Recently, the United States Pharmacopoeia published FAQ relative to USP General Chapter <62> - Microbial Enumeration of Nonsterile Products: Tests for Specified Microorganisms. Following you can find an excerpt:
Q. Can I use other strains than those that are cited in the USP?
A. You should use the strains that are cited in this chapter, or equivalent strains from other culture collections.
For example, if Pseudomonas aeruginosa ATCC 9027 is indicated, you should use this strain or strains from other culture collections claiming equivalence to ATCC 9027. Other strains such as ATCC 14149 are not appropriate.
Q. When are you actually supposed to do the negative control: when testing the suitability of the method, or when testing the product, or in both situations?
A. You are supposed to do the negative control at the same time as when you are testing the product.
Q. What is the purpose of the negative control?
A. The purpose of the negative control is to show that there is no contamination during the testing of the product. If a positive result is obtained with a negative control, the test can be regarded as invalid and may be repeated.
Q. Does it have to be done every time the product is tested or during the method validation or is it possible to do it periodically?
A. Negative controls should be included every time that the product is tested.
Q. Is it necessary to test the growth promotion on all received batches or does it serve just for microbiological validation?
A. Growth promotion must be checked for each new batch of medium.
Q. What are the specifications when we compare a freshly batch with a previous batch for growth promotion properties? Do we need to take a factor of 2 into account?
A. The factor of 2, as described in USP <61> can be used. No strict requirement was deliberately given in this chapter because the test is qualitative, not quantitative. You can define the comparability criterion yourself. For example, colony size at the shortest incubation time prescribed.
Q. You should not incubate more than the « incubation time prescribed ». How exactly are the given times (e.g. 18 - 24 h) to be followed?
A. For growth promoting properties of media you must incubate not more than 18 h (worst case conditions).
For inhibitory properties of media you must incubate not less than 24 h (worst case conditions). For indicative properties of the media you could incubate within the specified range (here, from 18 h to 24 h).
Q. In the growth promotion test of Rappaport Vassiliadis Salmonella enrichment broth there is no visible growth after the incubation time, but after subculturing on selective agar there is typical growth. Is this the case only in our laboratory?
Do we have to test systematically in parallel a previous and approved batch in order to compare with the new batch?
A. This can be observed, since for Rappaport Vassiliadis Salmonella enrichment broth, we inoculate low numbers of Salmonella sp (usually the inoculum is around 20 CFU per 10 mL Rappaport Vassiliadis Salmonella enrichment broth).Even if the enrichment broth seems clear, you must confirm recovery of Salmonella by subculturing the Rappaport Vassiliadis Salmonella enrichment broth to solid agar.
Please also see the complete FAQs "Frequently Asked Questions—USP General Chapter <62>—Microbial Enumeration of Nonsterile Products: Tests for Specified Microorganisms".
Axel H. Schroeder
CONCEPT HEIDELBERG (a service provider entrusted by the ECA Foundation)