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Microbiological assay: Its Definition Uses and methods

Updated on August 18, 2018

Why are microbiological assays necessary

Drugs like antibiotics, anti-fungals and anti-protozoans are highly specific for a set of microorganisms.

They can be effective against one microbe and ineffective on other.

Hence, before administering to a patient, they are tested for their effectiveness by use of microbiological assays.

They are routine procedure used in industry, quality control and research. For this purpose, there are even special microorganisms maintained by the government institutes.

Microbial assays or microbiological assays is a type of bioassay and are designed to analyze the compounds or substances which have effect on micro-organisms.

Microbial assay Definition: Microbiological assay is defined as the determination or estimation of concentration or potency of an antibiotic by means of measuring and comparing the area of zone of inhibition or turbidity produced by test substance with that of standard over a suitable microbe under standard conditions.

Hypothesis: So as definition says the hypothesis is that when an antibiotic is administered, there is inhibition in the growth of microbe as indicated by decrease in area of zone of microbial colony on nutrition media or decrease in turbidity due to decrease in microbial concentration.

Uses of microbial assay:

1. They help to estimate concentration: Concentration indicates the amount of substance in a given weight of sample. The more the molecules of a substance, the more is the concentration for given weight and vice-versa. This estimate can also be done by use of other methods like spectroscopy, HPLC etc. But since the substances like antibiotics act on microbes, this microbiological assay can also be used.

2. Determine the potency of anti-biotics. Potency indicates how effective is the substance cause an effect on the living being. One anti-biotics can definite number of microbes at little concentration. While other one can kill the same number of microbes at higher concentration. So the potency of the first is stronger than the latter.This estimation is possible only by use of microbiological assays.

3. Help in determination of the best anti-biotic suitable for patient recovery. When a person is infected with a microbe. He is given anti-microbial drugs. But not all the patients respond similarly to the drug. This is because of many factors and one of them is the resistance of microbe to that drug. Then the patients phlegm or urine is examined by biosaay to find the most effective antibiotic. The microbiological assay is done by using different anti-microbials on the patient serum or urine. The one which shows better effect on the microbe is selected for treatment. This determination is possible by immune assays like ELISA test for some diseases.

Books on microbiological assay

Microbiological Assay for Pharmaceutical Analysis: A Rational Approach
Microbiological Assay for Pharmaceutical Analysis: A Rational Approach

Guide for the evaluation of microbiological assays of Pharmaceutical Analysis


Microbial assays methods

Basically there are two methods of microbiological assays analogous to cell based assays

1. Agar Plate method

2. Turbidometric method

Agar Plate method : In this method the specified microbes are suspended in suitable & sterilized nutrition media. The this nutrition media with microbes is poured into a petri-dish and allowed to incubate at suitable temperature. This supports the growth of microbes exponentially.

The suitably cut circular filter paper pieces are immersed in the anti-biotic solutions of both test and standard. Then the pieces are placed on the microbial nutritional media all over with suitable gaps in between and incubated again.

After specified time of incubation the plates are removed and the diameter of the zone of inhibitions of test and standard samples are measured.

By comparing the areas of zone of inhibition of test with standard the concentration and potency of test samples are determined.

Sputum culture

Turbidometric method

Here the specified microbe is grown in a liquid form of nutrition media. Then the seeded media transferred into various test tubes required to be assigned into groups for standard and test samples evaluation.

Then different dilutions of test and suitable dilution of standard are taken for estimation into the microbe seeded media present in the test-tubes. After incubation at proper temperature for specified time the microbial growth inhibition of both test and standard samples are evaluated.

This is done by measuring the turbidity of the media in different test-tubes using either turbidometer if highly viscous or nephleometry if dilute.

The remaining process of comparison and estimation of concentration or potency is same as plate method.

This is the brief introduction to the microbial assay definition, uses and methods. For more details go through the theory and applications of microbiology.



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    • profile image


      3 years ago

      Thanks alot

    • profile image


      3 years ago

      can we know about interpretation of results

    • profile image


      4 years ago

      Its good i wana to knw also abnut microbiological assay of pencillin.

    • profile image


      4 years ago

      I want to know more about your services.How much time ansiyals take, costs, sample treatment that is needed.I do work on a food company, and my business area is confectionery (both sugar and chocolate).I want to know if you are able to do shelf life testing for chocolates, and for chewy candies (that includes fats) this means, if you have accelerated oxidative shelf life testings for this.Thanks.

    • profile image


      4 years ago

      Hi Yi,The software I used to craete the heatmaps was SigmaPlot, but any plotting software that can do XYZ pairs should work.No, I don't think adding extra liquid in between the wells will do much. If evaporation is truly a problem (I have never encountered an assay where that is true unless you incubate for a really long time), then you can use a humidity chamber to help somewhat. There are also gas permeable membranes that you can cover your plate with. They don't allow water vapor to pass through, but allows other gases to penetrate.I hope that helps,Dan

    • profile image

      Ajeet mcp 

      4 years ago

      Very good method in modern life

    • bheem profile imageAUTHOR


      4 years ago

      mubina! Thanks for the complements..

    • profile image


      4 years ago

      Simply superb

    • Tim Sandle profile image

      Tim Sandle 

      5 years ago from London, United Kingdom

      Very interesting review of current microbiological methods.

    • bheem profile imageAUTHOR


      7 years ago

      Hi guys! thanks for your complements...

    • profile image


      7 years ago

      it is helpful

    • profile image

      vinit sharma 

      7 years ago

      thnx 4 big job done by u

    • profile image


      7 years ago

      The book is rich.

    • profile image


      7 years ago

      assays can also be carried out on nutrients and not only antibiotics

    • profile image

      Ahamed Ismail Hossain 

      7 years ago

      These information are helpful. I hope i will manipulate these properly.


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