Growth Promotion Test (GPT) – this combination of words might make you wince, but the test is essential for labs following pharmacopeia guidelines.
GPT is more challenging than a qualitative or ecometric test. It is deemed successful if a small number of cells (100 CFU or less) can grow in or on the media being tested. So, why perform such a demanding test? Here are seven reasons why GPT is important:
1. Qualify new batches of media prepared in-house
Murphy’s Law says if something can go wrong it will go wrong. Things that can go wrong during media production include:
- Improper mixing
- Forgetting an ingredient
- Prolonged heating
- Over-sterilizing media
USP <61>, Microbiological Examination of Nonsterile Products: Microbial Enumeration Tests, states, “Test each batch of ready-prepared medium and each batch of medium prepared either from dehydrated medium or from the ingredients described.”
2. Qualify purchased media
Problems can occur during shipping. The media may be exposed to extreme heat or cold when it is left on a truck without temperature control, a shipping dock or in customs.
3. Ensure a strain is grown at the correct temperature and under the correct atmospheric conditions
A failed growth promotion test can be caused by reasons other than poor media. Other causes are:
- Incorrect incubation temperature
- Wrong atmospheric conditions
Examples of problems caused by using less than optimal growth conditions include:
- No growth of Bacillus subtilis when it was grown in broth with no airspace in the tube
- Aspergilllus brasiliensis that did not turn black when the petri dish was sealed with tape
- Staphylococcus aureus that recovered more slowly on Mannitol Salt Agar when the plates were stacked 20 high instead of 4 high
- Saccharomyces cerevisiae that will not grow at 35°C
4. Test the proficiency of laboratory technicians
Periodic internal proficiency testing ensures technicians can successfully perform the GPT. Get your lab up to speed with these 10 Best Practices for Proficiency Testing.
5. Determine the best media, incubation temperature and atmospheric conditions for an environmental isolate
6. Choose the best brand of media for your laboratory
Although the formulation may be the same from brand to brand, they may not be created equal. Adjustments may need to be made in the formulation to meet the manufacturer’s specification or components may come from different sources. Take time to test media from more than one manufacturer to find which will produce the desired results for your lab. Pick a primary and secondary supplier for media to ensure consistency and stock of the culture media.
7. Determine shelf-life of media prepared in-house
Shelf-life varies widely with the type of medium. Factors that influence shelf-life are sterilization method, preservation and packaging procedures, storage temperature and exposure to light. Each laboratory needs to determine the shelf-life of the media under their storage conditions.