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Antibiotic resistance develops by natural selection. Free GCSE Edexcel Biology revision notes with key definitions, plus 6 common questions answered.
How Antibiotic Resistance Develops is part of The development of understanding of genetics and evolution in GCSE Edexcel Higher Biology.
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Antibiotic resistance develops by natural selection. Free GCSE Edexcel Biology revision notes with key definitions, plus 6 common questions answered.
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Use this lesson to stay inside The development of understanding of genetics and evolution while connecting the detail back to the wider Topic 4: Natural selection and genetic modification topic.
Read the core explanations from How Antibiotic Resistance Develops before testing yourself from memory.
Imagine you're in hospital with a serious infection. The doctors give you antibiotics, but they don't work. The bacteria causing your infection have evolved to resist the very drugs designed to kill them. This isn't science fiction - it's happening right now in hospitals worldwide.
Antibiotic resistance is one of the biggest threats to modern medicine. To understand how we got here, we need to explore how bacteria evolve at lightning speed.
Bacteria have a massive evolutionary advantage over larger organisms - they reproduce incredibly quickly. While humans take years to reach reproductive age, bacteria can divide every 20 minutes under ideal conditions.
This rapid reproduction means evolution happens in fast-forward. In a simple doubling model, one bacterium dividing every 20 minutes could produce about 2^72 ≈ 4.7 × 10^21 descendants in 24 hours (before real-world limits reduce growth). Each generation is an opportunity for mutations to occur.
Not all bacteria are identical. Random mutations in their DNA create slight variations between individuals. Most mutations are harmless or even harmful to the bacteria. But occasionally, a mutation gives a bacterium a survival advantage.
When antibiotics are present, most bacteria die. However, any bacterium with a resistance mutation survives. This is natural selection in action - the environment (antibiotics) selects which traits help survival.
Worked example: The Evolution of Antibiotic Resistance
Trace how a resistant strain becomes dominant in a bacterial population
Answer: The resistant strain becomes the dominant population through natural selection
Trap: Many students think bacteria 'choose' to become resistant. Actually, resistance mutations happen randomly - antibiotics just kill the non-resistant ones, leaving resistant bacteria to multiply.
Once a resistant strain emerges, it spreads rapidly through human populations for two key reasons. First, people have no immunity to the new strain - our immune systems haven't encountered this exact variant before.
Second, there's no effective treatment available. The antibiotics that worked against the original strain are useless against the resistant version. This gives the resistant bacteria free rein to multiply and spread between hosts.
Hospitals are particularly vulnerable because they contain many sick people with weakened immune systems, and antibiotics are used frequently. This creates the perfect environment for resistant strains to emerge and spread.
MRSA (Methicillin-resistant Staphylococcus aureus) is a perfect example of how antibiotic resistance develops. Originally, Staphylococcus aureus was easily treated with penicillin and later methicillin.
Random mutations in some S. aureus bacteria happened to confer resistance; repeated antibiotic use in hospitals then selected for these resistant strains. These resistant strains survived, reproduced, and spread. Today, MRSA infections are much harder to treat and can be life-threatening.
Exam questions on antibiotic resistance often ask you to explain the process step-by-step. The key is to show you understand natural selection, not just memorise facts about bacteria.
Common mistake: Writing 'bacteria become resistant to survive'. This suggests bacteria choose to mutate, which is wrong. Instead write: 'Random mutations create resistant bacteria, which survive when antibiotics kill non-resistant ones'.
Resistance becomes common when selection pressure is repeated over time. Antibiotic treatment removes susceptible bacteria, while resistant bacteria survive, divide rapidly, and pass resistance genes on. Overuse or incorrect use of antibiotics increases this pressure and speeds up the spread of resistant strains in hospitals and communities.
Published by Revise Right. Use the official specification reference to check the requirements for your course. Content updated: 2026-03-23.
Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
MRSA (methicillin-resistant Staphylococcus aureus) is a strain of Staphylococcus aureus bacteria that has evolved resistance to methicillin and many other commonly used antibiotics. Repeated antibiotic use in hospitals selected for resistant strains, so MRSA infections are much harder to treat and can be life-threatening.
A superbug is a strain of bacteria that has evolved resistance to antibiotics, such as MRSA, which is resistant to methicillin and many other commonly used antibiotics. Infections caused by such strains are much harder to treat and can be life-threatening.
Antibiotic resistance develops by natural selection. Random mutations make a few bacteria resistant; when antibiotics are used, non-resistant bacteria die but resistant ones survive, reproduce rapidly without competition and pass on the resistance gene, so the resistant strain becomes the dominant population.
Bacteria can divide every 20 minutes under ideal conditions, and typically every 20-30 minutes in optimal conditions. This rapid reproduction means evolution happens in fast-forward, because each generation is an opportunity for mutations to occur.
Bacteria evolve quickly because they reproduce incredibly fast, dividing every 20 minutes under ideal conditions. Each generation is an opportunity for mutations to occur, so evolution happens in fast-forward, unlike in larger organisms.
Resistant strains spread rapidly because people have no immunity to the new strain and there is no effective treatment, since the antibiotics that worked against the original strain are useless. Hospitals are especially vulnerable because they contain many people with weakened immune systems and use antibiotics frequently.
Bacteria do not choose to become resistant to antibiotics. Resistance mutations happen randomly, and antibiotics simply kill the non-resistant bacteria, leaving resistant bacteria to multiply, so the antibiotic acts as a selection pressure.
In a simple doubling model, one bacterium dividing every 20 minutes could produce about 2⁷² ≈ 4.7 × 10²¹ descendants in 24 hours. Real-world limits reduce this growth.
Staphylococcus aureus was originally easily treated with penicillin and later methicillin. Random mutations gave some bacteria resistance, and repeated antibiotic use in hospitals selected for these resistant strains, producing MRSA.
People have no immunity to a new resistant strain because their immune systems have not encountered this exact variant before. With no effective treatment available either, this lets resistant bacteria multiply and spread between hosts.
Overuse or incorrect use of antibiotics increases resistance by increasing the selection pressure. Antibiotic treatment removes susceptible bacteria while resistant bacteria survive, divide rapidly and pass resistance genes on, speeding up the spread of resistant strains.