Bacteria to Consider for the Growth Promotion Test: Should Anaerobic Cultures Be Included?


The purpose of the Growth Promotion Test (GPT) is to determine the appropriateness of the culture media to be used in environmental monitoring sampling. The medium is challenged with a small number of microorganisms to assure the nutritive properties, functionality and media quality are adequate for the detection and enumeration of microbial contaminants.

The test is described in USP Chapter <61>, Microbiological Examination of Nonsterile Products: Microbial Enumeration Tests, USP Chapter <62>, Microbiological Examination of Nonsterile Products: Tests for Specified Substances, and USP Chapter <71>, Sterility Tests. It is also found in the corresponding European Pharmacopoeia chapters 2.6.12, 2.6.13, and 2.6.1.

The test may be run on solid media and in broths according to the European and United States Pharmacopoeias. GPT is performed by inoculating the product with ≤ 100 CFU of microorganisms, defined by the pharmacopoeia. The test can be quantitative, at which 50 – 200% of the inoculated microorganisms should be recovered after incubation of the media, or it can be qualitative, where the requirement is “growth” or “no growth”.

It is important to stress that the GPT must be tested for each new batch of medium. Growth promotion must be checked on agar media and nutritive broth but not on diluted broth.

The procedure to follow is summarized below:

  1. The new medium is inoculated with 100 or less CFU of microorganisms.
  2. The microorganisms recommended by the pharmacopeia are used in the test. Microorganisms must be no more than 5 passages removed from the original reference culture seed lot.
  3. After incubation, growth on the new batch of medium is compared to growth on the previously approved batch of medium.

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