Preparation of Antibiotic Stock Solutions and Discs

By Acharya Tankeshwar •  Updated: 05/30/21 •  1 min read

Preparation of antibiotic stock solutions

Antibiotics may be received as powders or tablets. It is recommended to obtain pure antibiotics from commercial sources, and not use injectable solutions.

Powders must be accurately weighed and dissolved in the appropriate diluents to yield the required concentration, using sterile glassware.

Standard strains of stock cultures should be used to evaluate the antibiotic stock solution. If satisfactory, the stock can be aliquoted in 5 ml volumes and frozen at -20ºC or -60ºC.

Stock solutions are prepared using the formula (1000/P) X V X C=W, where

  1. P= potency of the antibiotic base,
  2. V=volume in ml required,
  3. C=final concentration of solution and
  4. W=weight of the antimicrobial to be dissolved in V.

Preparation of dried filter paper discs

Whatman filter paper no. 1 is used to prepare discs approximately 6 mm in diameter, which are placed in a Petri dish and sterilized in a hot air oven. Dispense 0.005 ml (5 microliter) of antibiotic solultion using sterile micropipette tips.

Acharya Tankeshwar

Hello, thank you for visiting my blog. I am Tankeshwar Acharya. Blogging is my passion. As an asst. professor, I am teaching microbiology and immunology to medical and nursing students at PAHS, Nepal. I have been working as a microbiologist at Patan hospital for more than 10 years.

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6 responses to “E-TEST (Epsilometer): Principle, Procedure, Results”

  1. Anonymous says:

    can these strips be used for any kind of bacteria…or there are some specificities????? could i use them for Lactobacillus ???? if yes… and which media i can use????

    • Tankeshwar Acharya says:

      E-Test strip is being used for variety of Bacteria (both Gram Positive and Gram Negative). In our laboratory, we mostly perform disc diffusion method while testing for AST, and in some instances we use E-Test Strip. We do not isolate Lactobacillus often, so do not perform AST on them in routine basis. E-Test can be performed for Lactobacillus too. Read this article for more information:

  2. Darrell says:

    I worked in a clinical microbiology lab for a number of years. I initially did some MIC tests but gradually the lab ceased performing the test. Anyway, since I’m in a rush, I could not complete the reading, although I’m really keen to return to your blog, later. However, I wish you could share with me some disk diffusion techniques on ESBL.

  3. majida malik says:

    hello, i need to no the breakpoints for aminoglycosides in p. aeruginosa and staph.aureus

  4. Kishwar says:

    why the references of this article are not showing? I need references? can you mail me?

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