GPT in Microbiology

have one query regarding GPT

We perform GPT of R2A agar Medium and in that medium we also performed Water isolate…

So my question is, That water isolate is Gram Positive or Gram Negative??

Bcz only Gram Negative Bacteria exist in Water system Acc. To USP.

So only Gram Negative Bacteria will be use as in-house isolate,but we also use Gram positive bacteria in R2A Agar example- Bacillus, So what is the main logic for selecting Water In-house Isolate whether it is Gram positive or Gram Negative.

Please reply me asap.


R2A Agar is a low nutrient medium intended for culturing compromised microorganisms. Good growth of standard, healthy control organisms does not necessarily reflect the ability of the medium to recover stressed organisms.

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Yes sir and only Gram Negative Bacteria shows there presence in pharma water system…and for our water isolate in R2A Agar we use isolate which only comes from water system i.e. Gram Negative Bacteria but i have another query is that we perform GPT of R2A and we use Bacillus subtilis which is Gram Positive…So why we use that Gram Positive in GPT??


The assumption that gram positive bacteria never occur in pharma water is flawed. They hardly ever do, but when they do, it’s usually bacillus. Hence the presence of bacillus in the GPT.

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