Antibiotics

Biology AS · Infectious diseases. 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

An antibiotic is a drug that kills bacteria or stops their growth without harming the cells of the host. Penicillin works on the bacterial cell wall. It stops the enzymes that build cross-links between the peptidoglycan molecules, so a growing cell makes a weak wall. Water enters by osmosis and the cell bursts.

Viruses are not affected. They have no cell wall, no ribosomes and no metabolism of their own, so there is nothing for the antibiotic to act on.

Antibiotic resistance arises by mutation. When the antibiotic is used, resistant bacteria survive, reproduce and pass on the resistance gene, so the drug stops working.

Rules to remember

Penicillin inhibits the enzymes that form cross-links in the peptidoglycan cell wall; it acts only on growing bacteria.
Human cells have no cell wall, so penicillin does not harm them.
Resistance comes from a random mutation; the antibiotic is the selective agent and does not cause the mutation.
Vertical transmission: resistance genes pass to daughter cells at division.
Horizontal transmission: resistance genes on plasmids pass between bacteria, even of different species.
To reduce resistance: use antibiotics only when needed, finish the course, do not use them for viral infections, and use more than one antibiotic together where suitable.

Common mistake

Writing that bacteria become immune, or that the antibiotic makes them mutate. Bacteria are resistant, not immune, and the mutation is already present before the antibiotic selects for it.

Worked example

A patient with a bacterial infection feels better after three days and stops a seven-day course of an antibiotic. Why may this lead to resistance?

After three days the most susceptible bacteria are dead, so the symptoms ease.
The bacteria still alive are the ones least affected by the antibiotic.
With the drug removed, these survivors multiply and pass on their genes.
The new population is more resistant and can spread to other people.

Answer: Stopping early selects for the least susceptible bacteria, which then multiply.

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