Antibiotics and Painkillers

This lesson covers the important distinction between antibiotics and painkillers — two very different types of medicine that students often confuse. You also need to understand why antibiotic resistance is a growing global concern and how it links to natural selection. This topic is frequently examined in AQA GCSE Biology.

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What Are Antibiotics?

Antibiotics are medicines that kill or inhibit the growth of bacteria inside the body. They are one of the most important medical discoveries of the 20th century.

Feature	Detail
What they treat	Bacterial infections only
How they work	Kill bacteria or prevent them from reproducing
Example	Penicillin — discovered by Alexander Fleming in 1928
What they do NOT treat	Viral infections — antibiotics have no effect on viruses
Prescription required?	Yes — to reduce the risk of antibiotic resistance

Why Antibiotics Do Not Work on Viruses

This is one of the most commonly examined facts in GCSE Biology:

• Antibiotics work by targeting structures or processes found in bacterial cells (e.g. cell wall synthesis, protein synthesis, DNA replication)
• Viruses do not have their own cellular structures — they replicate inside host cells using the host's machinery
• Targeting the virus would mean destroying the host cell itself
• Therefore, antibiotics are completely ineffective against viral infections

Exam Tip: If the exam asks "Why can't antibiotics be used to treat influenza?", the answer must include: (1) influenza is caused by a virus, (2) viruses replicate inside host cells, and (3) antibiotics target bacterial cell processes, not viral replication.

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What Are Painkillers?

Painkillers (analgesics) are medicines that relieve the symptoms of disease but do not kill pathogens or cure the disease itself.

Feature	Detail
What they do	Relieve symptoms such as pain, fever, and inflammation
What they do NOT do	Kill pathogens or cure the underlying disease
Examples	Paracetamol, ibuprofen, aspirin
Available without prescription?	Yes — most painkillers are available over the counter
Used for	Headaches, muscle pain, fever, inflammation, toothache, and many other conditions

How Painkillers Work

Painkillers work in different ways depending on the type:

Type	Mechanism	Example
Paracetamol	Blocks chemical signals in the brain that produce the sensation of pain	Paracetamol
Anti-inflammatory	Reduces inflammation (swelling, redness) at the site of injury or infection	Ibuprofen, aspirin
Opioid	Blocks pain signals in the nervous system; much stronger; can be addictive	Morphine, codeine

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Antibiotics vs Painkillers — The Critical Comparison

This comparison is examined almost every year in some form:

Feature	Antibiotics	Painkillers
Purpose	Kill bacteria / cure bacterial infections	Relieve symptoms (pain, fever)
Target	The pathogen (bacteria)	The symptoms (pain signals, inflammation)
Effective against viruses?	No	Can relieve viral symptoms but do not cure
Cure the disease?	Yes (if bacterial)	No — only manage symptoms
Prescription needed?	Yes (in the UK)	Usually no (available over the counter)
Risk of resistance?	Yes — bacteria can develop antibiotic resistance	No — painkillers do not affect pathogens

Exam Tip: A classic exam question is: "A patient has influenza. The doctor prescribes painkillers but refuses to prescribe antibiotics. Explain why." Answer: (1) Influenza is caused by a virus; (2) antibiotics only work on bacteria and have no effect on viruses; (3) painkillers will help manage the symptoms (pain, fever) while the patient's immune system fights the virus.

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Antibiotic Resistance

Antibiotic resistance occurs when bacteria evolve to survive exposure to an antibiotic that previously killed them. This is one of the biggest threats to global health.

How Antibiotic Resistance Develops

The process follows the principles of natural selection:

graph TD
    A[Population of bacteria - most are susceptible to antibiotic] --> B[Random mutation occurs in one bacterium]
    B --> C[Mutation gives resistance to the antibiotic]
    C --> D[Antibiotic is used]
    D --> E[Non-resistant bacteria are killed]
    D --> F[Resistant bacterium survives]
    F --> G[Resistant bacterium reproduces rapidly]
    G --> H[Entire population becomes resistant]
    H --> I[Antibiotic is no longer effective]

Step-by-Step Explanation

1. Within a population of bacteria, there is natural variation — some may have random mutations
2. A mutation may give one bacterium resistance to an antibiotic
3. When the antibiotic is used, it kills the non-resistant bacteria
4. The resistant bacterium survives — it has a selective advantage
5. The resistant bacterium reproduces, passing on the resistance gene to its offspring
6. Over time, the entire population becomes resistant
7. The antibiotic is no longer effective against this strain of bacteria

This is an example of natural selection — the environment (antibiotic) selects for the fittest individuals (resistant bacteria).

Why Antibiotic Resistance Is Increasing

Cause	Explanation
Overprescription	Doctors prescribing antibiotics when they are not needed (e.g. for viral infections)
Not completing the course	Patients stopping antibiotics early, leaving some bacteria alive — including partially resistant ones
Overuse in agriculture	Antibiotics used in animal farming to promote growth, creating resistant bacteria
Poor hygiene in hospitals	Resistant bacteria (e.g. MRSA) spread between patients in healthcare settings
Lack of new antibiotics	Pharmaceutical companies have developed very few new antibiotics in recent decades

MRSA — A Case Study in Resistance

MRSA (Methicillin-Resistant Staphylococcus aureus) is a strain of bacteria that is resistant to several commonly used antibiotics. It is often called a superbug and is a particular problem in hospitals, where it can cause serious wound infections.

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How to Reduce Antibiotic Resistance