Estimate Microbial Number: MPN Method for CFU

[frankmp40]

Hi,

I just want to know should I only use counts in tube3 to estimate microbial number, or the average of tube2 and tube3 estimation.

Thank you!

 

[Yanick]

What do you think you should do? Do you have any reason to suspect doing it one way or the other?

Also, when you say averaging the counts do you mean the raw data from each tube? Just to be clear, are you saying you will calculate, for example, (42+4)/2?

 

[abitslow]

https://en.wikipedia.org/wiki/Colony-forming_unit#Theory
specifically, the second paragraph.

 

[frankmp40]

I mean now I have two estimation:

First one is No. of coliform = 39*dilution factor 1
The second one is No. of coliform = 241*dilution factor 2

Which estimation should I use, or I need to take the average of the two?

 

[Yanick]

We can't just offer you an answer, we are here to help you work through your problem and actually learn something.

So I will reiterate my questions above. Do you have any thoughts or reasoning for averaging or not averaging the two measurements? In an ideal world, do you think there should be some relationship between the counts in tube 2 and tube 3?

 

[frankmp40]

I think the relationship between tube2 and tube3 is a factor of ten.
I would prefer their average as it can minimize errors.

 

[Yanick]

I agree with both of the statements you made and would proceed accordingly if I were doing this.