Antibiotic Resistance Evolution

How Bacterial Populations Adapt to Antibiotics Over Generations

Start with mostly susceptible bacteria, then apply antibiotics across generations. Watch surviving mutants expand until resistant cells dominate, illustrating natural selection.

Blue = susceptible cells

Green = resistant mutants

More antibiotic pressure → stronger selection for resistant bacteria.

Susceptible bacteria (killed by antibiotic)

Resistant bacteria (survive antibiotic)

Killed cells / cleared space

Antibiotic pressure

Generations under antibiotic pressure

Antibiotic level

Higher = more susceptible cells die

60%

Mutation rate

Chance a child is resistant

0.2%

Gen: 0

Start at generation 0 with a mostly susceptible population and a few resistant mutants. Apply moderate antibiotic levels: most blue cells die, green survivors repopulate. Step forward to see resistance become more common by selection, not purpose.

Population snapshot

Total cells

After last generation

Resistant fraction

0.00

Resistant / total

Susceptible count

Blue cells

Resistant count

Green cells

How to explore evolution

Set antibiotic level (0–100%).
Set mutation rate (rare but non-zero is realistic).
Click “Step 1 generation” to apply:
- Antibiotic kills susceptible cells based on level.
- Survivors reproduce to refill space.
- Some offspring mutate into resistant form.
Use “Auto-run” to watch multiple generations in sequence.

What this illustrates

Mutations creating resistance are random, not triggered by the drug.
Antibiotics act as a filter: susceptible cells die, resistant ones survive.
Generation by generation, the population shifts toward resistance.
High antibiotic pressure with incomplete killing accelerates selection for resistant strains.
Stopping too early or under-dosing leaves resistant survivors to expand.

The numbers and motion are schematic, tuned for intuition: focus on the trend from mostly blue to mostly green as selection plays out.