Calculating sample size?

[Andy Resnick]

There has to be a simple way to calculate this, I just don't have a clue how.

The setup:

I'm teaching cell culture methods to some students, and part of the process is checking for contamination- after making culture media, a small volume is put into the incubator overnight and checked for bacterial growth in the morning.

The question (using 'normal' units for things)
"When you made 100 milliliters of solution, there could also be a small number of bacteria present, perhaps as much as 0.1 nanomolar. How much volume do you need to sample in order to be at least 99% confident that you captured at least 1 bacterium?"

Thanks in advance....

 

[jedishrfu]

It seems 1ml would work. I guess you could imagine from 0.1 nanomolar = numerically how many bacteria (6.9E12). Then assume they are evenly distributed.

Take your volume of 100ml and divide it into 100 1ml. So each 1ml would have 1/100th the number of bacteria (6.0E10).