Plate Counting 101 - (Oct/04/2004 )

Okay, y'all, here we go:

I'm coming in on the middle of an experiment. Being that I'm the luckiest person in the world, I've gotta crunch all the raw data. My problem is this: the former microbiologist only plated in duplicate and would frequently end up with one plate that was countable (b/w 30 - 300) and the other that was either TMTC or TFTC. When dealing with duplicates where both plates are countable, I'm taking the average of the plates and computing the CFU/ml of original sample from the plate count averages. However, I'm unsure as to how to calculate CFU/ml of original sample when one plate in the set is countable and the other is not. Any advice would be greatly appreciated.

Thanks

What are your test sets? And do you have several dilutions plated?

Generally I recommend to make at least 5 parallels in several dilutions to be able to make at least some statistics.
Otherwise just calculated the cfus for all dilutions and average them. If you end up with exactly one count for one test set the data usually isn't worth much.