Ch 2 · 10.2 Antibiotics · AS & A Level Biology 9700
Topic 2 of 2 in Infectious diseases — AS & A Level Biology 9700 — 57 lessons.
Introduction
Antibiotics have transformed the treatment of bacterial infections, but their overuse and misuse have driven the evolution of resistant bacterial strains, now a major threat to public health. (Syllabus section 10.2; how resistance evolves by natural selection is also examined in 17.2.)
Key ideas — Mode of action of antibiotics
Antibiotics are chemicals that kill bacteria, bactericidal, or inhibit their growth, bacteriostatic, without harming the host's cells. Penicillin, and related antibiotics, inhibit the enzymes that catalyse the final cross-linking stage of peptidoglycan synthesis in the bacterial cell wall. Because the cytoplasm of a bacterium has a lower water potential than its surroundings, water tends to enter by osmosis, and the bacterium relies on its rigid cell wall to stop it bursting; a weakened wall cannot resist water entering the cell; the cell swells and undergoes lysis, killing the bacterium. Penicillin therefore acts only on actively growing and dividing bacteria, since it is new wall synthesis that is blocked.
🧑🏫 Teacher's explanation
Imagine you are building a brick wall around a balloon. The balloon is the bacterium, and the bricks are the peptidoglycan layer. If you stop adding new bricks while the balloon is being inflated with air (water entering via osmosis), the wall will eventually fail, and the balloon will pop. This is exactly how penicillin works.
📖 Definitions
🔍 Worked example — Easy
State the difference between a bactericidal and a bacteriostatic antibiotic.
🔍 Worked example — Standard
Explain why penicillin is ineffective against a virus.
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🎯 Exam question
Key ideas — Why antibiotics do not affect viruses
Viruses lack a peptidoglycan cell wall, ribosomes and the other metabolic machinery that most antibiotics target, since they are not cells and rely on the host cell's own enzymes and ribosomes to replicate. Because antibiotics act on bacterial structures and processes such as cell wall synthesis, bacterial ribosomes or bacterial enzymes, none of which viruses possess or need, antibiotics have no effect on viral replication.
🧑🏫 Teacher's explanation
Think of a bacterium as a fully functional, self-contained factory. It has its own power generators, assembly lines (ribosomes), and a protective perimeter wall. Antibiotics are like specialized saboteurs designed to break specific machines or smash the walls of that factory. If you send a saboteur to destroy a factory's assembly line, the factory stops working.
📖 Definitions
🔍 Worked example — Easy
State the primary target of penicillin in a bacterium.
🔍 Worked example — Standard
Explain why penicillin is ineffective against the influenza virus.
🔍 Worked example — Exam-style
A population of bacteria is treated with an antibiotic. After several generations, the population grows despite the presence of the drug. Explain the biological basis of this phenomenon and suggest one way to reduce its occurrence.
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🎯 Exam question
Key ideas — Development of antibiotic resistance
Bacterial populations are genetically variable, partly due to random mutation. A mutation may, by chance, alter a bacterial protein in a way that reduces the effectiveness of an antibiotic against it, for example by changing the shape of the target enzyme or by producing an enzyme that breaks down the antibiotic. When a population of bacteria is exposed to an antibiotic, susceptible bacteria are killed, but any resistant bacteria survive and reproduce, passing the resistance allele to their offspring; this is natural selection acting on existing genetic variation. Resistance genes can also spread rapidly between bacteria, including between different species, by horizontal gene transfer via plasmids.
🧑🏫 Teacher's explanation
Imagine a city where every resident is wearing a specific type of uniform that makes them easy to spot. Suddenly, a new law is passed (the antibiotic) that targets anyone wearing that uniform. Most people are caught, but by pure luck, a few people are born wearing a different color, or they find a way to change their clothes instantly. Because the 'uniform-wearers' are gone, the 'different-color' people have no competition for food or space, so they quickly multiply until the whole city is wearing the new color.
📖 Definitions
🔍 Worked example — Easy
Explain why penicillin is ineffective against the influenza virus.
🔍 Worked example — Standard
A population of bacteria is treated with an antibiotic. Describe how the frequency of a resistance allele increases in the population.
🔍 Worked example — Exam-style
Compare the mechanisms by which resistance alleles can spread within a bacterial population.
⚠️ Common errors
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🎯 Exam question
(a) Explain why antibiotics are not effective in treating the flu. [2] (b) Describe how the use of antibiotics can lead to the evolution of antibiotic-resistant bacteria. [4] [6 marks]
Key ideas — Consequences of antibiotic resistance
Repeated or incomplete courses of antibiotics increase the selection pressure favouring resistant bacteria, since more susceptible bacteria are killed while resistant survivors are given more opportunity to multiply. Widespread use of antibiotics in agriculture, and poor infection control in hospitals, have contributed to the emergence of multi-drug-resistant strains such as meticillin-resistant Staphylococcus aureus, MRSA, which can no longer be treated with many standard antibiotics and causes serious, sometimes untreatable, infections, particularly in hospital patients with weakened immune systems.
🧑🏫 Teacher's explanation
Imagine you are playing a video game where you are trying to clear a level of monsters. If you use a 'weak' weapon that only kills 90% of the monsters, the 10% that survive aren't just lucky; they are the strongest, fastest, and toughest ones. Because they survived, they are the only ones left to breed, and their offspring will inherit those 'super-powers.' Soon, your weak weapon is useless because every monster in the level is a 'super-monster.'
📖 Definitions
🔍 Worked example — Easy
Explain why antibiotics are ineffective against viruses.
🔍 Worked example — Standard
A patient is prescribed a 7-day course of antibiotics for a bacterial infection. The patient feels better after 3 days and stops taking the medication. Explain the biological consequence of this action.
🔍 Worked example — Exam-style
Discuss the impact of antibiotic resistance on modern medical procedures such as organ transplants or chemotherapy.
⚠️ Common errors
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🎯 Exam question
Key ideas — Reducing the impact of resistance
The rate at which resistance develops can be reduced by prescribing antibiotics only when necessary, for confirmed bacterial infections rather than for viral infections such as colds; ensuring patients complete the full prescribed course, so surviving bacteria are not left to develop or spread resistance; restricting the routine use of antibiotics in agriculture and animal feed; using a combination of antibiotics with different modes of action where appropriate, to reduce the chance a bacterium survives all of them; and strict hygiene and infection control measures in hospitals to prevent the spread of resistant strains between patients.
🧑🏫 Teacher's explanation
Imagine you are trying to clear a field of weeds. If you only spray half the field, the toughest weeds survive, grow back stronger, and eventually take over the whole garden. This is exactly how antibiotic resistance works. If we don't kill every single bacterium, the 'strongest' ones survive and multiply.
📖 Definitions
🔍 Worked example — Easy
Explain why antibiotics are ineffective against the influenza virus.
🔍 Worked example — Standard
A patient is prescribed a 7-day course of antibiotics for a bacterial infection. They feel better after 4 days and stop taking the medication. Explain the consequence of this action.
🔍 Worked example — Exam-style
Compare the use of antibiotics in human medicine versus agriculture to reduce the impact of resistance.
⚠️ Common errors
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🎯 Exam question
(a) Explain why this practice may lead to the development of antibiotic resistance in the patient's body. [3] (b) Suggest one way in which the use of antibiotics in livestock can contribute to the problem of antibiotic resistance in humans. [1] [4 marks]
Worked example
Question. A hospital found that a strain of bacteria had become resistant to an antibiotic that was previously effective. Explain how this resistant strain could have arisen and increased in frequency in the hospital population of bacteria. [5]
🔍 Worked example — Easy
Why are antibiotics ineffective against viruses such as influenza or HIV?
🔍 Worked example — Standard
A population of bacteria has a 95\% susceptibility rate to an antibiotic. After exposure to the antibiotic, the susceptibility rate drops to 10\%. Calculate the percentage decrease in susceptibility.
🔍 Worked example — Exam-style
Explain how the misuse of antibiotics (e.g., not finishing a prescribed course) contributes to the spread of antibiotic resistance. [4]
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