Antibiotic resistance.

Step 1 of 6

A few mutants

In $1928$⁠ Alexander Fleming found that a mould in a lab dish killed bacteria, and from that mould came penicillin, the first antibiotic. As early as $1945$⁠, in his Nobel Prize lecture, Fleming warned that bacteria could become resistant to it. Today resistant bacteria kill more than a million people a year.

The Scene shows a dish with a colony of bacteria, drawn far larger than life. The green ones are ordinary. Days after the drug is fixed at Drug added.

Step 2 of 6

The selection window

Drug level is measured in units of MIC, the minimum inhibitory concentration: the lowest level that stops the ordinary bacteria growing. The resistant bacteria have an MIC of their own, much higher. Between the two values lies a selection window.

Drug level and Course are fixed, and Days after the drug will run two weeks ahead once you choose an answer.

Step 3 of 6

How fast

Beside the Scene is a chart on a logarithmic scale: the top line is the drug level against the window, and below it the two kinds of bacteria over two weeks. On a scale like that each equal step up is a multiplication by the same factor, so a hundred cells and a billion fit on the same chart.

Step 4 of 6

Finishing the course

Now the drug is above the window, and can wipe out the whole colony. The question is how long it needs.

Step 5 of 6

Below the MIC

Even a level that kills nothing does something. Drugs reach colonies of bacteria in many ways: antibiotics in farm animals’ feed, traces in sewage, and pills forgotten halfway through a course.

Step 6 of 6

Bacteria, not viruses

What you saw here is natural selection, as with the moths, only thousands of times faster, because a bacterium divides every few hours. Every use of an antibiotic selects resistant bacteria, even when it is needed. That is why it is used only where it can help.