Dear Nilesh,
For validating an enrichment test with a 10-gram sample amount into 90 or
490 ml of enrichment broth you should always use an inoculum of 10 to 100
CFU/0.1 ml for each test organism in order to ensure that you are able to
detect low levels by enrichment .
To answer your question about using a 1.0 gram or 10-gram test sample to
validate a plate count method for a 1:10 dilution, a different approach
needs to be made and the math is as follows for validating with less than
100 CFU.:
1.0 gram sample amount for each test microorganism:
1.0-gram of test sample + 8.9-ml of microbial count diluent + 0.1-ml of
inoculum with less than 1000 CFU = the final count should be less than 100
CFU/ml if there are no adverse effects.
Control:
9.9-ml of microbial count diluent + 0.1-ml of inoculum with less than 1000
CFU = final count should be less than 100 CFU/ml
For a 10-gram test sample, everything needs to be multiplied by 10 for each
test microorganism:
10-grams of test sample + 89-ml of microbial count diluent +1.0-ml inoculum
with less than 10,000 CFU = the final count should be less than 100
CFU/ml if there are no adverse effects
Control:
99.0-ml of microbial count diluent + 1.0-ml of inoculum with less than
10,000 CFU = the final count should be less than 100 CFU/ml
A comparison of the 1:10 dilutions for the test sample and control for each
test organism is made for each test microorganism to validate a plate count
procedure for a test sample.
From the math conducted above, it does not matter if a 1.0 or 10-gram test
sample is used for validating a test sample plate count method because the
results should be the same.
As far as incubation, I would always try to incubate the plate count
plates, enrichment broths and selective/differential media for the minimum
time periods that are present in USP Chapters 60, 61, and 62..
Regards,
Don
Donald J. English Microbiological Quality Consulting LLC
Florham Park, New Jersey 07932
On Wed, Nov 13, 2019 at 12:52 PM Nilesh Shah <[log in to unmask]> wrote:
> Dear Forum members,
>
> Query on Mlt validation.
> 1.Can I use 1 g sample in 9 ml diluent and after validation can we use 10 g
> sample in 90 ml diluent during routine testing.
>
> 2.for positive control and product positive control minimum incubation to
> be followed and for product control maximum incubation to be followed.
>
> 3.culture concentration used for specified organisms should be nmt 100 cfu
> in total volume.
>
> Thanks
> Nilesh
> 9820919185
>
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