4.6.3.7 - Resistant Bacteria

Rapid bacterial evolution

Evolution means a change in the inherited characteristics of a population over time. Bacteria can evolve rapidly because they reproduce at a fast rate, so there can be many bacterial generations in a short time.

Fast reproduction matters because any inherited change that helps a bacterium survive can be copied into many offspring quickly. One bacterium with a useful mutation can produce many descendants carrying the same useful change.

In this specification point, the mutation is important because it can produce a new strain of a bacterial pathogen. A strain is a type or variety within a species of bacterium. Different strains may not respond to the same antibiotic in the same way.

Resistant strains

Mutations of bacterial pathogens can produce new strains. Some strains might be resistant to an antibiotic, meaning the antibiotic does not kill them.

Be precise about the cause. The antibiotic does not make a bacterium deliberately change because it needs to survive. Instead, mutation produces variation. When an antibiotic is used, susceptible bacteria are killed, but bacteria with a resistance mutation are not killed.

[DIAGRAM: asset_name: resistant-bacteria-selection-flow - diagram 1; asset_slug: 093_4_6_3_7_resistant_bacteria_diagram1; file: diagram_assets/imagegen_regen_all/093_4_6_3_7_resistant_bacteria_diagram1_imagegen.png; recommended_method: codex_image_gen; description: Monochrome flow diagram showing a mixed bacterial population with mostly susceptible bacteria and one resistant mutant, antibiotic treatment killing susceptible bacteria, resistant bacteria surviving, surviving resistant bacteria reproducing so the resistant strain population rises, and the resistant strain spreading because people are not immune and there is no effective treatment. Keep the visual within AQA 4.6.3.7 and do not include detailed molecular resistance mechanisms.]

This is natural selection. The antibiotic is a selection pressure because it changes which bacteria are most likely to survive and reproduce. The resistant bacteria have the advantage in the antibiotic-treated environment.

After explaining it, check that your answer has the order: mutation, antibiotic selection, survival, reproduction, population rise.

Spread and MRSA

Once the population of a resistant strain rises, the strain can spread. AQA gives two reasons why this is especially serious: people are not immune to the resistant strain, and there may be no effective treatment.

MRSA is the named example in this specification point. For GCSE Biology, the key point is simple: MRSA is resistant to antibiotics.

Do not write that the person is resistant to antibiotics. The bacteria are resistant. If resistant bacteria cause disease, the usual antibiotic may not kill them, so the infection can be harder to treat and the resistant strain can spread to other people.