[Microbiology] Reporting of enumeration results

Loh Han Liat via microbio%40net.bio.net (by hanliat from tp.edu.sg)
Thu Nov 6 02:30:41 EST 2008


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
that.

 

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

 

Dilution

Plate 1 (0.4 ml)

Plate 2 (0.3 ml)

Plate 3 (0.3 ml)

10-1

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
results?

 

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!

 

Regards

HL



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