posted on Mar, 9 2016 @ 06:46 PM
a reply to: Noinden
Non-resistant bacteria commonly become resistant by several different means, most of which have nothing to do with mutations. Palumbi notes that in
‘most cases’ antibiotic resistance results from selection of anexisting genetic trait, especially those traits that are highly variable, such as
the natural defences that all organisms possess.13 An important mechanism by which bacteria become resistant is by obtaining one or more specific
resistance genes from other bacteria. This type of resistance can be obtained by the transfer of a plasmid (small circular units of DNA), already
existing in the bacterium gene pool, that carries a gene for an enzyme which either destroys or inactivates the antimicrobial substance.14 Many
resistance genes are also carried on self-transmissible genes known as transposable elements, that can jump between plasmids and chromosomes.15
Bacteria can obtain a new gene (or genes) by several methods:
Conjugation is the most common method. It is a complex system that transfers a copy of a plasmid from one bacterial cell (called the donor) to
another bacterium (called the recipient) (Figure 1). A tube-like structure known as pilus latches onto the recipient and is positioned in such a way
that a conjugation bridge can form, allowing for the transfer of genes for resistance and other purposes.16 A common example is bacterial resistance
to penicillin that is acquired by obtaining the gene for penicillinase as a result of conjugation. Penicillinase is an enzyme that alters the
penicillin molecule in such a way that it is rendered ineffective. The plasmid containing the drug resistance is called a Resistance (R)
factor.Transduction is a virus-mediated transfer of host DNA from one host to another. Bacterial viruses, known as bacteriophages, sometimes can
serve as intermediaries, picking up the resistance gene from a naturally resistant bacterium and then passing on this gene to non-resistant bacteria.
In this case the bacteria’s genomes gain information, but the source is not mutations. Instead, the new genetic material is derived from the genome
of another bacterium that already has the gene (or gene set) that confers resistance.Transformation (the process in which bacterium take up exogenous
DNA from its environment). Chromosomal or plasmid DNA can even be taken up and spread from dead to living bacteria.
Also, many gene sets called transposons are self-transmissible and can transfer from their normal location to other plasmids or chromosomes. In
bacteria, antibiotic-resistant genes are located on plasmids or transposons, small circular units of DNA that can even be spread from dead to living
I had that saved from somewhere...