Sterility testing - procedural requirements part-2

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Neutralization of antimicrobial agents in dosage forms1

The need of neutralization of antimicrobial agent before performing actual sterility test is to detect microorganisms which may present in formulation. The antimicrobial agent restrict the growth of microorganisms when one perform the sterility test by introducing formulation in culture medium and false results may obtained. Because of small size of culture medium, concentration of antimicrobial agent is sufficient to inhibit the growth of microbes but when the same formulation is introduced inside body immediately formulation is distributed and effect of antimicrobial agent inside the body get diluted in presence of large quantity of blood. So that time microbes which are inhibited/ restricted by the action of antimicrobial agent start growing rapidly inside the blood and travel across the body along with blood which may cause very harmful effects.

e.g. when a thief inside the custody he behaves like an innocent person due to control of police but when he will come out and got enough space to escape he shows his own nature and start stealing the things. Similar condition happens with microorganisms in culture medium and in body.

But, one has to see the presence of microbes and for that growth of microbes in culture medium should not be restricted by any means. So, if any antimicrobial or antibiotic which is present in formulation, first of all one has to neutralize this agent.

There are three main methods are usually employed;

Dilution method (generally used in case of phenols, alcohols, barbiturates etc)

Chemical neutralization (used for the substances which required high amount of medium for neutralization by dilution or substances which are adhere with bacterial cell membrane like mercuric compounds, arsenical compounds, quaternary ammonium compounds, sulphonamides, parabens, chlorhexidine etc)

Filtration method (most preferable method for antibiotics and most of the other antimicrobial agent)

Diluting Fluids2

Diluting fluids are also very important in case of sterility testing method. Official books were given the composition and method of preparation of two diluting fluids viz,. fluid A and fluid B.

Composition and method of preparation--

Fluid A:
Dissolve 1 gm of peptic digest of animal tissue i.e. bacteriological peptone in water and make up the volume upto 1000ml. Filter it and pH adjusted to 7.1+-0.2. Take fluid in 100 ml flasks and sterilised at 121C for 20min.

Fluid B: Same composition as fluid A with addition of 1ml polysorbate 80.

These fluids are used to dilute your formulations/ articles such as powder, suspensions, ointment, emulsion, cotton, surgical sutures/ligatures, surgical scissors, other instruments and many more whenever required so that uniformity of sample can be achieved.

So, these are some requirements for conducting actual sterility testing.


  1. Cooper and Gunn's Dispensing Pharmacy. CBS publisher, New Delhi, 12 th Ed: 541-572.
  2. Indian Pharmacopoeia. Govt. of India, ministry of health and family welfare. India, 2007: 2.2.11-56.

Note: This page does not contain any plagiarized material

About the Author

P.V.ABHIGNA's picture


Santosh kumar. JH's picture

Abhigna, Once again a informative blog from your side... Please add the compositions of diluting fluids: Fluid A and Fluid B
P.V.ABHIGNA's picture

Hi Santosh, I updated my page as per ur suggestion. Thanks for your valuable suggestion. regard,


Niklesh Rao V's picture

Very informative blog Abhigna. I liked the analogy of the thief. Are there dilution fluids in the USP too?

Regards, Niklesh Rao V

P.V.ABHIGNA's picture

Niklesh, Firstly thank you for the support.This information is actually well described in IP. Regards,


Sanjay Patel's picture

Thanks for compiled information

Sanjay Patel M.Pharm, LL.B (Continue) Indian Patent Agent

P.V.ABHIGNA's picture

Dear Sir, Thanks for your comment. I Also try to update if you have any suggestions. please. Regards,'


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