Antibiotics

Biology · Disease and immunity. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

A drug is any substance taken into the body that changes or affects the chemical reactions in the body. An antibiotic is a drug used to treat infections caused by bacteria. It kills the bacteria, or stops them growing, without harming the cells of the patient.

Antibiotics do not work on viruses. A virus has no cell wall and no metabolism of its own for the drug to attack, and it reproduces inside our own cells.

Some bacteria are resistant: the antibiotic no longer kills them. Every time an antibiotic is used, resistant bacteria are the ones that survive and multiply. MRSA is a resistant strain of Staphylococcus aureus. Using antibiotics only when they are really needed slows this down.

Rules to remember

Drug: any substance taken into the body that modifies or affects chemical reactions in the body.
Antibiotics treat bacterial infections only.
Antibiotics kill bacteria but do not affect viruses, so they do not cure a cold, influenza or HIV.
MRSA is a bacterium that is resistant to many antibiotics.
To limit resistance: use antibiotics only when essential and finish the whole course.
In a disc test on agar, a wider clear zone means the antibiotic is more effective against that bacterium.

Common mistake

Do not write that people become resistant or immune to antibiotics. It is the bacteria that become resistant, not the patient.

Worked example

An antibiotic cured almost every case of a bacterial infection twenty years ago. Today it fails in many patients. Explain why.

In a large population of bacteria, a few are resistant to the antibiotic by chance.
When the antibiotic is used, the non-resistant bacteria are killed and the resistant ones survive.
The survivors reproduce, so their offspring are resistant too.
After many years of heavy use, most of the bacteria causing the infection are resistant.

Answer: Frequent use of the antibiotic has left a population made mostly of resistant bacteria, so the drug no longer works.

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