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.
Understanding Biology: Antibiotic Resistance as Evolution
Resistance can work in several different ways. Some bacteria make enzymes that break apart an antibiotic before it reaches its target. Others alter the cell structure that the drug normally binds to.
A drug may no longer fit the altered target well enough to stop growth. Some cells use protein pumps in their outer membrane to push the medicine back out. Bacteria can even form biofilms, which are sticky communities attached to surfaces such as teeth, catheters, or lungs.
Inside a biofilm, drugs may penetrate poorly and cells can grow slowly. Slow-growing cells are often less affected by medicines that target active cell processes.
Evolution is measured across a population, not within the body of one individual cell. A bacterium does not decide that it needs a useful mutation. Mutations happen during DNA copying, before they are useful or harmful.
Most mutations do not help under a particular drug. A rare change can become important when conditions shift. If treatment removes many competing cells, a resistant cell has more food, space, and chances to divide.
Its descendants can make up a large share of the population in a short time because bacteria reproduce quickly. This is why resistance can become noticeable over days, weeks, or months rather than over thousands of years.
Gene sharing makes bacterial evolution unusually fast. A plasmid is a small loop of DNA that can carry one or more resistance genes. During cell contact, one bacterium can transfer a plasmid to another, even when the two cells are not close relatives.
Bacteria can sometimes collect DNA released by dead cells from their surroundings. Viruses that infect bacteria can move DNA between cells as well. One plasmid may carry genes for resistance to several drug types.
When one antibiotic is used, it can therefore favor bacteria that survive other antibiotics too. This pattern is called co-selection and it makes treatment choices more complicated.
Students meet this topic in everyday health decisions. Antibiotics treat bacterial infections, not illnesses caused by viruses such as most colds and flu. Using them when they cannot help exposes harmless bacteria in the body to the drug and may select resistant strains among them.
Infection prevention reduces the need for treatment in the first place. Handwashing, safe food handling, vaccination, clean water, and careful hospital hygiene all matter. When learning this topic, separate an individual bacterium from the population over generations.
Pay attention to the source of variation, the inherited trait, the environmental pressure, and the change in how common that trait becomes. Those four parts provide strong evidence for evolution by natural selection.
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.