CRITERION™ Antibiotic Medium #19

Cat. no. C8321 CRITERION™ Antibiotic Medium #19 500gm


Hardy Diagnostics CRITERION™ Antibiotic Medium #19 is recommended for determining antibiotic potency by the microbiological assay technique as described by the United States Pharmacopeia (USP <81>).(4,6,8)

This dehydrated culture medium is a raw material intended to be used in the making of prepared media products, which will require further processing, additional ingredients, or supplements.


Microbial and biological assays to establish the inhibitory effect on microorganisms remain the gold standard for determining antibiotic potency. Subtle changes in potency may be undetectible by chemical means; consequently, evaluating antimicrobial efficacy by microbial methods may resolve concerns related to potential loss of activity. Two methods are outlined by the United States Pharmacopeia (USP) General Chapter <81>: the cylinder-plate or "plate" assay and the turbidimetric or "tube" assay.(8)

The cylinder plate method was first described by Abraham et al. for the determination of penicillin and was later modified by Foster and Woodruff and Schmidt and Moyer.(1,3,7) This method employs diffusion of an antibiotic solution from a cylinder through a solid agar medium to produce a "zone" of inhibited growth around the region of the medium containing the antibiotic. The cylinder plate method is useful for determining commercial preparations of antibiotics, in addition to quantitative determination of antibiotics in bodily fluids, animal feeds and other materials.

In contrast, the turbidimetric method depends on the inhibition of growth of a culture in a uniform solution of antibiotic, which is added to a growth promoting broth medium. Turbidimetric methods have an advantage over the cylinder plate method by providing results after three to four hours. However, this method is prone to interference from solvents or other inhibitory materials that may also influence turbidimetric assays. Thus, turbidimetric methods are most appropriate when the test sample is initially optically clear. However, with either method, the use of standardized culture media and careful and consistent control of all test conditions is essential in order to achieve satisfactory and meaningful results.

CRITERION™ Antibiotic Medium #19 was originally described by Kirshbaum and Arret and is prepared according to the specifications set forth by the USP, European Pharmacopeia (EP) and AOAC International.(2,5,8) Antibiotic Assay Media are traditionally identified numerically as assigned by Grove and Randall, with the exception of Antibiotic Medium #19 which was described by Kirshbaum and Arret.(4,6) The formulation contains peptone, yeast and animal extracts, and dextrose (D-glucose) to provide essential nutrients and other factors required for growth. Agar is the solidifying agent.


Gram weight per liter: 60.0gm/L
Dextrose 10.0gm
Sodium Chloride 10.0gm
Peptone 9.4gm
Yeast Extract 4.7gm
Beef Extract 2.4gm
Agar 23.5gm

Final pH 6.1 +/- 0.1 at 25ºC.

* Adjusted and/or supplemented as required to meet performance criteria.


Store the sealed bottle(s) containing dehydrated culture medium at 2-30ºC. Dehydrated culture medium is very hygroscopic. Keep lid tightly sealed. Protect dehydrated culture media from moisture and light. The dehydrated culture media should be discarded if it is not free-flowing or if the color has changed from its original light tan.

Store the prepared culture media at 2-8ºC.



1. Suspend 60.0gm of the dehydrated culture media in one liter of distilled or deionized water. Stir to mix thoroughly.

2. Heat to boiling for one minute to dissolve completely.

3. Sterilize in the autoclave at 121ºC. for 15 minutes.

4. Cool to 45-50ºC. and pour desired volume into pre-sterilized petri dishes.


For information on procedures and interpretation of results, consult listed references.



Standard microbiological supplies and equipment such as autoclaves, incinerators, and incubators, etc., are not provided.


Test Organisms Inoculation Method* Incubation Results
Time Temperature Atmosphere
Saccharomyces cerevisiae
ATCC ® 2601
A 48hr 30°C Aerobic Growth

User Quality Control


1. Abraham, E.P., E. Chain, C.M. Fletcher, A.D. Gardner, N.G. Heatley, M.A. Jennings, and H.W. Florey. 1941. Further Observations on Penicillin. Lancet 2:177-188.

2. Council of Europe. 2002. European Pharmacopeia, 4th ed. Council of Europe. Strasbourgh, France.

3. Foster, J.W. and H.B. Woodruff. 1943. Microbiological Aspects of Penicillin; Methods of Assay. J. Bacteriol. 46:187-202.

4. Grove, D.C. and W.A. Randall. 1955. Assay Methods of Antibiotics. New York, NY. Medical Encyclopedia, Inc.

5. Horwitz, W. (ed.). 2005. Official Methods of Analysis, 18th ed. Association of Analytical Communities (AOAC) International, Gaithersburg, MD.

6. Kirshbaum, A. and B. Arret. 1967. Outline of detail for official microbiological assays of antibiotics. J. Pharm. Sci. 56(4):511-515.

7. Schmidt, W.H. and A.J. Moyer. 1944. Penicillin; Methods of Assay. J. Bacteriol. 47:199-209.

8. The Official Compendia of Standards. 2008. USP General Chapter <81> Antibiotics - Microbial Assays. USP27-NF22. United States Pharmacopeial Convention, Rockville, MD.

ATCC is a registered trademark of the American Type Culture Collection.