Implications
Gene transfer
When humans reproduce, some of the DNA of both parents is replicated and then passed on to their children. Figure 5 shows the stages in meiosis, the production of sex cells( eggs and sperm). In the first phase all the DNA is duplicated; the parent cell then divides into four cells, which are the sex cells, shown at the bottom of the diagram, each with some of the parent cell’ s DNA. At fertilisation a sex cell from the mother( egg) joins with a sex cell from the father( sperm), creating a new parent cell with some genes identical to those of the mother and father.
In addition to the standard kind of gene acquisition, such as that in humans, where genes are passed to new generations through reproduction, bacteria can transfer genes between each other in the same generation, by what is known as HGT, which is responsible for widely distributing antibiotic resistance genes among bacteria( de la Cruz and Davies, 2000).
Figures 6a-6c show one method of HGT between bacteria. First a bacteriophage carrying DNA from a donor bacterium binds to the surface of the recipient bacterium( 6a). The bacteriophage then inserts the donor DNA into the recipient bacterium( 6b), and this DNA then becomes integrated into the recipients own DNA( 6c).
In HGT, DNA mutations, as discussed earlier, are not required in an organism for a transferred gene to be acquired by it; instead, the resistance gene is acquired from another organism. The newly acquired gene can become part of the DNA of the bacterium, or it can be introduced via a plasmid, which is kept separate and does not become part of the bacterium’ s DNA.
The new gene can either be acquired from a living organism, or a dead organism. A big issue with HGT is that resistance can not only be distributed within a single bacterial population, but unlike in gene transfer through reproduction, it can also be distributed to another species of bacteria( Kaiser, 2012)
Implications
HGT means that bacteria can become resistant to an antibiotic without ever being exposed to that antibiotic, but by merely being in the presence of bacteria which are already resistant to it. An example of a horizontally transferred gene is CTnDOT, which gives resistance to the antibiotic tetracycline( Gardner et al., 2005; Gui-Rong et al., 2009).
Although its presence is not necessary for HGT, tetracycline use stimulates HGT of CTnDOT from tetracycline resistant bacteria. Overuse of antibiotics not only increases frequency of resistance by mutation, but also frequency of resistance by HGT( Sapkota et al., 2011).
The development of antibiotic resistance has meant that there are very few antibiotics available now that are effective against bacteria. In the example bacterium H. pylori, rates of antibiotic resistance are in some places as high as 95 %. There have been many studies which show the negative effects of resistance on the success rates of antibiotic based therapies; for example, in nitroimidazole therapies, success rates have been recorded as 90 % vs 73 %; in bismuth therapies, 89 % vs 53 % and in macrolide therapies, 68 % vs 33 %, in non-resistant vs resistant kinds of H. pylori.
The clear lack of effectiveness means that treating bacterial infections often requires combinations of drugs rather than just a single antibiotic, known as combination therapy, as a single antibiotic drug is not effective enough alone. The components of the drug combination are one or more antibiotics and, for example, an acid suppressive drug, which is a drug that weakens stomach acid( Gerrits, 2006).
Antibiotic resistance is responsible for large numbers of deaths around the world; figures 7a and 7b illustrate these numbers. Antimicrobial resistance( AMR) is resistance of all microorganisms, such as bacteria and fungi, to drugs that are designed to kill them; antibiotics are a type of antimicrobial.
Figure 7a shows that although there are around 700,000 deaths per year in 2017, this is still relatively low when compared to cancer and traffic accidents.
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