You notice something is growing in a 100ml pot of liquid soap. You take 1ml of this liquid soap and perform a serial dilution and plate to determine the number of bacteria per 1ml in the liquid soap. On the 1:100,000 dilution you plate out 1ml and get a count of 37 colonies. How many bacteria were present in 1ml of soap (cfu/1ml)?

3'700,000 cfu

Explanation:

One way to count the amount of bacteria in a medium is by doing a dilution of the sample and count how many colonies growth. Each colony is a cfu (Colony forming units).

In the problem, you count 37 colonies. The dilution was 1:100,000. That means the bacteria present in the soap is:

37 colonies × (100,000 / 1) = 3'700,000 cfu

I hope it helps!