A culture holds 1,000,000 bacteria. By chance, 100 of them carry a gene for resistance to an…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A culture holds 1,000,000 bacteria. By chance, 100 of them carry a gene for resistance to an antibiotic, and the other cells are sensitive. A dose of the antibiotic kills 99.99 per cent of the sensitive cells, and does not harm the resistant ones. The survivors then grow back to 1,000,000 cells in the same ratio, and a second dose is given.
- How many sensitive cells survive the first dose?
- What percentage of the cells that survive the first dose are resistant?
- After the culture has grown back and the second dose is given, what percentage of the survivors are resistant?
Need a reminder? The key ideaShow
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Given. sensitive and resistant cells. The dose kills per cent of the sensitive cells.
Plan. Count the survivors of each kind after each dose.
Step 1. (a) Sensitive survivors: cells.
Step 2. (b) Resistant cells: . Share: per cent.
Step 3. (c) The culture grows back to cells, half of them resistant, so resistant and sensitive. The second dose leaves sensitive cells. Share: per cent.
Answer: cells; per cent; per cent.
Check. The resistant cells went from per cent to per cent to per cent. The antibiotic did not cause the resistance. It selected the cells that had it.
Watch out. A weaker dose that is stopped early leaves many resistant cells behind. This is why a full course is advised.
Discussion (0)
Log in to join the discussion.
Loading…