Preparation of antibiotic stock solutions and dried paper discs

This post was most recently updated on May 13th, 2017

Preparation of antibiotic stock solutions
Antibitiotics 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.