Mutations are random changes that occur to the DNA sequence. A change in DNA results in different protein being produced, this changes the characteristics of an organism.
Variation is caused by two things: mutations; recombination of existing DNA.
Bacteria do not sexually reproduce, so until recently it was unknown how bacteria were so diverse without recombination of existing DNA, however, now we know that there is recombination of existing DNA but not from two parents to an offspring, it happens from one existing bacteria to another: plasmids are transferred between them in a process known as conjugation:
- One bacteria starts to grow a tube called a pilus or conjugation tube
- It also starts to replicate a plasmid
- The tube reaches another bacteria and the DNA starts to move through it as a line
- It enters the new bacteria and forms a ring (the new plasmid)
- The tube is broken down
This process takes a matter of seconds and allows the transfer of genetic material from one existing bacteria to another- this is known as horizontal gene transmission (because it occurs across a generation). In this way a mutation could be passed between the same type of bacteria, to a different strain, or even to a different species. Like this resistance to an antibiotic could be gained by a species.
Bacteria also pass DNA on by asexual reproduction, this is known as vertical gene transmission as DNA is passed down from one generation to another.
In 1946 Lederberg and Tatum designed an experiment to prove that DNA was transferred horizontally between bacteria:
- Take two strains of E. coli, one that can synthesise everything but methionine and biotin and one that could synthesise everything but threonine and leucine
- This meant that when they were placed in a medium without the nutrients neither strain could grow
- They mixed the two together and left them for several hours
- They put the mixture back onto a medium with no nutrients
- Some bacteria were able to survive
- This meant that there must have been DNA exchanged so that the strains had the ability to synthesise the nutrients they needed to survive
Most of the time mutations are not beneficial to an organism, they stop it from being able to function properly, however, occasionally a mutation happens that can increase the success of an organism.
In bacteria mutations can occur that make them resistant to antibiotics, this means that if they are in an infected person who is taking antibiotics then all the other, non mutated bacteria will die off, the one with the mutation will survive and replicate passing on the resistant gene and making the resistant bacteria more common.
This will only occur because of the presence of antibiotics giving the resistant strains an advantage, therefore the more antibiotics are used, the more resistance happens.
An example of this is the mutation of an enzyme in a bacteria that changed an enzyme to make one which could break down penicillin (an antibiotic) before it could harm the bacteria. This meant that it was not destroyed by the antibiotic and lived on to reproduce successfully. The gene was then passed on to future generations of the bacteria to make a resistant population, but it was also transferred horizontally so that other species were also resistant.
A bacteria is placed in a Petri dish with an antibiotic. If the bacteria is not resistant then it will not be able to grow near the antibiotic, and it will be visible everywhere apart from near the antibiotic. It the bacteria is resistant it will grow even in the area of the antibiotic and therefore will be visible over the whole dish.
To treat the bacterial infection of tuberculosis (TB) you have to take a 6-9 month course of antibiotics. Often people start to feel better a little way into the course because a lot of bacteria has been killed off so they stop taking the antibiotic. However, even though a lot of bacteria had been killed off, some would not yet have died- this is the more resistant stuff that is harder to kill. As a consequence the resistant bacteria are left in the body, free to multiply and spread. This is known as selection pressure.
As a result, TB is resistant to most known antibiotics- the definition of a super bug.
Methicillin-resistant staphylococcus aureus (MRSA) is also a super bug. It is found in hospitals where: people tend to be ill anyway, making them more susceptible to infection; people are in close proximity; there is a lot of contact (doctors patients); there are a lot of antibiotics being taken (which make resistant strains more successful.