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Topic 9- Antibiotics
Some antibiotics are naturally produced by bacteria or fungi in order to reduce competition.
How They Work...
Antibiotics can kill bacteria in 2 ways:
They kill them directly (they are bactericidal).
o Damage the cell wall- movement of water into the cell causes osmotic lysis.
o Damage the membrane.
o Stop enzymes functioning and therefore stop life processes.
They stop them reproducing (bacteriostatic).
o Damage DNA so they can't replicate.
Broad spectrum- they kill a wide range of bacteria (e.g. penicillin).
Narrow spectrum- they kill only a few specific types of bacteria.
By interfering with the cell walls, antibiotics have the least effect on us, because our cells have no cell walls.
Protein synthesis antibiotics target smaller ribosomes and so don't affect us.
If antibiotics target DNA it could also affect us because our cells also have DNA. This causes side effects.
Some antibiotics are used less than others because of the side effects they cause.
What is Resistance and How does it Work?
Bacteria can evolve solutions so that they are no longer affected by a drug.
Increase production of blocked chemicals leading to overproduction.
Finding an alternative source of blocked chemicals e.g. absorbing it from the environment.
Modify the antibiotic so that the bacterium is no longer affected, e.g. adding a phosphate group.
Bacterial enzymes to break down the antibiotic.
Prevent the antibiotic entering the cell e.g. using cell capsules.
Spontaneous mutation- very small probability of a positive mutation but because of high numbers it works.
Pass on resistance between bacteria.
How Does Resistance Spread?
Vertical Gene Transmission
Bacterium with resistance survives antibiotic attack whereas unresistant bacteria are killed off. The resistant bacterium
survives to pass on the mutation to its offspring so the next generation also possess resistance to that antibiotic.
Horizontal Gene Transmission- Conjugation
Horizontal gene transmission is the process of passing a resistance mutation onto a peer
through the sharing of DNA rather than through reproduction. This process is unique to
1. The resistance gene occurs in the plasmid.
2. The plasmid replicates.
3. A pilus forms between two bacteria.
4. The replicated plasmid is sent through the pilus into the second bacterium which now
possesses the resistance gene.
The combining of DNA means that one bacterium can have a mixture of DNA from other
bacteria which provides an evolutionary advantage. Conjugation can happen between
bacteria of different species. MRSA conjugates easily and therefore collects a lot of
Ethics of Antibiotic Use
The more we use antibiotics the more resistance there is.
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When you have the choice of using a drug to treat someone do you only use it when it's absolutely necessary?
Not finishing a course of antibiotics can make the problem worse because not all the bacteria are killed off and so some
may develop resistance.
All antibiotics are similar to each other.…read more