SKIM MILK AGAR

Cat. no. G138 Skim Milk Agar, 15x60mm Plate, 11ml 10 plates/bag

INTENDED USE

Hardy Diagnostics Skim Milk Agar is used to determine proteolysis by microorganisms and is recommended for the cultivation and confirmation of Pseudomonas aeruginosa in the testing of recreational waters. (1,2) This media is also useful for the detection of Pseudomonas aeruginosa in the food and dairy industry. (3,5)

SUMMARY

Many different methods have been used to enumerate Pseudomonas aeruginosa from water samples. The most-probable-number (MPN) procedures result in satisfactory recovery of Pseudomonas aeruginosa , but are not suitable for large-volume water testing and lack precision. The membrane filter (MF) techniques eliminate these deficiencies. Skim Milk Agar acts as a differential and confirmatory agar for the identification of Pseudomaonas aeruginosa in water. (1)

Pseudomonas species are possibly the organism most often isolated from large bodies of water. Some Pseudomonas species have been linked to eye, ear and skin infections after exposure to recreational bodies of water and thus may serve as an indicator of recreational water quality. Pseudomonas aeruginosa is commonly found in drinking water. Pseudomonas aeruginosa has been found to be very resistant to ozonation processes and chemical disinfection in swimming pools and thus underscores its importance. (2)

Psychrotrophic bacteria, such as Pseudomonas aeruginosa , are strongly proteolytic and are often responsible for the spoilage of both meat and dairy foods. (3,4) This spoilage can result in a stale, bitter or rancid taste and smell. (3) It is generally known that Pseudomonas species are the organisms most often responsible for the spoilage of fish. (5)

The hydrolysis of casein (a primary milk protein) is often used to evaluate proteolytic activity of organisms. The enzyme caseinase hydrolyzes the protein casein. Hardy Diagnostics Skim Milk Agar is an improved formulation from standard skim milk formulas, demonstrating a greater sensitivity. (11) It contains casein and glucose as carbon sources for growth promotion. Yeast extract is added to the medium as a vitamin source. Pseudomonas aeruginosa hydrolyzes casein and is indicated by a zone of clearing around the colonies. There may also be a yellow to green pigment diffused in the media. (1)

FORMULA

Ingredients per liter of deionized water:*

Dry Milk, Instant Nonfat 50.0gm
Pancreatic Digest of Casein 5.0gm
Yeast Extract 2.5gm
Glucose 1.0gm
Agar 12.5gm

Final pH 6.8 +/- 0.3 at 25ºC.

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

STORAGE AND SHELF LIFE

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.

PRECAUTIONS

PROCEDURE

For the presumptive detection of Pseudomonas aeruginosa from a recreational water, a 200-500ml sample of water is filtered through a sterile membrane filter. The membrane is then placed on a poured plate of Modified m PA (Cat. no. G133). Inoculated plates are inverted and incubated at 41.5ºC. for 72 hours. Colonies are isolated from m PA and sub cultured onto the Skim Milk Agar (Cat. no. G138) making a single streak 2 to 4cm long and incubated at 35ºC. for 24-48 hours as a confirmatory test. (1)

For food and dairy testing of Pseudomonas aeruginosa using Skim Milk Agar (Cat. no. G138), see listed references below. (3,5)

INTERPRETATION OF RESULTS

A positive reaction is indicated by a clearing in the media surrounding the colonies. Pseudomonas aeruginosa will hydrolyze casien and may produce a yellow to green diffusible pigment. (1)

LIMITATIONS

MATERIALS REQUIRED BUT NOT PROVIDED

Standard microbiological supplies and equipment such as loops, other culture media, swabs, applicator sticks, incinerators, and incubators, etc., as well as serological and biochemical reagents, are not provided.

QUALITY CONTROL

Test Organisms Inoculation Method* Incubation Results
Time Temperature Atmosphere
Pseudomonas aeruginosa
ATCC ® 27853
* 24-48hr 35°C Aerobic Clear zone surrounding the colonies, may have a yellowish to green pigment
Escherichia coli
ATCC ® 25922
* 24-48hr 35°C Aerobic No clear zone

User Quality Control

PHYSICAL APPEARANCE

Skim Milk Agar should appear opaque, and white in color.

P. aeruginosa growing on Skim Milk Agar

Pseudomonas aeruginosa (ATCC ® 27853) growing on Skim Milk Agar (Cat. no. G138). Incubated aerobically for 24 hours at 35ºC.

E. coli growing on Skim Milk Agar

Escherichia coli (ATCC ® 25922) growing on Skim Milk Agar (Cat. no. G138). Incubated aerobically for 24 hours at 35ºC.

REFERENCES

1. American Public Health Association. Standard Methods for the Examination of Water and Wastewater, APHA, Washington, D.C.

2. Hurst, C.J., Crawford, R.L., Knudsen, G.R., McInerney, M.J., Stetzenbach, L.D. 1996-2005. Manual of Environmental Microbiology . 2nd ed. ASM Press, Washington, D.C.

3. American Public Health Association. Standard Methods for the Examination of Dairy Products, APHA, Washington, D.C.

4. Hui, Y.H., Pierson, M.D., Gorham, J.R. 2001. Foodborne Disease Handbook . 2nd ed. Marcel Dekker, Inc., New York, NY.

5. APHA Technical Committee on Microbiological Methods for Foods. Compendium of Methods for the Microbiological Examination of Foods, APHA, Washington, D.C.

6. 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.

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

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

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

10. MacFaddin, J.F. 1985. Media for Isolation, Cultivation, Identification, Maintenance of Bacteria , Vol. I. Williams & Wilkins, Baltimore, MD.

11. Martley, F.G., et al. 1969. J. Appl. Bact. ; 33:363-370.


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