Estimate Microbial Number: MPN Method for CFU

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Discussion Overview

The discussion revolves around the estimation of microbial numbers using the Most Probable Number (MPN) method, specifically focusing on whether to use counts from a single tube or an average from multiple tubes. The context includes considerations of methodology and reasoning behind the choice of estimation.

Discussion Character

  • Homework-related
  • Debate/contested
  • Exploratory

Main Points Raised

  • One participant questions whether to use counts from tube 3 alone or the average of counts from tube 2 and tube 3 for estimating microbial numbers.
  • Another participant asks for clarification on the averaging process, specifically if it involves raw data from each tube.
  • A participant presents two different estimations based on counts from different tubes and seeks advice on which to use or whether to average them.
  • One participant emphasizes the importance of reasoning behind the choice to average or not, suggesting that there should be a relationship between counts in tube 2 and tube 3.
  • Another participant suggests that the relationship between tube 2 and tube 3 could be a factor of ten and expresses a preference for averaging to minimize errors.
  • A later reply indicates agreement with previous statements and suggests proceeding accordingly.

Areas of Agreement / Disagreement

Participants express differing views on whether to use a single count or an average for estimation, indicating that multiple competing views remain without a clear consensus.

Contextual Notes

There are unresolved considerations regarding the assumptions behind averaging counts and the potential relationship between the counts in different tubes.

frankmp40
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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!
 

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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?
 
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?
 
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?
 
I think the relationship between tube2 and tube3 is a factor of ten.
I would prefer their average as it can minimize errors.
 
I agree with both of the statements you made and would proceed accordingly if I were doing this.
 
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