Growth promotion testing (GPT) of selective media following Microbiological Examination of Non- Sterile Products: Tests for Specified Microorganisms Ph.Eur.2.6.13 / USP<62> / JP 4.05 can be quite challenging. Achieving proficiency in selective media GPT requires a strong understanding of the pharmacopeia guidelines and the role of selective media.
Selective media are designed to provide a conducive environment for growth of some types of microorganisms but not to others. They provide the ability to control what will and will not grow on a culture plate. However, it is important to note that a microorganism will not grow as well on a selective media compared to a non-selective media. This is entirely due to non-selective media being well balanced with all the elements that most bacteria need to grow and flourish, whereas in selective media, due to the presence of either activating or inhibiting elements, selected microorganism will grow but not thrive.
Therefore, compendial test requirements for selective media are not quantitatively evaluated. Instead, pharmacopeia requirements for selective media testing state:
Growth of the microorganism comparable to that previously obtained with a previously tested and approved batch of medium occurs.
No percent recovery requirement must be achieved when testing a selective media batch. The qualification is based on growth of the new batch of selective media comparable to growth of previously tested and approved batch of the same selective media.