Microbiology

Bacteria Horizontal Gene Transfer



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Although bacterial reproduction results in the generation of clones (vertical gene transfer), prokaryotes can undergo genetic recombination through the horizontal gene transfer methods of transformation, transduction, and conjugation.

* Vertical Gene Transfer (VGT) *

Bacteria reproduce through binary fission, a process of asexual reproduction in which the bacterium divides, producing an exact copy. When organisms replicate their genomes and provide copies to descendants (future generations), this is considered vertical gene transfer.

* Horizontal Gene Transfer (HGT) *

HGT is a process in which a prokaryotic cell can acquire genes from other microbes of the same generation, which, in some cases, can be a different species, or even a different genus than the donor. There are three types of horizontal gene transfer.

* Transformation
* Transduction
* Conjugation

Bacterial Transformation: This is the process in which a recipient cell takes up DNA from the environment (such as DNA released from a dead organism). In 1928, Frederick Griffith discovered transformation while trying to develop a vaccine for pneumonia caused by Streptococcus pneumoniae. Griffith was working with two strains of Strep. Colonies of the first strain appeared shiny and smooth (strain S) because the bacteria had a protective capsule, and this strain caused deadly pneumonia when injected into mice.

Colonies of the second strain had a rough appearance (strain R) because they were mutants that could not make the protective capsule. This strain did not cause deadly disease because the white blood cells of the mice could easily destroy these unprotected bacteria.

Griffith did a number of experiments in which he demonstrated that the live R strain cells could acquire the smooth, virulent feature when exposed to dead S strain cells. His experiments indicated that the R strain was taking up genetic material of the dead, broken down S strain bacteria.

Bacterial Transduction: Another method of horizontal gene transfer is called transduction. This process involves the transfer of DNA from one cell to another via a replicating virus. Transduction can occur between prokaryotic cells or between eukaryotic cells. The following is an example of transduction in bacterial cells by bacteriophage (phage) virus.

When a phage is being replicated inside a host cell, the new viruses self-assemble from proteins and viral nucleic acid (genetic material) that the host cell has produced. Sometimes some of the DNA of the host, which had been chopped up during the lytic replication process, gets inside a new virus during viral self-assembly. When that phage then infects another cell, the new host may incorporate the donated DNA into its chromosome by recombination.

Bacterial Conjugation: The third method of gene transfer in prokaryotes is called conjugation. Unlike the processes of transformation and transduction, the donor cell is not killed during this type of gene transfer. This process involves one bacterium making a copy of a portion of DNA, called a plasmid, and transferring that copy to another bacterium.

A plasmid is a small, circular molecule of DNA that replicates independently of the bacterial chromosome. Plasmids carry information required for their own replication, and often for one or more cellular traits which are not essential for normal bacterial metabolism, growth, or reproduction, but confer survival advantages to the bacteria that possess plasmids.

Plasmid copies can be transferred during the process of conjugation, during which the donor bacterium extends a rod-like conjugation pilus that connects with the recipient bacteria. The plasmid is transferred via this extension. After conjugation the bacteria separate. Conjugation is about as close as bacteria can get to having sex.

* Sources *

Bauman, R. (2005) Microbiology.
Park Talaro, K. (2008) Foundations in Microbiology.

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