What is the difference between Growth promotion test and Positive control?

In Microbiology, Growth promotion test and positive control is common terminology. Sometime these terms are used interchangeably but there is difference between these two terms.

                                                 E.coli on MacConkey agar

Growth promotion test is used to qualify the media and growth promotion test has to be performed before using that media in any testing. As per pharmacopoeia, growth promotion test has to be performed for every new lot/batch of the media and on the basis of GPT of one container , whole lot/batch of the media could be approved. But in pharmaceuticals different companies have different practices like some perform GPT for every new container or every autoclaved load of the media and this is a good practice though. GPT is just to check whether media is supporting the growth of microorganism or not by inoculating not more than 100 cfu's of particular microorganism. Another term is positive control, which is used to check any positive outcome. For example if we are testing any product for E.coli. Then during observation of characteristic growth on MacConkey agar plate, another plate should be there which is having growth of E.coli streaked on the media for comparison purpose. As we know that E.coli gives brick red coloured colony on MacConkey agar media so by observing both product plate and positive plate (which is having characteristic growth of E.coli) we can differentiate whether the product is having E.coli or not by matching both plates. If characteristic growth matches in both plates then it means product is having E.coli contamination and if characteristic growth does not match then E.coli is absent form the tested product.

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