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?
Murphy's Law says if something can go wrong it will go wrong. Things that can go wrong during media production include:
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.
A failed growth promotion test can be caused by reasons other than poor media. Other causes are:
Examples of problems caused by using less than optimal growth conditions include:
Periodic internal proficiency testing ensures technicians can successfully perform the GPT.
A little experimentation may be needed to choose the best media and growth conditions for difficult-to-grow environmental isolates such as Burkholderia cepacia and moulds.
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.
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.