Antibiotic-Resistant Bacteria & Selection (Cambridge O Level Biology)

• While natural selection in most species occurs over many years, in species with short generation times it can take place very quickly
• The development of antibiotic resistance in bacteria is an example of natural selection that occurs over easily observable time frames
  • E.g. methicillin-resistant staphylococcus aureus, or MRSA, are resistant to the antibiotic methicillin
• The development of antibiotic resistance can be explained as follows:
  1. Variation exists in a bacterial population; some have an allele that provides resistance to an antibiotic while others do not
     • This allele will have arisen due to random mutation
  2. Bacteria reproduce, resulting in many individuals, not all of which will survive
  3. The bacteria compete for survival
  4. The bacteria with alleles for antibiotic resistance are more likely to survive when exposed to the antibiotic, and these surviving individuals are more likely to reproduce
  5. The reproducing bacteria pass on the allele for antibiotic resistance to their offspring
  6. Over many generations, the number of antibiotic resistant bacteria in the population increases
     • This may only take a few hours if the bacteria are reproducing quickly

Antibiotic resistance diagram

The development of antibiotic resistance is an example of natural selection