Team:Paris Bettencourt/Human Practice/HGT

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Introduction: Cells, Bacteria & DNA

The building block of any living organism is the cell (just like the building block of a house is the brick). Human and plants are complex organisms: they are made up of many cells. Bacteria are more simple organisms: they are made up of just one cell.

In every cell, we can find some ‘genetic information’. This information (coupled to environmental stimuli) will tell the cell how to function.

Let’s looks specifically at bacteria. A bacteria is made up of one cell that contains genetic information to tell it how to function. This genetic information is called DNA, and is organized in the following way: Most of it is condensed in the form of a stick called a chromosome. A small part of it can exist as a plasmid: a small circular fragment that can be easily exchanged between bacteria. This is for example how bacteria exchange information on antibiotic resistance: they exchange plasmids containing this information.

Bacteria are living organisms so they are able to feed, move, and reproduce. All these functions are encoded in the bacteria’s DNA. A bacteria will reproduce by the process called ‘asexual reproduction’: it will grow a bit, duplicate its genetic information, and then split in two identical clones: one ‘mother cell’ gives two identical ‘daughter cells’. This type of reproduction does not call for much diversity. Yet, diversity is essential for evolution and survival and there is in fact a lot of it in bacterial population. Horizontal gene transfer (and mutations) contributes to this diversity.

A general explanation of horizontal gene transfer

    A Bacteria can pass some fragments of its genetic information to other bacteria. This is called Horizontal Gene Transfer (HGT). This mechanism enables bacteria to exchange information on how to resist to certain antibiotic (antibiotic resistant genes) for example. Exchange of genetic information, or HGT, can happen in 3 different ways, called: transformation, conjugation and transduction.

• Transformation: Once a bacteria dies, its limiting membrane loses its integrity and its genetic information gets degraded into smaller fragments. These fragments can remain in the environment for a long time. If they come in contact with a living bacteria, they can sometime penetrate the living bacteria and be integrated to its chromosome.

• Conjugation: Two living bacteria come in direct contact via a type of bridge called a ‘pillus’. Through this pillus, one of the bacteria can give its plasmid to the other bacteria.

• 'Transduction: a phage, which is a virus that operates in bacteria, will take some genetic information from a bacteria and transfer it to another bacteria (just like a mosquito can transmit a disease from one human being to another when it feeds on them).

Source for the image HYPERLINK "http://science.kennesaw.edu/~jdirnber/Bio2108/Lecture/LecBiodiversity/BioDivProkaryotes.html" http://science.kennesaw.edu/~jdirnber/Bio2108/Lecture/LecBiodiversity/BioDivProkaryotes.html

A more technical and historical explanation of HGT

    Fred Griffith discovered Horizontal Gene Transfer (HGT) in 1928 when he reported the transfer of genetic material from heart-killed virulent Streptococcus pneumoniae to an avirulent form of the bacterium by a process he described as transformation (Bushman, 2002). 

    Much later, in 1946, other forms of HGT, such as conjugation and transduction were discovered.

    In 1980, the terms “horizontal gene transfer” and “lateral gene transfer” appeared (Gogarten et al, 2002; Kooning et al, 2001; Ochman et al, 2000; Syvanen, 1994), to designate all form of gene exchange that occurs between organisms in nature without recourse to reproduction [26].

Source: HYPERLINK "http://textbookofbacteriology.net/HorizontalTransfer.gif"http://textbookofbacteriology.net/HorizontalTransfer.gif

• Transformation is the uptake of free DNA by competent bacteria. Competence is a physiological state, often highly dependent of the environment, which enables bacteria to uptake macromolecules that bind to its surface. It does not require a close contact between bacteria and the free DNA usually comes from long dead ones. In the presence of homologies, there can be recombination and integration of the free DNA into the bacterial genome. This can give the bacteria a new function, or disrupt an already existing one, depending on where the insertion takes place.

• Conjugation consists in gene transfer by the means of plasmids and conjugative pili.

It requires a close contact between bacteria. It occurs primarily between closely-related strains or species, although it can occur between distantly-related species. The donor needs to possess the capacity to create conjugative pili, a capacity not shared by all bacteria. Conjugation is very wide spread between bacteria and is the most important mechanisms for translocating DNA between bacteria (Espinose-Urgel, 2004; Grohmann et al, 2003). In addition, conjugation is also used by certain phytopatogen such as Agrobacterium to insert DNA into plants cells as part of the infectious process [26]

Schematic drawing of bacterial conjugation

Drawing: HYPERLINK "http://en.wikipedia.org/wiki/Bacterial_conjugation"http://en.wikipedia.org/wiki/Bacterial_conjugation Caption: HYPERLINK "http://www.slic2.wsu.edu:82/hurlbert/micro101/pages/Chap9.html"http://www.slic2.wsu.edu:82/hurlbert/micro101/pages/Chap9.html

• Transduction: gene transfer mediated by a bacteriophage, that is by a virus that is specific to bacteria [22]

HGT plays a key part in bacterial evolution and has enabled bacterial populations to occupy entirely new niches (Burrus and Waldor, 2004). For example, under an intense selection pressure due to wide use of antibiotics, multi resistant plasmid arose from various origins and antibiotic resistance, mainly from these mobile genetic elements, spread in less than 5 decades (Hartl and Dykhuizen, 1984; Davies, 1994) [27].