How do antibiotics treat bacterial infections, and why is antibiotic resistance a growing problem?
Explain how antibiotics treat bacterial infections and how antibiotic resistance arises and spreads
A focused answer to the H2 Biology Infectious Disease and Immunity outcome on antibiotics. How antibiotics target bacteria-specific processes, why they do not work against viruses, how resistance arises by mutation and selection, how it spreads (including by plasmids), and how to slow it.
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What this dot point is asking
SEAB wants you to explain how antibiotics treat bacterial infections (and why they do not work against viruses), how antibiotic resistance arises by mutation and natural selection, how it spreads (including by plasmids), and what can be done to slow it. It applies natural selection to a major public-health problem.
The answer
How antibiotics work
Antibiotics are chemicals that kill bacteria or stop them multiplying by targeting structures or processes specific to bacterial cells: the synthesis of the peptidoglycan cell wall, the function of the 70S ribosome, or bacterial enzymes. Because these targets are not found in the host's own cells, antibiotics have selective toxicity.
Antibiotics do not work against viruses, which lack these structures and replicate using the host cell's machinery.
How resistance arises
Resistance originates by chance mutation, which may change the antibiotic's target or let the bacterium make an enzyme that breaks the antibiotic down. When the antibiotic is used, it acts as a selection pressure: non-resistant bacteria are killed, resistant ones survive and reproduce, passing on the resistance allele. Over generations the resistant proportion rises. This is natural selection.
How resistance spreads
Resistance genes are often carried on plasmids, which can be passed from one bacterium to another (even between species) by conjugation. This horizontal transfer lets resistance spread rapidly, not only by inheritance.
Slowing resistance
- Prescribe antibiotics only when needed (not for viral infections), to reduce selection pressure.
- Complete the full course, so resistant survivors are not left to multiply.
- Use targeted (narrow-spectrum) antibiotics where possible.
- Improve hygiene and infection control to limit spread, and develop new antibiotics.
Examples in context
Example 1. Hospital-acquired resistant infections. Resistant bacteria can spread between patients in hospitals, where antibiotic use is heavy and vulnerable people are concentrated. Strict hygiene and careful antibiotic stewardship are the main defences, illustrating why limiting spread matters as much as limiting development.
Example 2. Multidrug resistance via plasmids. A single plasmid can carry resistance genes to several antibiotics at once, and conjugation can pass it between bacterial species. This explains how bacteria can rapidly become resistant to multiple drugs, making infections very hard to treat.
Try this
Q1. Explain why antibiotics do not work against viral infections. [2 marks]
- Cue. Antibiotics target bacteria-specific structures such as the cell wall and 70S ribosomes, which viruses do not have; viruses replicate using the host cell's machinery.
Q2. State how a resistance gene can be passed from one bacterium to another of a different species. [1 mark]
- Cue. On a plasmid, transferred by conjugation.
Q3. Explain why prescribing antibiotics only when necessary helps reduce resistance. [2 marks]
- Cue. Unnecessary use applies a selection pressure that favours resistant bacteria; using antibiotics only when needed reduces this pressure and slows the rise of resistance.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original5 marksExplain how a population of bacteria can become resistant to an antibiotic, and how resistance can spread between bacteria.Show worked answer →
The answer should cover the origin by mutation and selection, then transfer.
Resistance originates by chance mutation. Within a bacterial population, a random mutation may produce an allele that makes a bacterium resistant to the antibiotic, for example by changing the antibiotic's target or producing an enzyme that breaks the antibiotic down.
When the antibiotic is used, it kills the non-resistant bacteria but the resistant ones survive (the antibiotic acts as a selection pressure). The survivors reproduce and pass on the resistance allele, so over successive generations the proportion of resistant bacteria increases. This is natural selection.
Resistance can also spread horizontally between bacteria. The resistance gene is often carried on a plasmid, which can be passed from one bacterium to another (including between different species) by conjugation, so resistance spreads quickly through and between populations.
Markers reward mutation as the origin, the antibiotic as a selection pressure favouring resistant survivors, inheritance increasing the resistant proportion, and plasmid transfer (conjugation) spreading resistance horizontally.
Original4 marksDescribe measures that can be taken to reduce the development and spread of antibiotic resistance, and explain why each helps.Show worked answer →
The answer should give measures each with a reason.
Only prescribe antibiotics when they are needed (for bacterial infections, not viral ones), because unnecessary use applies needless selection pressure that favours resistant bacteria.
Always complete the full prescribed course, because stopping early may leave the more resistant bacteria alive to multiply and spread.
Use narrow-spectrum antibiotics targeted to the specific bacterium where possible, to reduce selection pressure on other bacteria.
Improve hygiene and infection control in hospitals, to reduce the spread of resistant bacteria between patients. Rotate or develop new antibiotics so that bacteria are not continually exposed to the same drug.
Markers reward at least three sensible measures, each with a correct explanation linking it to reducing selection pressure or limiting the spread of resistant bacteria.
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