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Top 10 Tips for Microbial Limit Test

These tips shall help the analyst to reduce the analytical errors in Microbial Limit Test and help to get accurate and error free results.
The microbial limit test is the quantitative and qualitative assessment of the microbes or microbial contamination test in a sample. This test is performed for:- estimation of the total viable count of fungi and bacteria i.e. Quantitative estimation. Identification of each of the microbes of importance by culturing on selective media i.e. Qualitative estimation.

In quantitative estimation, two tests are carried out, one total fungal count and other is total bacterial count. For determination of total fungal and total bacterial population size in a given sample. Total four methods are there for microbial limits test: A) Pour Plate Method B) Filtration Method C) Serial Dilution Method D) Spread Plate Method

Each of these methods has their own error susceptibility but here we will talk in general about avoiding errors. Top 10 tips for the microbial limit test in pharmaceuticals include:
Microbial Limit Test
1. Media: For the cultivation of test organism, select the media which favors the fast growth of the microbes. The recommended media are Soybean casein digest broth/agar, and Sabouraud's dextrose agar broth/media.

2. Growth Promotion Capability of the Media: The media we are using for the growth of organisms, we need to choose it wisely. Media need to promote the inoculation of the specific microbes of interest. Antimicrobial effectiveness testing must be done for the product.

3. Time Consumption: Traditional method of Microbial Limit test requires a trained microbiologist analyzing the culture plates after an interval of time. So by using the Growth Direct System, we can reduce the time required for testing by about 50%. This system records the growth of the culture plate every four hours. And it also can detect any contamination. This system analyzes hundreds of culture plates at a time saving the time of microbiologist.

4. Limited Accuracy: Colonies need to grow up to millions of cells before they can be visible to the naked eyes, so a well trained and experienced person may mistake in counting. So the specialist needs to be highly alert and try to avoid errors.

5. Errors of Transfer: Some of the sensitive tests requires serial incubation where samples are delivered from one incubator to another in a specific time at different temperature. So it better to use the Growth Direct system, which automates the process and reduces the error of human effort.

6. Data Entry: After the technician has performed the counting then it is must avoid data entry errors. Technicians can make errors while dealing with hundreds of samples, this error of data entry must be avoided because it will alter the output of the well-performed test.

7. Temperature: Range of temperature must be considered depending on the experimental needs. It depends on the microorganism and the culture media.

8. Atmosphere: Proper atmospheric conditions like aerobic, anaerobic, facultative aerobic/anaerobes, capnophilic and obligate anaerobic conditions must be considered.

9. pH: A appropriate range of pH is required for any microorganism to grow. Any deviation from the pH will inhibit the growth of microbes even if other conditions are favorable. An appropriate pH is required for enzymatic functioning and metabolic activities of microbes.

10. Sterile Conditions: While performing the MLT, it is essential to maintain sterile/aseptic conditions. Any contamination shouldn't invade into the test, otherwise, it would affect the result.

Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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