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Coliform, E. coli results between microbial test report and COA

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Posted 02 July 2020 - 01:18 PM

Apologize for my wording or grammar. I have some question that I confuse to explain to team.


Customer request for coliform testing by use desoxycholate agar that the results will interpret to <10 cfu/g.

But the customer want to show in COA as Negative/0.1 g.



1. I would like to know can I interpret from <10 cfu/g to Negative/0.1 g or not?

    If I cannot, Do any one have the correct or standard method that can test coliform

    and E. coli to negative/0.1 g.


2. MPN method also, I reported <3 MPN/g to coliform and E. coli in FG products. 

    But why COA show Negative or Negative in 0.3 g. Is it correct? Or it is not

    correct for microbial testing method. But can assume they are the same results for

    business/marketing reason.


Please help me to clarify. I would to find the reason that I will explain to customer, R&D or QA.


Thank you everyone in advance for your advice.

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Edited by Microbial29, 02 July 2020 - 01:28 PM.


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Posted 02 July 2020 - 03:48 PM



it sounds to me like the customer does not entirely understand the testing being done. A <10 cfu/g should not be interpreted as NEGATIVE because it means that there may be 1-9 cfu/g still there. Reporting as 0.1g would be <1, but still not 0, and also, unless the testing is actually done with 0.1g of product, the results should not be divided. On that same note, I would have more confidence in a 1g sample than a 0.1g sample to actually detect anything.


For  E. coli and coliforms, these results are industry standard, so I'm not sure why the customer is asking for "negative". Same for the MPN. It sounds like they are trying to divide to get a result that's <1 and then calling that negative, and this is not correct. 



You can always reach out to your lab to see if they offer different methods for testing, they could do a qualitative test for E. coli, but I don't think that's truly needed. I think your customers just need to be more educated on the types of results they should expect. 


Hope this helps!


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Posted 02 July 2020 - 06:03 PM

IIRC this is kind of "twisted" micro. semantics. It may also (originally) derive from an older kind of measurement procedure, I forget.


Regardless, the "negative" form may be considered as " nominally"  "interpretable/equatable" to "not detected" in 0.1 gram or  <1cfu in 0.1 gram or <10cfu/gram


Operationally, and assuming a plate count   is involved, <10cfu/g typically means a negative result (ie no colonies) on the 10(-1) plate. The (calculative) logic here  is that assuming a 1ml sample is pipetted,  1 colony would yield a result of 10 cfu/gram so negative means <10cfu/gram.   <10 cfu/gram is IMO a more preferable format assuming the above procedure was used.


similarly negative in 0.3 gram can be interpreted as <1 cfu in 0.3 gram, or <( 1/0.3) cfu in 1 gram or approx < 3 cfu/gram

But I think comparing  to MPN is a bit more questionable since procedure-wise this is not a direct count.


Similarly the use of "Negative" for salmonella is nowadays usually frowned on since a sample from a parent lot can not prove total absence. Accordingly  preference is given to  "not detected in 25g" (which is sometimes equated to < 0.04 cfu/gram by same logics as above :smile: )


These "manipulations" are occasionally explained in micro. textbooks but IMEX the "negative" style is rarely used in current Literature.  (However the one for salmonella is still  utilised IIRC in  quantitative MRAs).


PS -  i think using desoxycholate agar for coliform is rather unusual, what is the product ?

Kind Regards,



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