Bacterial DNA Transfer Mechanisms

• Bacteria exist in a haploid state.
• There are four methods to exchange genetic fragments.

Transformation

• Transformation is the process where naked DNA fragments from a bacterium, released during cell lysis, bind to another bacterium’s cell wall.
• The recipient bacterium must be competent to bind and uptake the DNA intracellularly, and is usually of the same species as the donor.
• The incoming DNA can incorporate itself into the recipient's genome given enough homology between strands.
• The famous example of this experiment was conducted by Frederick Griffith in 1928.
• Griffith used smooth encapsulated pneumococci, which cause infection and is lethal to mice; as well as rough nonencapsulated pneumococci, which do not kill mice.
• Griffith heat-killed the smooth encapsulated bacteria and injected them, along with the live rough nonencapsulated bacteria, into mice.
• The mice died, and when cultured out of the blood, he found live smooth encapsulated pneumococci.
• The capsule gene released from the heat-killed bacteria was incorporated into the living rough nonencapsulated bacteria.
• Thus transforming the rough bacteria into virulent encapsulated smooth bacteria.

Transduction

• Transduction occurs when a bacteriophage carries a piece of bacterial DNA from one bacterium to another.
• Integrated temperate phage genome is called a prophage.
• Lysogenic immunity is the ability of the prophage to block subsequent infection by a similar phage.
• The first temperate phage to infect a bacteria produces a repressor protein, ensuring it is the sole occupant.

Generalized Transduction

• After phage penetration into a host bacterium, the phage DNA is transcribed, replicated, and translated into capsids and enzymes.
• At the same time, bacterial DNA is repressed and eventually destroyed. Sometimes pieces of the bacterial DNA are left intact.
• If these pieces are the same size as the phage DNA, they can accidentally be packed into the phage capsid head.
• After lysis of the cell and release of the phages, the phage with bacterial DNA in its head can then infect another bacterium.
• If there is homology between the new injected strand and the recipient bacterial genome, the piece may become incorporated.

Specialized Transduction

• There is an error in splicing. A piece of bacterial DNA that lies at one side of the prophage will be cut, replicated, and packaged with the phage DNA.
• This results in the transfer of that piece of bacterial DNA to another bacteria.
• Virulent phages participate in generalized transduction and temperate phages in specialized transduction.

Transposons

• Transposons are mobile genetic elements
• Transposons insert into the DNA of phages, plasmids, and bacterial chromosomes.
• Transposons do not replicate independently but are copied during their host's DNA transcription.
• When transposons leave the DNA they are incorporated in, there is often aberrant excision and the transposon can carry new DNA away to another site.
• Transposons are visualized as DNA pieces with legs.
• Pieces of DNA can insert themselves into a donor chromosome without DNA homology.
• Pieces of DNA can carry genes for antibiotic resistance and virulence factors.
• Clinically, a transposon gene that confers a particular drug resistance can move to the plasmids of different bacterial genera. This results in the rapid spread of resistant strains.

Conjugation

• DNA is transferred directly by cell-to-cell contact, resulting in a very efficient exchange of genetic information.
• Bacteria that carry F plasmids are F(+) cells.
• A bacterium must have a self-transmissible plasmid, or F plasmid, for conjugation to occur.
• In conjugation, an F(+) donor cell passes its F plasmid to an F(-) recipient cell, thus making the recipient F(+).
• The self-transmissible (F plasmid) has a gene that encodes enzymes and proteins that form the sex pilus.
• The long protein structure protrudes from the cell surface of the donor F(+) bacterium and binds to and penetrates the cell membrane of the recipient bacterium.
• After a conjugal bridge has formed, a nuclease cleaves one strand of the F plasmid DNA. This single strand of DNA passes through the sex pilus to the recipient bacterium.

Hfr Conjugation

• A rare occurrence: An extra-chromosomal F plasmid integrates into the neighboring bacterial chromosome much like a temperate bacteriophage.
• A bacterial cell is called a Hfr cell (high frequency of chromosomal recombinants) when this occurs.
• The integration can result in two unique mechanisms of DNA transfer
  • The F plasmid that is now with the circular bacterial DNA undergoes normal conjugation with an F(-) cell.
  • The entire bacterial chromosome, including, the integrated F plasmid transfers from the Hfr cell to the recipient cell.
  • The integrated F plasmid in the Hfr cell may be excised at a different site than that of integration This can result in an F plasmid also containing a segment of chromosomal DNA.
• These plasmids are called F' (F prime) plasmids.
• This F' conjugation is analogous to specialized transduction since, in both situations, a nearby segment of chromosomal DNA is picked up and can be transferred to other bacterial cells.

Plasmids

• Some plasmids are non-self-transmissible plasmids.
• These do not have the genes to direct conjugation.
• They replicate within their host bacterium and are passed on as bacteria divide in binary fission.

Explore bacterial DNA transfer mechanisms like transduction, transformation, and conjugation. Questions cover requirements for recipient bacteria, phage involvement, and DNA integration. Includes Griffith's experiment and virulence factors.