Antibiotic resistance is a direct example of EVOLUTION BY NATURAL SELECTION occurring in our lifetimes.
The process:
1. VARIATION — within a population of bacteria, individuals vary slightly due to random mutations. A very small number may have a mutation that gives RESISTANCE to a particular antibiotic.
2. SELECTION PRESSURE — when antibiotics are used, they KILL bacteria that are NOT resistant. The resistant bacteria SURVIVE.
3. REPRODUCTION — resistant bacteria reproduce (bacteria can double every 20 minutes). They pass on the resistance gene to offspring.
4. SPREAD — resistant bacteria become increasingly common in the population. The antibiotic no longer works.
This is natural selection in real time — the antibiotic acts as the selection pressure. The resistant mutation existed BEFORE the antibiotic was used — the antibiotic didn't create the mutation, it selected for it.
MRSA — A Serious Example
MRSA (Methicillin-resistant Staphylococcus aureus) is a bacterium that has developed resistance to many commonly used antibiotics.
Staphylococcus aureus is normally a relatively harmless skin bacterium found on about 30% of people.
MRSA has acquired resistance through repeated exposure to antibiotics — particularly in hospital settings where antibiotics are widely used.
Why MRSA is dangerous:
Difficult to treat — standard antibiotics don't work.
Can cause serious infections: bloodstream infections (sepsis), pneumonia, wound infections.
Particularly dangerous for hospital patients who are already weakened or have wounds.
Few effective antibiotics remain — some strains are resistant to almost everything.
MRSA shows what can happen when antibiotic resistance goes unchecked — a common bacterium becomes a serious, life-threatening pathogen.
Why Antibiotic Resistance is a Global Crisis
Antibiotic resistance is one of the greatest threats to global health, food security and development — according to the World Health Organisation.
The problem is accelerating because:
OVER-PRESCRIPTION — antibiotics given for viral infections (where they have no effect) or 'just in case'.
INCOMPLETE COURSES — stopping antibiotics early leaves the most resistant bacteria alive to reproduce.
AGRICULTURAL USE — antibiotics widely used in livestock farming to prevent disease and promote growth → resistant bacteria enter food chains.
GLOBAL SPREAD — resistant bacteria travel with people internationally.
SLOW DEVELOPMENT of new antibiotics — pharmaceutical companies have reduced investment because new antibiotics are not as profitable as drugs for chronic diseases.
What can be done:
Prescribe antibiotics ONLY when necessary.
Patients must COMPLETE FULL COURSES.
Develop NEW antibiotics and alternative treatments (e.g. phage therapy — using viruses that kill bacteria).
Reduce agricultural antibiotic use.
Better hygiene to prevent spread of resistant bacteria.
⚠️ Common Mistake
Antibiotics do NOT CAUSE mutations — they SELECT resistant bacteria that already exist. The mutation happened randomly, long before the antibiotic was used. The antibiotic is the selection pressure that determines which bacteria survive. This is a key distinction — evolution acts on existing variation, it doesn't create new variation in response to need.
📌 Key Note
Resistance develops by natural selection: random resistance mutation exists → antibiotic kills non-resistant bacteria → resistant bacteria survive and reproduce → resistance spreads. Antibiotics select for resistance — they don't cause it. MRSA = serious hospital-acquired resistant bacterium.
🎯 Matching Activity — Steps in Developing Antibiotic Resistance
Put the steps of antibiotic resistance development in order by matching each step. — drag the symbols on the right to match the component names on the left.
Step 1 — Variation
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Step 2 — Selection pressure
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Step 3 — Reproduction
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Step 4 — Population change
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Resistant bacterium reproduces — passing resistance gene to millions of offspring
A random mutation gives one bacterium resistance to an antibiotic — before any antibiotic is used
Antibiotic is used — kills all non-resistant bacteria, resistant one survives
Resistant bacteria dominate the population — the antibiotic no longer works
⭐ Higher Tier Only
MRSA developed resistance through natural selection in hospital environments where antibiotics are heavily used. Strategies to manage resistance: prescribe only when necessary, complete courses, reduce agricultural use, develop new antibiotics, explore alternative therapies (bacteriophage therapy — viruses that infect bacteria). Students should be able to explain why resistance is evolutionarily inevitable if antibiotics are overused — resistant variants will always be selected for.
🎯 Test Yourself
Question 1 of 3
1. How does antibiotic resistance develop in bacteria?
2. Why is it important to complete a full course of antibiotics even if you feel better?
3. MRSA is resistant to many antibiotics. How did this resistance arise?
⭐ How Well Do You Understand This Topic?
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