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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