Chapter 16 - Adaptation and selection

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Bacteria are one of the most diverse and adaptable groups of organisms. Their adaptation to a changing environment can be seen in their ability to develop resistance to antibiotics. Genetic variation can arise in bacteria as a result of : mutation changing the quantity or structure of the DNA of an organism, or recombining the existing DNA of two individuals - this occurs during sexual reproduction. Bacteria also increase diversity by changing their DNA in a similar manner to other organisms, namely by mutations and conjugation.

> Mutations

changes in DNA that result in different characteristics. Mutations arise in many ways, for example, one or more bases in a DNA sequence may be added, deleted or replaced by others during replication. Any differences in the base sequence of a DNA may result in a different sequence of amino acids. This will lead to a different polypeptide, and hence a different protein being produced. If this protein is an enzyme it is likely to disrupt the metabolic pathway, leading to the production of other substances, including proteins. As proteins are responsible for the characteristics of an organism, it follows that changes to DNA are likely to alter an organism's characteristics.

Conjugation - occurs when one bacterial cell transfers DNA to another bacterial cell

- one cell produces a thin projection that meets another cell and forms a thin conjugation tube between the two cells

- the donor cell replicates on of its plasmids

- the plasmid is broken to make it linear before it passes along the conjugation tube into the recipient cell

- Contact between the cell is brief, leaving only time for a portion of the donor's DNA to be transferred

- In this way the recipient cell acquires new characteristics from the donor cell

DNA can be passed on from one species to another species - Horizontal gene transmission

DNA can be passed down from one generation of a species to the next generation - Vertical gene transmission.

16.2 Antibiotics

> How antibiotics work - in bacterial cells water constantly enters by osmosis, this entry of water would normally cause the cell to burst, a process called osomotic lysis, however it doesnt burst due to the wall that surrounds all bacterial cells, this wall is made up of a tough material that is not easily stretched, being inelastic, the wall halts the further entry of water, preventing osmosis. Certain antibiotics kill bacteria by preventing them forming cell walls, they inhibit the synthesis and assembly of important peptide cross linkages in bacterial cell wall, this makes the cell unable to withstand pressure, so they are unable to prevent water entering and therefore osmotic lysis occurs, killing the bacterium. Antibiotics are only effective when bacteria are growing.

> Antibiotic resistance.

It is not the presence of antibiotics that cause bacteria to mutate. Mutations occur randomly and are very rare, however because there are so many bacteria around, the total number of mutations is large. Many of these mutations will not be advantageous to the bacterium. Occasionally these mutations will be advantageous. For example a bacteria may mutate to produce an enzyme that will cause the antibiotic to be ineffective, in this case the resistant bacteria will grow and survive and reproduce. Antibiotic resistance can also be carried in the circular loops of DNA called plasmids, these can then be transferred from cell to cell y conjuhgation. Resistance can therefore find its way into other bacterial species by horizontal gene transmission, HGT can lead to certain bacteria accumulating DNA that ives them resistance to a range of antibiotics, these are called "SUPERBBUGS". The more we use antibiotics, the greater the chance that the mutant bacterium will gain an advantage over the normal variety, in time the mutant will out-compete and replace the normal variety.

16.3

> antibiotic resistance and tuberculosis - TB is a bacterial disease of the lungs, treatment involves the use of antibiotics. In order for the antibiotics to be effective, they must be taken for a long time (6-9 months). When individuals are ill they willingly take the antibiotics as they are keen to recover. Antibiotics initially destroy the least resistant strains of mycobacterium tuberculosis. After a number of months the patients feel better because the vast majority of the bacteria have been killed, so patients stop taking the antibiotics. However this is dangerous as the few bacteria that do remain are the resistant bacteria. So the resistant strains survive and multiply and spread to others. These strains can then interchange genes for resistance with other strains by conjugation . It is in this way that multiple-antibiotic-resistant strains of TB have developed.

> Antibiotic resistance and MRSA - many people carry a bacterium belonging to the genus staphylococcus in their throats. This is a bacteria that causes only minor symptoms in healthy individuals, if it becomes a major health risk it can be treated with antibiotics. MRSA is the name given to any strain of this bacterium that is resistant to one or more antibiotics , MR is especially prevalent in hospitals and can present a particular danger because

- People in hospital tend to be older sicker and weaker than the general population, making them vulnerable to the infection

- Many people live close together and are examined by doctors and nursers who have just touched other patients, this is perfect for the transmission of infections

- Many antibiotics are used in hospitals, and as a result any mutant strain of the bacterium has an advantage over non-mutant strains. With many different antibiotics being used, strains can more easily develop multiple resistance to antibiotics. MRSA is difficult to treat, some strains have developed resistance to almost every known antibiotic.