to the repair of any mismatched regions.
• Transformed cells can now take up the new DNA.

Recombination

• If the transformed DNA is homologous to a region in the bacterial chromosome, it can be incorporated by homologous recombination.
• If the DNA is not homologous, it may be incorporated at a random site on the chromosome through nonhomologous recombination.

Taking up DNA from the same species

• Some bacteria preferentially take up DNA released from dead bacteria of the same or a related species.
• Streptococcus pneumoniae secretes a competence-stimulating peptide, leading to competence only when many cells of the same species are nearby.
• Other species use DNA uptake signal sequences that are repeated throughout the genome and are preferentially taken up.

Artificial Transformation

• Laboratory methods are commonly used to get plasmids into cells, such as treating with calcium chloride and a high temperature shock or electroporation.
• These methods make the cells permeable to small DNA molecules.

Medical Relevance of Bacterial Genetic Transfer

• Horizontal gene transfer is the process in which an organism receives genes from another organism without being a direct offspring.
• Conjugation, transformation, and transduction are examples of horizontal gene transfer, which can occur within and between species.
• Horizontal gene transfer has contributed to the phenomenon of acquired antibiotic resistance, where bacteria acquire genes that break down antibiotics, pump them out of the cell, or block their inhibiting effects.

Transduction

• Transduction is the transfer of DNA from one bacterium to another via a bacteriophage, a virus that specifically infects bacterial cells.
• Bacteriophages that can transfer bacterial DNA include P22, which infects Salmonella typhimurium, and P1, which infects Escherichia coli.
• The process of transduction involves a donor cell with a his+ gene on a chromosome fragment being packaged into a phage particle, which then infects a recipient cell, making it recombinant and his+.

Transformation

• Transformation is the process by which a bacterium takes up extracellular DNA released by a dead bacterium.
• It was discovered by Frederick Griffith in 1928 while working with strains of Streptococcus pneumoniae.
• There are two types of transformation: natural and artificial.
• Natural transformation occurs naturally in a wide variety of bacteria, while artificial transformation occurs with the help of experimental techniques.
• Competent cells are able to take up DNA and have genes that encode proteins called competence factors, which facilitate the binding, uptake, and incorporation of DNA into the bacterial chromosome.

Steps of Transformation

• Extracellular DNA is broken down into smaller fragments, and one strand is degraded and transported into the competent bacterial cell.
• Homologous recombination can occur, leading to the repair of any mismatched regions.
• Transformed cells can now take up the new DNA.

Recombination

• If the transformed DNA is homologous to a region in the bacterial chromosome, it can be incorporated by homologous recombination.
• If the DNA is not homologous, it may be incorporated at a random site on the chromosome through nonhomologous recombination.

Taking up DNA from the same species

• Some bacteria preferentially take up DNA released from dead bacteria of the same or a related species.
• Streptococcus pneumoniae secretes a competence-stimulating peptide, leading to competence only when many cells of the same species are nearby.
• Other species use DNA uptake signal sequences that are repeated throughout the genome and are preferentially taken up.

Artificial Transformation

• Laboratory methods are commonly used to get plasmids into cells, such as treating with calcium chloride and a high temperature shock or electroporation.
• These methods make the cells permeable to small DNA molecules.

Medical Relevance of Bacterial Genetic Transfer

• Horizontal gene transfer is the process in which an organism receives genes from another organism without being a direct offspring.
• Conjugation, transformation, and transduction are examples of horizontal gene transfer, which can occur within and between species.
• Horizontal gene transfer has contributed to the phenomenon of acquired antibiotic resistance, where bacteria acquire genes that break down antibiotics, pump them out of the cell, or block their inhibiting effects.

Interrupted Conjugation Technique

• Developed by Elie Wollman and François Jacob in the 1950s
• Purpose: to map the order of genes along the E. coli chromosome
• Method: interrupting conjugations at different times to determine the order of genes
• The time it takes genes to enter the recipient cell is directly related to their order along the chromosome

Strains Used in Interrupted Mating

• Donor (Hfr) strain:
  • thr + : able to synthesize threonine
  • leu + : able to synthesize leucine
  • azi s : sensitive to azide
  • ton s : sensitive to T1 bacteriophage
  • lac + : able to metabolize lactose
  • gal + : able to metabolize galactose
  • str s : sensitive to streptomycin
• Recipient (F-) strain:
  • thr - : unable to synthesize threonine
  • leu - : unable to synthesize leucine
  • azi r : resistant to azide
  • ton r : resistant to T1 bacteriophage
  • lac - : unable to metabolize lactose
  • gal - : unable to metabolize galactose
  • str r : resistant to streptomycin

Goal of the Experiment

• Determine the times at which genes azi s, ton s, lac +, and gal were transferred
• Use streptomycin to kill the donor cell following conjugation

Results and Conclusion

• A genetic map was constructed based on the data
• The order of genes along the E. coli chromosome was determined
• Various Hfr strains with different origins of transfer were identified and compared to demonstrate the order of genes