About Growth promotion test i have question when you use inoculum standarized.
Currently in the lab we used EZ-CFU One step microorganisms form microbiologics. This strains have a CoA where certify the inoculum, for example A. brasiliensis 50+/-2 CFU/0.1 ml.
Taking in account this information, we also should the recovery of the microorganisms in an old batch of media? or we can use the CoA and asume that the acceptance criteria will be 25-100 cfu ( 2 factor) ?
The doubt born in the chapter 61 and FAQs from USP that established the following:
USP Growth Promotion of the Media Chapter <61> (harmonized).
1) "For solid media, growth obtained must not differ by a factor greater than 2 from the calculated value for a standardized inoculum. For freshly prepared inoculum, growth of the micoorganisms comparable to that previously obtained with a previously tested and approved batch of medium occurs."
In the case 1, USP makes a differences between the use of standarized inoculum and freshly prepared inoculum but not stablish if we need to do the recovery in other batch also. The same for FAQs below:
2) Is there a method that provides verification that there is 100 CFU in the inoculum?
You may establish a turbidimetric calibration curve or use another suitable method and then you will be able to get an estimate of the concentration of your inoculum. This can be later confirmed by standard plating methods, such as described in USP General Chapter <51>. You can also use ready-to-use certified strains.
Finallly in one FAQs, USP states the following:
7. Do we have to test systematically in parallel a previous and approved batch in order to compare with the new batch?
You do not have to test a previous batch in parallel. You can do the comparison 'on paper' if growth was clearly described.
This last statement is more confuse because we can do the comparison on paper independent if we use ready-to-use certified strains or freshly prepared inoculum?
i am open for your comments and recommendations.
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