What it shows
Antibiotics are chemicals, first found in microbes, that kill other microbes or stop them from growing. A simple way to see how well one works is the disc diffusion test: spread bacteria all over a Petri dish of agar, press on small paper discs soaked in antibiotics, and leave it warm overnight. The bacteria grow into a cloudy lawn everywhere except in a clear ring round each disc that works: the zone of inhibition.
This page simulates such a plate with four made-up antibiotics (P, Q, R, S) and a plain paper control. It works out each zone from a simple diffusion model: the antibiotic spreads out from the disc and gets weaker with distance, and the germ is stopped wherever the concentration is above its MIC (minimum inhibitory concentration). A second graph shows that concentration against distance, with the MIC line and the zone edge.
It fits Class 12 Biology: microbes in human welfare (antibiotics, Fleming and penicillin) and links to evolution (antibiotic resistance as natural selection). How the body’s own defences work is in immune response timeline, and how resistance spreads by selection is in natural selection.
This is a teaching model with made-up antibiotics and numbers. Real laboratories follow standard methods and official tables. Not medical advice.
How to use it
- Choose the germ: Strain 1 is an ordinary germ; Strain 2 has become resistant to some antibiotics.
- Slide the antibiotic in each disc (micrograms) and the agar depth.
- Tap a disc, a table row, or use Look closely at to see its concentration profile and working.
- Read the table: each zone’s diameter and a teaching verdict (≥ 18 mm sensitive, 14 to 17 mm intermediate, below 14 mm resistant; made-up cut-offs).
Worked example
Disc P on Strain 1, with 10 µg in the disc, agar 4 mm deep, D = 0.5 mm²/h and Tc = 10 h. The concentration next to the disc is c₀ = M / (4π·D·Tc·h) ≈ 39.8 µg/mL. The germ’s MIC is 0.5 µg/mL, so radius² = 4 × 0.5 × 10 × ln(39.8 / 0.5) ≈ 87.6 mm², radius ≈ 9.36 mm and the zone is about 18.7 mm across: sensitive. On Strain 2 (MIC 32 µg/mL) the zone shrinks to nothing: resistant.
What changes the zone?
- More antibiotic in the disc: bigger zone, but only slowly (the radius² grows with the logarithm of the amount).
- Thinner agar: the same antibiotic is less diluted, so the zone is bigger. That is why real tests fix the depth.
- Big molecules spread slowly (small D), so the zone can be small even for a strong antibiotic. Zone size compares germs on one antibiotic better than antibiotics with each other.
- A resistant germ has a higher MIC, so the zone shrinks or disappears.
Check yourself
Five fresh questions: radius to diameter, which antibiotic works better, a zone radius from the model, Fleming, and what no zone means. Nothing is saved.
FAQ
Why is there a control disc? To show that the paper itself does not stop the bacteria. Its zone should be zero.
Why does resistance matter? Using antibiotics when they are not needed, or stopping a course early, helps resistant germs survive and spread. Then the medicines stop working.
Is this from my textbook? It covers the standard school topic with its own model and examples. It is an independent study aid.
Where the numbers come from
- Disc diffusion and the zone edge from the critical concentration: the Cooper relation x² = 4·D·Tc·ln(c₀/MIC) with c₀ = M/(4π·D·Tc·h), from the standard theory of diffusion from a point source in a thin layer. Written for fibub; no textbook text, figures or exercises are used.
- The antibiotics P, Q, R and S, the two strains, their MICs, diffusion coefficients, the 10-hour critical time and the 18 and 14 mm cut-offs are all made up for teaching; they are not real breakpoints. Not medical advice.
- Checked against scipy.optimize.brentq solving c(x) = MIC on the same profile for five cases.
Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.
Last updated 2026-11-09.