Standard medium for use in antibiotic assays
FORMULA IN g/l
Final pH 7.0± 0.2 at 25ºC
Suspend 17.5 grams of medium in one liter of distilled water. Soak for 10-15 minutes. Mix well and dissolve by heating with frequent agitation. Boil for one minute until complete dissolution. Sterilize in autoclave at 121ºC for 15 minutes. The prepared medium should be stored at 8-15°C. The color is clear amber.The dehydrated medium should be homogeneous, free flowing and beige in color. If there are any physical changes, discard the medium
ANTIBIOTIC MEDIUM Nº 3 is prepared according to the formula specified by the Food and Drug Administration (FDA) and the United States Pharmacopoeia (USP).
Gelatin peptone, Yeast extract and Beef extract provide nitrogen, vitamins, minerals and amino acids essential for growth. Potassium phosphates act as a buffer system. Dextrose is the fermentable carbohydrate providing carbon and energy. Bacteriological Agar is the solidifying agent.
The potency of an antibiotic can be demonstrated under appropriate conditions by its inhibitory effect on microorganisms. Reduction in antimicrobial activity may reveal changes not demonstrated by chemical methods.
Antibiotic Medium N° 3 can be used with the following microbiological methods for Antibiotic Assays:
1. Cylinder method in plates.
2. Serial dilution method.
3. Turbidimetric method.
In the cylinder method in plates, Antibiotic Medium N° 3 is used to resuspend the inoculum in the potency assay for penicillin, erythromycin, neomycin, chlortetracycline and chloramphenicol.
The serial dilution method is used for penicillin assay.
Lastly, this medium can also be used in the turbidimetric determination of the potency of bacitracin, streptomycin and terramycin. The turbidimetric method is based on the inhibition of growth of a microbial culture in a fluid medium containing a uniform solution of an antibiotic. Use of this method is appropriate only when test samples are clear.
Plates are prepared and incubated following the FDA and the USP guidelines. The use of standardized culture media and strict control of all test conditions are essential requirements in the microbiological assay of antibiotics in order to obtain satisfactory test results.
The following results were obtained in the performance of the medium from type cultures after incubation at a temperature of 3 5 ± 2 ° C and o bser v ed a f t e r 24 – 48 h ou r s .
Micrococcus luteus ATCC 9431
Staphylococcus aureus ATCC 6538
Klebsiella pneumonia ATCC 10031
Grove and Randall. Assay Methods of Antibiotics, Medical Encyclopedia Inc. New York 1955. United States Pharmacopoeia Convention. 1955. The United States Pharmacopoeia, 23rd Ed. Biological Tests and Assays, p. 1690-1696. The United States Pharmacopoeia Convention, Rockville, Md
Once opened keep powdered medium closed to avoid hydration.