In D. Dilution series, you want to determine the original cell number of the culture you plated out. Suppose that for dilution 10-3 you can not see individual colonies (600). For dilution 10-4 you can count 180 colonies and for dilution 10-5 there are 20 colonies. Which dilution will you take to calculate the initial cell number and why?

Question

In D. Dilution series, you want to determine the original cell number of the culture you plated out. Suppose that for dilution 10-3 you can not see individual colonies (>600). For dilution 10-4 you can count 180 colonies and for dilution 10-5 there are 20 colonies. Which dilution will you take to calculate the initial cell number and why?

anonymous · Answer

please help me T_T

thomaster · Answer

1,000x diluted = not useful
10,000x = 180
100,000x = 20

You can see the 100,000x dilution is almost 1/10th of the 10,000, which is what you'd expect theoretically.

Just calculate it back to the original concentration by multiplying them by the dilution factor.
So for the 10^-4 dilution you just multiply 180 with 10,000 which equals 18,000,000 or 1.8*10^7

Do the same for the 10^-5 and you'll get 100,000 * 20 = 20,000,000 or 2.0*10^7

When I'm counting bacteria and the colony count 1 dilution further would almost be a perfect 1/10th I'd just take the average which is 1.9*10^7, but I don't know if your school has specific rules regarding which dilution to use.