Organisms That Grow on a Nutrient Agar Plate

Nutrient agar is a type of general purpose complex medium used mainly for the cultivation of a variety of microorganisms.

Complex media microbiology speaking, are growth media that are composed of a range of available nutrients and protein molecules in unknown concentrations. Nutrient agar is used for culturing nonfastidious microbes such as bacteria and yeast.

What Grows on Nutrient Agar?

Microorganisms need food, water and a suitable environment in order to survive and grow. Nutrient agar provides these resources for many types of microbes, from fungi like yeast and mold to common bacteria such as Streptococcus and Staphylococcus.

The microbes that can be grown on complex media such as nutrient agar can be described as nonfastidious organisms. Nonfastidious organisms are microbes that are able to grow and thrive without special nutritional or environmental conditions.

Some bacteria cannot be grown with nutrient agar medium. Fastidious organisms (picky bacteria) may need a very specific food source not provided in nutrient agar. One example of a fastidious organism is Treponema pallidum, bacteria that causes syphilis. Scientists have been trying unsuccessfully to grow this bacteria in a culture for over 100 years.

Nutrient agar can be used to culture microorganisms for several purposes. One use is the cultivation and maintenance of specific colonies of nonfastidious organisms for scientific study or identification. Another use is to detect and quantify the presence of potentially harmful bacteria or other microbes in water, sewage, shellfish, meat, dairy and other food products.

The medium must be kept sterile in order to ensure that the only microbes growing are the ones intended for culture, and not growing as a result of contamination of the medium.

Nutrient Agar Ingredients

The main ingredients in nutrient agar are peptone, beef extract and agar. These ingredients are powdered and then added to distilled water. The specific composition of nutrient agar can vary slightly depending on the manufacturer and source of the ingredients used.

The composition of prepared nutrient agar medium is 0.5 percent peptone, 0.3 percent beef extract (or yeast extract), 1.5 percent agar and 0.5 percent sodium chloride.

To Define agar

To define agar, think of a complex carbohydrate sourced from marine red algae that acts as a solidifying agent in the nutrient medium. It has no nutritional value for the microbes to be cultured.

Agar gels at a temperature of 113 degrees Fahrenheit (45 degrees Celsius) and melts at 203 degrees Fahrenheit (95 degrees Celsius). Agar also includes food-grade varieties that can be used as a thickening ingredient for soups, jellies and other foods.

Peptone Definition

The definition of peptone is a soluble protein formed in the early stages of protein breakdown during the process of digestion. It is made by partially digesting protein materials such as meat, gelatin and casein using acids or enzymes.

The purpose of peptone in the nutrient agar medium is to provide the primary source of organic nitrogen for the growing microbial culture, and it can also be a source of carbohydrates and vitamins. The exact composition of peptone in the nutrient medium will differ depending on the protein source and method of digestion.

Beef Extract

The beef extract used to make the nutrient agar medium is a mixture of water soluble particles of animal tissue, carbohydrates, organic nitrogen compounds, vitamins and salt.

Yeast extract can be used as well to make nutrient agar and provides similar compounds. The purpose of the compounds in beef extract is to aid in the growth of the bacteria being cultured on the nutrient agar medium.

Distilled Water

Distilled water is typically used to make the nutrient agar medium. Distilled water is water that has been processed to remove any dissolved contaminants and minerals.

The distilled water present in nutrient agar medium is necessary for the life processes of the microorganisms growing there, just as water is essential for the life processes of all living things. The addition of sodium chloride to the mixture makes the culture environment similar to cytoplasm.

Preparing the Nutrient Agar Medium

In order to properly prepare a nutrient agar mixture for bacterial culture, it is important to refer to specific instructions provided by the nutrient manufacturer. Some ingredients or amounts may vary slightly depending on the source of the nutrient agar.

  1. Dissolve 28 grams of the nutrient agar powder in one liter of distilled water.
  2. Heat the mixture to a boil, stirring constantly. Continue to boil and stir the mixture for about one minute or until all the powder is dissolved. 
  3. Autoclave the dissolved mixture for 15 minutes at 249.8 degrees Fahrenheit (121 degrees Celsius). 
  4. Allow the agar mixture to cool slightly. Dispense into plates or tubes. Leave to solidify.
  5. Replace lids and store in a cool, dark and sterile environment such as the refrigerator. Store agar plates (Petri dishes) upside down to prevent condensation forming on the medium.

The final pH of the prepared nutrient agar medium should be 6.8. The medium should be a light amber color and have the consistency of firm gelatin. Prepared nutrient agar medium should last in the refrigerator for up to two years, unless there is a marked change in the medium's appearance which would indicate contamination.

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