Resistant bacteria: Mutations of bacterial pathogens produce new strains.

The idea

A mutation is a random change in the genetic material of a bacterium. When a bacterial pathogen reproduces, most offspring are identical, but occasional mutations alter their characteristics. These changes can produce a new strain, which is a variant of the species with different features, such as the ability to resist an antibiotic or to produce a different toxin. For example, a mutation might change the shape of a protein that an antibiotic normally targets, so the drug no longer binds and the bacterium survives. Mutations occur at random and are not caused by the antibiotic itself; the antibiotic merely selects the strains that already carry a useful mutation. New strains can therefore spread through a population and make an infection harder to treat.

What earns the marks

• Defines a mutation as a random change in the genetic material of a bacterium.
• Explains that mutations can alter bacterial characteristics, such as resistance or toxin production.
• Defines a strain as a variant of a species with different inherited features.
• Links mutation to the formation of new strains that can spread through a population.
• States that mutations arise at random and are not directed by the environment or by antibiotics.

Common mistakes and how to put them right

• Saying antibiotics cause the mutation.

  Put it right: Mutations occur at random and antibiotics select resistant strains.

• Treating a strain as a different species.

  Put it right: A strain is a variant within a species.

• Believing all mutations are harmful.

  Put it right: Some are neutral or beneficial to the bacterium.

Exam tips

• Use the term random when describing mutation.
• Distinguish clearly between mutation, which creates variation, and selection, which favours it.
• Give a named example, such as a change in a target protein, to show how a new strain arises.