Antibiotic resistanceSpec 3.39
In short
Antibiotic resistance evolves by natural selection. A mutation produces an allele that makes a bacterium resistant to an antibiotic. When the antibiotic is used, it kills the non-resistant bacteria, but the resistant bacterium survives, reproduces with less competition and passes on the allele. The population becomes mostly resistant, so infections are difficult to control.
The increase in antibiotic-resistant bacteria is an example of natural selection that can be seen happening within a human lifetime. Bacteria reproduce very quickly, so changes in a population happen fast.
- In a population of bacteria, a mutation produces an allele that makes one bacterium resistant to an antibiotic. This is variation.
- The antibiotic is used. It kills the non-resistant bacteria, but the resistant bacterium survives. The antibiotic is the selection pressure.
- The resistant bacterium reproduces with less competition and passes the resistance allele on to its offspring.
- Over time the population becomes mostly resistant bacteria, so the antibiotic no longer works against the infection.
As a result, infections caused by resistant bacteria are difficult to control. The antibiotic does not work, so the infection can spread to other people. Using antibiotics only when they are needed, and finishing the course, helps to slow the increase of resistance.
Percentage of resistant bacteria (practice data)
A sample of 300 bacteria had 12 resistant to an antibiotic. After several rounds of treatment, a sample of 300 bacteria had 192 resistant. Calculate the percentage resistant each time.
- Before: 12 ÷ 300 × 100 = 4%
- After: 192 ÷ 300 × 100 = 64%
Answer: 4% before and 64% after.
Antibiotics do not cause the mutation, and bacteria do not 'get used to' antibiotics. The resistant bacteria were there by chance and are selected by the antibiotic.
Quick check
How many cells does mitosis produce, and how do they compare with the parent cell?
Show answer
Two cells, genetically identical to the parent cell.
What is the diploid number and the haploid number of chromosomes in human cells?
Show answer
Diploid 46 and haploid 23.
Name the type of cell division that produces gametes.
Show answer
Meiosis.
Give two causes of variation within a species.
Show answer
Genetic (alleles inherited) and environmental.
Written and checked against the Edexcel IGCSE Science Double Award (4SD0) specification · Updated October 2026