By the way, I’m answering questions like this for my hw. One of the questions is “stefanie and Bradley are lab partners. Using a 10^-2 dilution, Stefanie counts 57 cells in a Petroff-Hauser chamber. Bradley counts 63 cells using the same dilution. What was the average number of cells per mL in the original sample? assuming 25 squares and standard volume.” So I already know how to do answer this kind of question thanks to your video. But I’m not sure how many mL of the dilution they put on the plate or what they mean by “standard volume.” Could you help me with this? It’s for my microbiology class.

Maybes it your prof

Yes he did! My professor tried teaching us without using any units. Talk about confusing :(

That is exciting! The biotech field can be very rewarding . I wish you the best.

It's my pleasure

You well be great. Let me know if you get the job!

Its so so helpful, thank you

Beverley P. , that’s awesome. Thank you!

There’s an increase of error because we’re dealing with small numbers. If you have a plate with 10 colonies and a second one with 20 colonies, this isn’t big a difference until you start multiplying and the small error now becomes multiplied.

What type of error are you referring to when you mentioned the small error ? I mean error due to what ?

It’s all mathematical. There’s less of a difference between 250 CFU and 275 CFU than if your talking about the difference between 5 and 30 CFU. If you count either 250 or 275CFU from a sample diluted 10^6 times than your answer is either 2.5 x10^8 CFU or 2.75 x 10^8 CFU. But if you use data values such as 5 or 30 CFU from a sample that’s similarly diluted 10^6 than your calculated answers would be either 5 x 10^6 or 3 x 10^7 which are more different. Thus there’s a bigger error between to two answers. To make your data more accurate you would average data from triplicate plates with CFU between 30 and 300. This is pretty much standard in most labs. I sometimes hear that scientist will use plates with CFU counts between 20 and 200.

You can transfer any volume on a plate. But anything more than 0.1 ml might add too much liquid to the plate and the bacterial colonies may run into each other as they grow. This makes it difficult to count them efficiently. I generally like to use a volume between 0.05 - 0.1 ml.

That is not correct. 1ml (from tube 1) was added to tube 2 (containing 9ml). Tube 2 was mixed a bit and 0.1ml from tube 2 was than added to tube 3 (containing 0.9ml).

Good question. You would use both plates to calculate your answer, then average the two for a better answer.