Culture media, also known as growth media, are specific mixtures of nutrients and other substances that support the growth of microorganisms such as bacteria and fungi (yeasts and molds). Culture media are used for quality control tests of nonsterile raw materials and finished products as well as for microbial contamination (sterility) tests in applications such as hygiene monitoring, sterilization process validation and determination of the effectiveness of preservatives and antimicrobial agents.
Microbial enumeration tests for nonsterile products are performed according to the harmonized test procedures of the European, US and Japanese pharmacopoeias. These tests, formerly known as Microbial Limit Tests (MLT), determine the bioburden of the product sample. This is achieved by counting the number of colony forming units that have grown on the culture media for a known sample size (CFU/g). Acceptance criteria are based on the Total Aerobic Microbial Count (TAMC), the Total Yeast and Mold Count (TYMC). Furthermore, specific tests are undertaken to confirm the presence or absence of certain microorganisms that may prove to be objectionable.
The culture media ingredients vary according to the test being performed and the microorganism of interest. Most commonly, culture media are based on a nutrient broth (liquid), often mixed with agar and prepared in petri dishes (they may be semi-solid or solid). For enriched or selective media, further specific components are added.
Culture media preparation must be carried out accurately to ensure microbiological growth is correctly promoted. The individual ingredients of the culture media (powders, gels and liquids) must be carefully weighed out according to the culture media formulation recipe. A precision balance with readability from 1 mg up to 10 mg is typically used for the main components. An analytical balance may be required for weighing out trace elements such as copper and zinc. If a smaller amount of culture media is required, care must be taken to correctly recalculate the ingredient quantities and a balance with a higher readability may be required in order to fulfil accuracy requirements.