Resistant bacteria

Inheritance, variation and evolution · The development of understanding of genetics and evolution · note 4 of 4

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Resistant bacteriaSpec 4.6.3.7

In short

Antibiotic-resistant bacteria are strains that are not killed by antibiotics, such as MRSA. Bacteria evolve rapidly because they reproduce at a fast rate. A mutation produces a resistant strain. When antibiotics are used, non-resistant bacteria are killed but resistant ones survive and reproduce, so the resistant strain spreads. Appropriate prescribing and completing the course of antibiotics slow this down.

Bacteria can evolve rapidly because they reproduce at a fast rate. Mutations of bacterial pathogens produce new strains.

A mutation produces a strain that is resistant to antibiotics.
When antibiotics are used, non-resistant bacteria are killed. The resistant bacteria are not killed.
The resistant bacteria survive and reproduce, so the population of the resistant strain rises.
The resistant strain spreads, because people are not immune to it and there is no effective treatment.

MRSA is resistant to antibiotics.

Reducing the rate of development of resistant strains

Doctors should not prescribe antibiotics inappropriately, such as for non-serious or viral infections.
Patients should complete their course of antibiotics so all bacteria are killed and none survive to mutate and form resistant strains.
The agricultural use of antibiotics should be restricted.

The development of new antibiotics is costly and slow. It is unlikely to keep up with the emergence of new resistant strains.

Common mistake:

People do not become resistant to antibiotics. The bacteria are resistant. The resistant bacteria survive and reproduce, passing on the resistance.

Quick check

Why is the fossil record incomplete?
Show answer
Early life was soft-bodied and left few traces, and many traces were destroyed by geological activity.
Why do resistant strains of bacteria spread so quickly?
Show answer
Bacteria reproduce quickly, people are not immune and there is no effective treatment.
Give one way to reduce the development of antibiotic resistance.
Show answer
Do not prescribe antibiotics inappropriately, complete the course, or restrict agricultural use.

Written and checked against the AQA GCSE Combined Science (8464) specification · Updated October 2026

Frequently asked questions

What is the evidence for evolution?

The evidence for evolution includes the fossil record, which shows how much or how little organisms have changed as life developed on Earth, the discovery that characteristics are passed on to offspring in genes, and antibiotic resistance in bacteria, which shows natural selection happening in a short time. This is why the theory is now widely accepted.

How are fossils formed?

Fossils form in three ways: from parts of organisms that have not decayed because one or more of the conditions needed for decay are absent, from parts that are replaced by minerals as they decay, or as preserved traces such as footprints, burrows and rootlet traces. Fossils are found in rocks and show how organisms have changed.

How does antibiotic resistance develop in bacteria?

Antibiotic resistance develops by natural selection. A mutation produces a strain of bacteria that is resistant to antibiotics. When antibiotics are used, non-resistant bacteria are killed but the resistant bacteria survive and reproduce, so the resistant population rises. Because bacteria reproduce at a fast rate, resistant strains such as MRSA can spread quickly, with no effective treatment.

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