The microbial limits test as described in European Pharmacopoeia Chapter 2.6.12 and 2.6.13 and USP Chapter 61 and 62 is used to test the microbiological content of non-sterile pharmaceutical products or raw materials used in the manufacture of other products.
The EP and USP Microbial Limit Test is used to estimate the total number of viable micro-organisms present and for the absence of specific pathogens.
The principle of the test is that any viable microbial organism that may be present in a sample will form a visible separate colony when incubated in appropriate agar medium.
After incubation for appropriate time, enumeration of these colony-forming units (cfu) will give an estimate of the bioburden of the product under the specified incubation conditions.
There are 2 general methods used for microbial limits testing: pour plate, where sample is diluted in sterile medium and poured directly onto agar plate; or membrane filtration where sample is diluted in medium and filtered through a membrane filter which will retain any organisms. Filters are then applied to the surface of agar plate and incubated at appropriate temperature to encourage growth of aerobic bacteria and yeasts and moulds.
If required non-selective enrichment, selective enrichment, selective plating and confirmatory testing will allow specific micro-organisms to be detected.
The presence of micro-organisms in purified water can also be assessed by performing a total viable count using the membrane filtration method.
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