Microbiology Dilution Factor Question?
Hello. I was wondering if anyone knew how to solve this or if there is some kind of formula to use? Thank you!
If a plate were to yield 40 CFUs and have a dilution factor of 10 to the 3rd power, should there be more or less growth on the 10 to the 1st power plate?
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If the dilution factor was 10^3, that means it was a 1000-fold dilution. So, if the UNDILUTED stuff is 1x concentration, the 1000-fold dilution is 1/1000x, or 10^-3 x. That gave 40 CFU.
If the dilution factor was 10^1, that means it was a 10-fold dilution--1/10 x = 10^-1 x, or 100 times less dilute than the one above. So 40 * 100 = 4000 CFU.
Even without doing any calculations, it should be just common sense: the one that is diluted LESS (the 10-fold dilution), will have more CFU than the one that was diluted MORE (the 1000-fold dilution).
If you want an equation, it's just an ordinary proportion, the "tricky" part is to understand that a 1/1000 dilution = 0.001, and a 1/10 dilution = 0.1. Once you understand that, then you just plug the numbers into the proportion and solve for the unknown (x):
40 / 0.001 = x / 0.1
x = 0.1 (40 / 0.001)
x = 0.1 * 40,000
x = 4000