Microbiological Plating Question

dr. Good Bull paojgre at tamu.edu
Tue Jul 18 14:05:00 EST 2000

In article <20000717205202.13122.00001418 at ng-bg1.aol.com>, netomoney at aol.com 
>A reply would be greatly appreciated.
>In a standard 1:10 dilution, two duplicate plate counts of 1mL are prepared. 
>The plate count results are zero, therefore the result is reported as less 
>If a 1:300 dilution is performed, six 5 mL replicate plates are prepared, and
>no counts are observed.  Which is correct to report?  Less then 60 or less 
>If replicate counts are performed, does that increase the sensitivity of the
>test?  For a normal 1:10 dilution with 2 replicate 1 ml counts performed, 
>it correct to report results as less then 5, since 2 mL was actually plated, 
>are the number of replicates disregarded?

Above questions are "interesting" ONLY as thought 'games."

As a practical matter there are very few instances where it makes much
(if any difference) how the above are reported.  The bottom line is that
NO bacteria were recovered-- which just might mean you used the wrong
stuff/conditions etc. to grow them; or it might mean there really is nothing
there; or it might mean less than some number.

Yes, I do understand that for certain water or food quality testing 
there are a maximum allowable number of organisms.  These tests are however
so standardized that you don't really need to bend your mind worrying
about the theory.

Did the above come up on a quiz?

I've seen worse sins than minor inaccuracies in reporting as could happen
in the above scenario. One 'screw-up" is the guy who puts something
in enrichment broth and then does a plate counts on the enrichment broth.
Then, claims that this is valid because if you start with a higher number
of organisms in a sample placed in enrichment, the ultimate number of organisms
found in there would be a function of what was put in.

Keep your eye on the ball. Pay attention to what is really important.
That is usually the practical information acquired by doing a test.

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