Antibiotics and resistanceEdexcel A-Level Biology B: Revision notes
Section 1
Bactericidal and bacteriostatic antibiotics
Antibiotics are chemicals that kill or stop the growth of bacteria without harming the host's cells. They work by targeting structures or processes that bacteria have and human cells lack, or that differ from those in human cells.
- A bactericidal antibiotic kills bacteria, for example penicillin.
- A bacteriostatic antibiotic stops bacteria multiplying but does not kill them, for example tetracycline. The immune system must remove the remaining bacteria.
Antibiotics are not effective against viruses, which have no cell wall and use the host's cell machinery.
Section 2
Penicillin: a bactericidal antibiotic
Bacterial cell walls contain peptidoglycan, whose chains are held together by cross-links. Penicillin inhibits the enzyme that forms these cross-links, so the wall cannot be built or repaired as the cell grows.
The weakened wall can no longer resist the pressure from water entering by osmosis, so the cell swells and bursts (lyses). Penicillin acts on growing cells. It does not harm human cells because they have no cell wall.
Section 3
Tetracycline: a bacteriostatic antibiotic
Tetracycline binds to the bacterial ribosome and stops the translation of mRNA, so the bacterium cannot make proteins it needs. It cannot grow or divide, but it is not killed at once, so tetracycline is bacteriostatic.
Bacterial ribosomes are structurally different from human ribosomes, so human protein synthesis is largely unaffected at normal doses. If the drug is removed, growth can restart.
Remember the pair: penicillin acts on the cell wall and kills; tetracycline acts on ribosomes and stops growth.
Section 4
How antibiotic resistance develops
Within a large bacterial population, a few cells may carry a gene that gives resistance, for example an enzyme that destroys the antibiotic. Such genes arise by random mutation.
When the antibiotic is used it acts as a selection pressure. Non-resistant bacteria are killed, resistant bacteria survive and reproduce, and they pass the gene to their offspring. The proportion of resistant bacteria then increases. This is natural selection, and bacteria show it quickly because they reproduce rapidly.
Section 5
How resistance spreads
Resistance genes are often carried on plasmids. Bacteria can pass plasmids to each other by horizontal gene transfer (conjugation), including between different species. A recipient becomes resistant without mutating or being the offspring of a resistant cell.
Resistant bacteria spread between people, animals and food, and travel makes this international. MRSA, a Staphylococcus aureus resistant to methicillin and other antibiotics, is an example in hospitals.
Do not say bacteria become resistant because they get used to the drug. Resistance arises by mutation and selection, not by individuals adapting.
Section 6
Controlling resistance and why it is difficult
Measures include:
- prescribing antibiotics only when needed and not for viral infections
- patients completing the full course
- restricting use in agriculture
- hygiene, screening and isolating infected patients
- developing new antibiotics, or using more than one antibiotic together
Difficulties include patient expectation and non-compliance, slow diagnosis, the cost to farmers, staff and money needed for hygiene, the high cost of finding new drugs, and the fact that resistance crosses borders through travel and trade, so international action is needed.
Must Know
- Bactericidal antibiotics kill bacteria; bacteriostatic antibiotics stop them multiplying
- Penicillin stops cell wall cross-links forming, so cells burst by osmosis
- Tetracycline blocks translation at the bacterial ribosome
- Resistance arises by mutation and spreads by selection and plasmid transfer
- Overuse and incomplete courses increase selection pressure
- Controlling resistance is hard because it needs behaviour change, money and international cooperation
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Antibiotics and resistance
- A microbiologist adds penicillin to a growing culture of bacteria and tetracycline to a second, identical culture. In the penicillin culture the number of living cells falls to almost zero within several hours. In the tetracycline culture the number of living cells stays roughly constant but does not increase. When the tetracycline is washed out, the cells begin to divide again.Explain how penicillin kills growing bacteria but does not harm human cells.2 marks
- Researchers are testing a new compound, X, that binds to a site on bacterial ribosomes and prevents them from reading mRNA. Bacteria exposed to X do not die at once but cannot grow. Human liver cells exposed to the same dose of X are unaffected.Compound X is bacteriostatic. Explain why a patient's own immune system is still needed to clear the infection.2 marks
- A patient with an infected wound is treated with an antibiotic. At first the infection responds, but after several days the bacteria in the wound are no longer killed. Tests show that the bacterial population now contains a gene for an enzyme that destroys the antibiotic. The gene is carried on a plasmid. Before treatment only about one bacterium in ten million carried it.Explain how treatment with the antibiotic led to a bacterial population that was mostly resistant.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).