Antibiotic-resistant bacteria

How a population becomes resistant

Tap a stage for more on exactly what changes — and what doesn't.

1. 1A random mutationBacteria copy their DNA every time they divide, and the copying isn't perfect. A mutation is a random change in a bacterium's genes. It happens whether or not an antibiotic is anywhere nearby — the drug doesn't cause it and the bacterium isn't responding to it.+
2. 2The antibiotic selects — it doesn't createWhen the antibiotic is used, it kills the bacteria whose genes make them vulnerable to it. A small number of bacteria are already resistant because of the mutation in stage one, and an antibiotic can't kill what it can't act on — those bacteria are left completely untouched.+
3. 3The resistant strain takes overWith the non-resistant bacteria gone, the resistant strain has the space and nutrients to itself. Each survivor divides in two, and every new generation inherits the same resistance — so with each generation, the resistant strain becomes a bigger share of the population.+
4. 4It spreadsThe population is now mostly the resistant strain, and it doesn't stay contained: people are not immune to it, and there's no effective treatment to stop it moving from person to person. MRSA is resistant to antibiotics — a real example of exactly this.+

Check the reversal

You've just walked through the mechanism above. Before you move on, commit to an answer — this exact question is where the idea usually slips.

A population of bacteria meets a particular antibiotic for the first time. What is really happening to the bacteria that survive?

Three practices, one shared pressure

Select a practice to see why it raises the rate at which resistant strains develop.

What you need to know

• Bacteria reproduce at a fast rate, so mutations appear and spread through a population within a timescale short enough to observe.
• Mutations of bacterial pathogens produce new strains — some of these strains happen to already be resistant to a particular antibiotic.
• An antibiotic kills the bacteria it can act on; a resistant strain is not killed, so it survives and reproduces, and its share of the population rises.
• The resistant strain can then spread, because people are not immune to it and there's no effective treatment for it — MRSA is a real example of a resistant strain.
• Doctors not prescribing antibiotics inappropriately, patients completing their full course, and restricting agricultural use, all reduce the rate at which new resistant strains develop.
• Developing new antibiotics is costly and slow, and is unlikely to keep up with the rate at which new resistant strains emerge.

The big picture

Bacteria evolve resistance to antibiotics through natural selection: a random mutation that already exists in the population is not created by the drug. The antibiotic simply kills everything else, leaving the resistant strain free to take over and spread.

⚠ Watch out

Thinking the antibiotic makes bacteria resistant, or that it's the patient who becomes resistant or immune. In reality, the resistant mutation already existed in a few bacteria before the antibiotic was ever used — the antibiotic just removed the competition. It's the bacterium that is resistant, never the person.

Memory hook

The antibiotic is a filter, not a trainer: it doesn't toughen bacteria up, it just decides who was already tough enough to survive.

Check yourself

Explain, in order, how a single resistant mutation ends up spreading between people — and name the one step the antibiotic is responsible for.