Antibiotic resistance sim

What the sim shows

Antibiotic resistance: a model. One population of one species of bacteria at an infection site, shown as a culture dish. Each drawn bacterium stands for about a million real ones (illustrative); the close-up is magnified and not to scale (a real bacterium is about 1 to 5 µm long). Most bacteria are sensitive to the antibiotic. On day 0, 2 of the 120 carry a resistance allele that arose earlier by random mutation, not because of any antibiotic. Each day, in this order: if you take the dose, it kills ≈ 45 % of the sensitive bacteria and only ≈ 12 % of the resistant ones (a low dose: ≈ 30 % and ≈ 4 %); the immune system destroys 4 bacteria (drawn) a day, so it can clear a small population but not a large one; then each survivor divides into two with a chance of ≈ 35 % while there is room (at most 160 living). A new bacterium has its parent's allele, except that the daughter of a sensitive bacterium carries the resistance allele with a chance of 4 in 1000, by random mutation, on every day, with or without the antibiotic. The antibiotic does not cause mutations: it selects the bacteria that already resist it, so the resistant fraction rises (natural selection). Real resistance often builds up in small steps, which low doses can favour; this model has one allele so cannot show that. You have symptoms while 45 or more are living. A course is one dose a day for 7 days. A cold is caused by a virus: antibiotics do not affect viruses, and the immune system clears the cold by day 9 either way. In the cold run the dish and the close-up show the body's own normally harmless bacteria, which the immune system leaves alone. Simplified on purpose: illustrative numbers; one species; one resistance allele; no gene transfer between bacteria (real bacteria can also pass resistance genes to each other, for example on plasmids); the immune system simplified to a fixed number a day; the doses and kill rates are illustrative (≈); resistant bacteria divide as fast as sensitive ones; a dose acts once a day; illness shown only as ill or better, from a single threshold; the luck is fixed (seeded), so the same choices always give the same result; the infection and the dose strength are set before day 1. The real effect sizes differ from these, and real infections vary widely.

• Lit ring: where to drop the capsule to take today's dose
• Close-up: the bacteria in the ringed area, magnified
• Resistant bacteria: orange with two dark bands; sensitive: plain green

Use it in your lessons

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