[Microbiology] Re: Reporting of enumeration results

N10 via microbio%40net.bio.net (by limbic_lesion from hotmail.com)
Thu Nov 6 18:58:51 EST 2008

"Loh Han Liat" <hanliat from tp.edu.sg> wrote in message 
news:mailman.643.1225999747.29717.microbio from net.bio.net...
Hi All,

I've got questions regarding the reporting of enumeration results based
on plating. The same issue occurs with several testing methods from
FDA-BAM as well as AOAC, and I really need some good advice.

I'll use the example of enumerating for S. aureus, according to FDA-BAM.

Accordingly, I'll divide 1 ml of inoculum over 3 Baird-Parker agar
plates. After incubation, I'll select plates that have 20-200 colonies,
add up the S. aureus colonies on the 3 plates and report as count per g
X the dilution factor. Of course prior to this, I would have determined
the fraction of "true" S. aureus among the presumptive colonies and all

Now, the problem is I never get anywhere close to 20 colonies on any of
my plates, for example, as in the table below.


Plate 1 (0.4 ml)

Plate 2 (0.3 ml)

Plate 3 (0.3 ml)


5 colonies

4 colonies

5 colonies

Assume that I've already confirmed all 14 colonies are S. aureus, how do
I report my results?

There's a clause somewhere in the manual that says if the count from the
lowest dilution is < 20, I could still use the plates. Does this mean I
could report the results as 140 cfu/g? If not, how should I report the

Also how should I report the results if there're no colonies on any of
the plates?

Any advice, opinions or comments from anyone with any experience in this
area would be greatly appreciated.

Thank you very much!



Hi Hl

If you are following  your protocol exactly then you are no doubt returning 
the correct result. The problem with  microbiological assays of this type is 
that at low levels of isolation there is usually a high degree of " 
Uncertainty"  associated with the result of obtained   At  high  levels of 
uncertainty you really should report your result  with a caveat expresing 
the range for  upper an lower limits within which  results might reside.

YOu need to carry out a validation exercise to calculate the degree of 
uncertainty associated with this assay in your lab and possibley with 
respect to  concensus results in ring trials.   Uncertainty is a statiscally 
quanity related to  confidence limits ( therefore functions of varience and 
SD)  and which takes into account repeatability, reporducability , precision 
and bias. There are no set rules for the calculation of uncertainty  but 
some stable guide lines do exist based on sound statiscal criteria.

Thsi link might help as will many other ; google "Microbiological 
uncertainty of meadsurement


Until then report your results :- Any isolation of  confirm SA is 
significant ..usually.

Hope this helps

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