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Puzzling Results Across Dilution Factors


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Brothbro

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Posted 23 December 2021 - 12:42 AM

Hello All, reaching out with what I'm sure is a common occurrence in food micro, but did not see it asked on this board. Here is the situation:

 

A food sample is tested for e.coli via petrifilm at a 10^-2 and 10^-3 dilution. The -2 plate (less diluted) shows no e.coli growth, but on the -3 plate (more diluted) there is a single e.coli colony. Our specification is that e.coli should not be detected in the sample. This is puzzling, because you would expect that a single colony on a -3 plate would be backed up by at ~10 colonies of e.coli on the -2. I would think that lab error would be the root cause of this single colony. However, I do know resampling in the face of detected pathogens is a sensitive issue. Would this case be considered unique from a regulatory perspective, where the spread of results from initial testing just don't seem to make scientific sense, so retesting would be in order?



Charles.C

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Posted 23 December 2021 - 03:38 AM

Hello All, reaching out with what I'm sure is a common occurrence in food micro, but did not see it asked on this board. Here is the situation:

 

A food sample is tested for e.coli via petrifilm at a 10^-2 and 10^-3 dilution. The -2 plate (less diluted) shows no e.coli growth, but on the -3 plate (more diluted) there is a single e.coli colony. Our specification is that e.coli should not be detected in the sample. This is puzzling, because you would expect that a single colony on a -3 plate would be backed up by at ~10 colonies of e.coli on the -2. I would think that lab error would be the root cause of this single colony. However, I do know resampling in the face of detected pathogens is a sensitive issue. Would this case be considered unique from a regulatory perspective, where the spread of results from initial testing just don't seem to make scientific sense, so retesting would be in order?

Hi Brothbro,

 

A few comments.

 

I assume the observations relate to a Procedure for evaluation of generic E.coli.

 

A MPN-type Procedure is preferable for the numerical assessment of low levels of generic E.coli which is not, per se, regarded as a pathogen..

 

IMEX It is usual to seed duplicate plates for each dilution. I deduce the other 3 plates involved were all negative.

 

You have probably discovered a statistical aberration but which could reflect a genuine low level of generic E.coli.

 

I suggest to repeat the evaluation using more than one sample / a more statistically precise Procedure if the Specification requirement is significantly important to you.


Edited by Charles.C, 23 December 2021 - 04:35 AM.
edited

Kind Regards,

 

Charles.C


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Brothbro

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Posted 23 December 2021 - 04:22 PM

Hi Brothbro,

 

A few comments.

 

I assume the observations relate to a Procedure for evaluation of generic E.coli.

 

A MPN-type Procedure is preferable for the numerical assessment of low levels of generic E.coli which is not, per se, regarded as a pathogen..

 

IMEX It is usual to seed duplicate plates for each dilution. I deduce the other 3 plates involved were all negative.

 

You have probably discovered a statistical aberration but which could reflect a genuine low level of generic E.coli.

 

I suggest to repeat the evaluation using more than one sample / a more statistically precise Procedure if the Specification requirement is significantly important to you.

 

Hi Charles.C, I appreciate the quick response! Re-evaluation with a greater sample size would indeed give us more insight into whether the single colony reflects lab error, an aberration, or an issue with the manufacturing process. 






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