Composition of Nutrient Agar

Composition of Nutrient Agar: Nutrient agar is a common laboratory medium (Table.1) used for the growth of many bacterial species. It is important to note that not a single media is suitable for the growth of all microorganisms. 

Table.1: Composition of Nutrient Agar

Composition of Nutrient agar

Common Ingredients of Media

1. Water: Tap, pure, or distilled water may be used for the preparation of culture media by dissolving various organic and inorganic compounds. In the protoplasm of a cell, 70 to 80% water is present and it acts as a vehicle for the flow of nutrients. All enzymatically controlled chemical reactions occur within the cell in the presence of water. Copper-distilled water cannot be used for the preparation of media because copper inhibits bacterial growth. 

2. Peptone: It is a complex mixture of partially digested proteins obtained from lean meat, heart muscle, casein, fibrin, soya meal, etc. The important constituents are proteases, amino acids, inorganic salts which include phosphates, potassium, and magnesium, and growth factors including nicotinic acid and riboflavin. Peptones mainly supply nitrogenous material and also act as a buffer. Peptone is stored in a tightly closed container because it is hygroscopic and becomes sticky when exposed to air. 

3. Yeast extract: Yeast extract is prepared from cells of baker’s yeast or Saccharomyces.  It contains carbohydrates, amino acids, growth factors (vitamin B group), and inorganic salts. Yeast extract is used mainly as a source of vitamins and may be substituted for meat extract. 

4. Meat extract: Meat extract is prepared from fresh lean meat by a hot water extraction process. It contains gelatin, peptones, proteoses, amino acids, creatine, creatinine,  purines, mineral salts, carbohydrates, and growth factors include thiamine, nicotinic acid, riboflavin, pyridoxine, and pantothenic acid. 

5. Agar: Agar is a long-chain polysaccharide obtained from seaweed algae (class:  Rhodophyta). These algae which yield agar are called Agarophytes which include:  Gelidium, Gracilaria, Hyphea, Gelidiella, Ceramium, etc. Agar is a mixture of two polysaccharides such as agarose (70%) and agaropectin (30%). It also contains calcium, chloride, magnesium, sulfate, iron, etc. Agar or agar-agar is now commonly used for the preparation of solid media. The properties of agar are as follows: 

  • Prepared from different seaweed algae. 
  • It acts as a good solidifying agent (2%). 
  • It has no nutritional value in the media. 
  • Bacteriologically inert. 
  • Resistant to the action of all microorganisms. 
  • It is stable or firm at different temperatures used for incubation. 
  • It melts at 95 to 98°C and remains liquid up to 40 to 42°C. 
  • It gets solidified below 40°C. 
  • Easily available and economical.
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