Antibiotic resistance is a clear example of evolution by natural selection happening fast enough for humans to observe. In a bacterial population, some cells may already have traits that let them survive a drug. When an antibiotic is used, susceptible bacteria die while resistant bacteria survive and reproduce.
Over time, the population becomes more resistant because the survivors pass on the traits that helped them live.
Resistance can begin with random mutation, but it can also spread when bacteria share DNA through horizontal gene transfer. Antibiotics do not intentionally create stronger bacteria, but they create a powerful selective pressure that favors resistant ones. This matters in medicine because resistant infections are harder to treat and can spread through hospitals, farms, communities, and the environment.
Slowing resistance requires using antibiotics only when needed, finishing prescribed treatments, preventing infections, and limiting the spread of resistant strains.
Key Facts
- Natural selection occurs when variation, heritability, and differential survival or reproduction act together.
- Antibiotics kill susceptible bacteria, but resistant bacteria can survive and reproduce.
- A random mutation can change a bacterial protein so an antibiotic no longer works well.
- Horizontal gene transfer lets bacteria share resistance genes through plasmids.
- Change in resistance frequency can be described as new frequency = resistant bacteria / total bacteria.
- Selection pressure from antibiotic exposure increases the relative fitness of resistant bacteria.
Vocabulary
- Antibiotic resistance
- Antibiotic resistance is the ability of bacteria to survive or grow in the presence of a drug that would normally kill them or stop their growth.
- Natural selection
- Natural selection is the process in which organisms with helpful inherited traits survive and reproduce more successfully in a specific environment.
- Mutation
- A mutation is a random change in DNA that can sometimes create a new trait, such as resistance to an antibiotic.
- Plasmid
- A plasmid is a small circular piece of DNA in bacteria that can carry genes, including genes for antibiotic resistance.
- Horizontal gene transfer
- Horizontal gene transfer is the movement of genetic material between organisms that are not parent and offspring.
Common Mistakes to Avoid
- Saying antibiotics make bacteria mutate because they need to survive is wrong because mutations occur randomly before or during exposure, and the antibiotic selects which bacteria live.
- Thinking individual bacteria evolve resistance during their lifetime is wrong because evolution is a change in the genetic makeup of a population across generations.
- Assuming all bacteria die if the correct antibiotic is used is wrong because a few resistant cells may already be present and can survive treatment.
- Stopping antibiotics early because symptoms improve is wrong because surviving bacteria may remain, reproduce, and increase the chance of a resistant infection returning.
Practice Questions
- 1 A dish contains 1,000,000 bacteria. If 0.01% are resistant before antibiotic exposure, how many resistant bacteria are present?
- 2 After antibiotic treatment, 500 resistant bacteria survive and reproduce by binary fission every 30 minutes. Starting with 500 cells, how many resistant bacteria are present after 3 hours?
- 3 Explain why antibiotic resistance is considered evolution by natural selection rather than a choice made by bacteria.