Growth promotion test (GPT)

A test showing that a batch of culture medium supports growth of specified organisms at a low inoculum, not more than 100 CFU. On solid media, recovery must be within a factor of 2 of an inoculum control or approved batch; in liquid media, growth must be clearly visible. Selective and differential media also need inhibitory and indicative checks. GPT is done on every batch, whether prepared in-house or bought ready to use.