Serial dilution and Spread plating, Biology

Question

The undiluted bacterial concentration in a Laurel Creek water sample is 5.0 x 105CFU/ml. Which final dilution factor test tube would yield the countable agar plate?

Assuming that 0.1mL of water is added to each plate.

Answer 1

So here we have 5X10^5 CFU/ml. We need to get it to be 5x10^1. If our final test tube is 5 x 10^2 and we add 0.1ml of that to a plate, then our final plate will be 5x10^1 which will be 50CFU which is in our countable window.

So our final test tube is going to be 5x10^2 CFU/ml.

Hope this helps!