Skim Milk Agar Composition: Principle

Skim Milk Agar is used to cultivate and enumerate microorganisms in milk and dairy products. Skim milk to a culture medium with a superior nutrient base optimally adapts it to the neutral conditions experienced by microorganisms that grow in milk. More colonies grow and a wider range of bacteria can be cultured than in other cultures media is intended for the same purpose.

Skim Milk Agar composition

Skim Milk Powder28.0
Yeast Extract 2.5
Skim Milk Agar composition

Skim Milk Agar princeple

Skim Milk Agar contains casein enzymic hydrolysate which provides amino acids and other complex nitrogenous substances. Yeast extract supplies vitamin B complex. The addition of skim milk in the medium makes the conditions optimal for microorganisms encountered in milk; it is a source of lactose casein and other growth nutrients. Dextrose acts as the carbon and energy source.

Skim Milk Agar procedure

  • Prepare the all ingredients.
  • Suspend 51.5 grams of in 1000 ml distilled water.
  • Heat to boiling to dissolve the medium completely.
  • Sterilize by autoclaving at 15 lbs pressure (121°C) for 15 minutes.
  • Cool to 45-50°C. Mix well and pour into sterile Petri plates.

Skim Milk Agar Specimen Type

Dairy Sample

Skim Milk Agar Specimen Collection

For Dairy samples, follow appropriate techniques for sample collection and processing as per guidelines. After use, contaminated materials must be sterilized by autoclaving before discarding.

Skim Milk Agar Quality Control

Sr. No.OrganismsInoculum (CFU)GrowthRecoveryProteolytic activity
1.Bacillus subtilis50 – 100good-luxuriant>=70%positive reaction,
clear zone surrounding
2.Enterococcus faecalis50 – 100good-luxuriant>=70%negative reaction, no
clear zone surrounding
3.Escherichia coli50 – 100good-luxuriant>=70%negative reaction, no
clear zone surrounding
4.Proteus mirabilis50 – 100good-luxuriant>=70%positive reaction,
clear zone surrounding
5.Pseudomonas aeruginosa50 – 100good-luxuriant>=70%positive reaction,
clear zone surrounding
6.Serratia marcescens50 – 100good-luxuriant>=70%positive reaction,
clear zone surrounding

Skim Milk Agar Limitations

  • Skim Milk supports the growth of many microorganisms.
  • For identification, organisms must be in pure culture. Morphological, biochemical, and/or serological tests should be performed for final identification.
  • Consult appropriate texts for detailed information and recommended procedures.
  • Some formulations may require a settling period before pH testing of the prepared medium. If the pH is tested immediately after preparation and is out of specification, retest the medium after 24 hours to obtain final pH results. Always take pH readings at room temperature.

Skim Milk Agar Storage

Store between 10-30°C in a tightly closed container and the prepared medium at 2-8°C. Use before expiry date on the label. On opening, product should be properly stored dry, after tightly capping the bottle inorder to prevent lump formation due to the hygroscopic nature of the product. Improper storage of the product may lead to lump formation. Store in a dry ventilated area protected from extremes of temperature and sources of ignition Seal the container tightly after use. Use before expiry date on the label. Product performance is best if used within stated expiry period.

Skim Milk Agar Disposal

Users must ensure safe disposal by autoclaving and/or incineration of used or unusable preparations of this product. Follow established laboratory procedures in disposing of infectious materials and materials that come into contact with clinical samples must be decontaminated and disposed of following current laboratory techniques.

Skim Milk Agar Precautions

This product may contain components of animal origin. Certified knowledge of the origin and/or sanitary state of the animals does not guarantee the absence of transmissible pathogenic agents. Therefore, it is recommended that these products be treated as potentially infectious, and handle observing the usual universal blood precautions. Do not ingest, inhale, or allow to come into contact with skin.

Skim Milk Agar Keynotes

S M Agar is commonly used to demonstrate proteolysis by organisms capable of hydrolyzing casein. Proteolytic bacteria use the enzyme caseinase to hydrolyze casein and form soluble nitrogenous compounds displayed as a clear zone around colonies. This clearing is much more pronounced on agar-containing milk if the bacteria can produce acid from fermentable carbohydrates in the medium.

Moreover, the hydrolysis of casein is often used to evaluate the proteolytic activity of psychrotrophic microorganisms of significance to food, water, and dairy industries.

Psychrotrophs, such as Pseudomonas aeruginosa, are strongly proteolytic and often responsible for the spoilage of meat and dairy foods; this spoilage can result in a stale, bitter, or rancid taste and smell. It is generally known that Pseudomonas species are most often responsible for the spoilage of fish.

In addition, pseudomonads are often isolated from large bodies of water, and some species have been linked to eye, ear, and skin infections from recreational water use. Thus, pseudomonads may serve as an indicator of recreational water quality. P. aeruginosa is also commonly found in drinking water and is known to be very resistant to ozonation and chemical disinfection in swimming pools.

Many methods have been used to enumerate P. aeruginosa from water, but the most-probable-number (MPN) method results in satisfactory recovery of the organism. However, this method is not suitable for large-volume water testing and lacks precision. The membrane filtration (MF) technique eliminates these deficiencies.

The medium is a modification of Brown and Foster’s formulation and can be used as a differential and confirmatory test for the identification of P. aeruginosa in water. P. aeruginosa hydrolyzes casein as indicated by a zone of clearing around the colonies. There may also be a yellow-to-green pigment diffused into the medium.

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