Starch Agar

Cat. no. G294 Starch Agar, 15x100mm Plate, 18ml 10 plates/bag


Hardy Diagnostics Starch Agar is used in determining starch hydrolysis by microorganisms.


Many bacteria produce extracellular enzymes used to catalyze chemical reactions outside of the cell. In this manner, nutrient sources, such as starch, that are too large to be absorbed through the cell membrane can be broken down into smaller molecules and transported into the cell via diffusion.

The base medium of Starch Agar is Nutrient Agar, to which soluble starch has been added. Beef extract and pancreatic digest of gelatin provide nitrogen, vitamins, carbon and amino acids. Agar is the solidifying agent and starch is the carbohydrate. When starch is present, it forms a complex with Gram's iodine to yield a blue color. Organisms capable of hydrolyzing starch through amylase production will produce a clearing zone around the inoculum while the remaining medium is blue.


Ingredients per liter of deionized water:*

Potato Starch 10.0gm
Pancreatic Digest of Gelatin 5.0gm
Beef Extract 3.0gm
Agar 15.0gm

Final pH 6.8 +/- 0.2 at 25ºC.

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


Storage: Upon receipt store at 2-8ºC. away from direct light. Media should not be used if there are any signs of deterioration (shrinking, cracking, or discoloration), contamination, or if the expiration date has passed. Product is light and temperature sensitive; protect from light, excessive heat, moisture, and freezing.



Specimen Collection: Specimens should be submitted directly to the laboratory without delay and protected from excessive heat and cold. If there is a delay in processing, specimens should be inoculated into an appropriate transport medium and refrigerated until inoculation. Consult listed references for more information.(2-5,7)

Method of Use:

1. Allow the plate to warm to room temperature prior to use.

2. Inoculate the medium with an 18-24 hour old isolated colony of the test organism using a single heavy streak or a spot inoculum of approximately 5mm diameter.

3. Incubate plates at 35ºC. for 24-48 hours in an aerobic atmosphere.

4. Following incubation, flood the streak or spot growth with Gram's iodine (Cat. no. I008N) to fully cover the inoculum and neighboring agar. Note: Iodine reacts with starch to form a dark blue-colored complex.


Any zone of clearing surrounding growth of the culture after the addition of Gram's iodine is positive for starch hydrolysis due to the production of amylase, an extracellular enzyme. Cultures negative for starch hydrolysis would lack a clearing zone.


Colonies cannot be subcultured from the medium after the addition of Gram's iodine due to the oxidative nature of the reagent and the resulting cell death.


Standard microbiological supplies and equipment such as loops, swabs, applicator sticks, other culture media, incinerators, Gram's iodine (Cat. no. I008N), and incubators, etc., as well as serological and biochemical reagents, are not provided.


Test Organisms Inoculation Method* Incubation Results
Time Temperature Atmosphere
Bacillus subtilis
ATCC® 6633**
E 24-48hr 35°C Aerobic Growth; colorless zone demonstrating positive starch hydrolysis
Escherichia coli
ATCC® 25922**
E 24-48hr 35°C Aerobic Growth; no colorless zone demonstrating negative starch hydrolysis

** Recommended QC strains for User Quality Control according to the CLSI document M22 when applicable.


Physical Appearance

Starch Agar should appear slightly hazy, and colorless.


1. Anderson, N.L., et al. Cumitech 3B; Quality Systems in the Clinical Microbiology Laboratory, Coordinating ed., A.S. Weissfeld. American Society for Microbiology, Washington, D.C.

2. Jorgensen., et al. Manual of Clinical Microbiology, American Society for Microbiology, Washington, D.C.

3. Tille, P., et al. Bailey and Scott's Diagnostic Microbiology, C.V. Mosby Company, St. Louis, MO.

4. Isenberg, H.D. Clinical Microbiology Procedures Handbook, Vol. I, II & III. American Society for Microbiology, Washington, D.C.

5. Koneman, E.W., et al. Color Atlas and Textbook of Diagnostic Microbiology, J.B. Lippincott Company, Philadelphia, PA.

6. Quality Assurance for Commercially Prepared Microbiological Culture Media, M22. Clinical and Laboratory Standards Institute (CLSI - formerly NCCLS), Wayne, PA.

7. U.S. Food and Drug Administration. Bacteriological Analytical Manual. AOAC, Arlington, VA.

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