The potency of an antibiotic can be determined by chemical, physical and biological means. An assay is made to determine the ability of an antibiotic to kill or inhibit the growth of living microorganisms. Biological tests offer the most convenient means of performing an assay (1), since a reduction in the antimicrobial activity of a specific antibiotic reveals changes not usually displayed by chemical methods (2). Antibacterial susceptibility testing may be performed by either dilution (turbidimetric) or diffusion methods. The choice of methodology is often based on many factors, including relative ease of performance, flexibility and use of automated or semi-automated devices for both identification and susceptibility testing (3). Grove and Randall have elucidated those antibiotic assays and media in their comprehensive treatise on antibiotic assays (4). Antibiotic Assay Medium No.1 is used in the microbiological assay of ß-lactam and other antibiotics. These media are prepared according to the specifications detailed in various pharmacopoeias (2,5,6) and by the FDA (7).
Freshly prepared plates should be used for antibiotic assays. Test organisms are inoculated in sterile seed agar cooled to 40-45oC and spread evenly over the surface of solidified base agar. After incubation the concentration of the antibiotic being assayed is determined by measuring the zone of inhibition obtained, with that of reference standard antibiotic. All conditions in the microbiological assay must be carefully controlled. The use of standard culture media in the test is one of the important steps for good results.
Nutrients and growth factors are supplied by the ingredients like peptic digest of animal tissue, casein enzymic hydrolysate, yeast extract and beef extract. Dextrose is supplemented as a carbon and energy source.
Storage and Shelf-life:
Store below 30°C in tightly closed container and use freshly prepared medium. Use before expiry date on label.
References:
1. Pelczar M. J. Jr., Reid R. D., Chan E. C. S., 1977, Microbiology, 4th Edi, Tata McGraw-Hill Publishing Company Ltd, New Delhi
2. The United States Pharmacopoeia, 2009, The United States Pharmacopoeial Convention,Rockville, MD.
3. Murray P. R., Baron J. H., Pfaller M. A., Jorgensen J. H. and Yolken R. H., (Eds.), 2003, Manual of Clinical Microbiology,8th Ed., American Society for Microbiology, Washington, D.C.
4. Grove and Randall, 1955, Assay Methods of Antibiotics Medical Encyclopedia, Inc.New York.
5. European Pharmacopoeia, 2009, European Department, for the Quality of Medicines
6. British Pharmacopoeia, 2009,The Stationery office British Pharmacopoeia
7. Tests and Methods of Assay of Antibiotics and Antibiotic containing Drugs, FDA, CFR, 1983 Title 21, Part 436, Subpart D, Washington, D.C.: U.S. Government Printing Office, paragraphs 436, 100-436, 106, p. 242- 259 (April 1).