Microbiologists have several methods at their disposal when they have to cultivate microbes, such as bacteria. Generally, bacterial culture involves the use of agar media plates or Petri dishes. Microbiologists fill the Petri dishes with agar media which forms a surface for bacteria to grow on. There are several types of nutrient agar like nutrient agar, blood agar, MacConkey agar, etc. In this article, we give you a brief idea of the two most commonly used agar plates.

What is Agar?

Agar is a complex polysaccharide, extracted from marine algae. Agar itself does not provide any nutrient support for the bacteria but helps in forming a surface of the bacterial colonies to grow. Agar usually liquefies when it reaches around 100° Celsius and does not solidify until the temperature is brought down to 40° Celsius. The ability of agar (special heat tolerant) to remain solid at higher temperatures also makes it ideal for growing thermophilic bacteria. 

Nutrient Agar Media

Bacteria need nutrients for growing and reproducing but for creating a solid surface for bacteria to grow in colonies, microbiologists use agar with a nutrient broth, containing beef extract and peptone. This is called nutrient agar media. The beef extract is the source of carbohydrates, vitamins, salts, and organic nitrogen trace amounts whereas peptone is the main source of organic nitrogen, long-chained peptides, and amino acids. 

Nutrient Agar- Complex Media

Nutrient agar plates can be used for growing most types of bacteria of non-fastidious and heterotrophic bacteria for practical purposes. “Non-Fastidious” refers to non-selective and “heterotrophic” refers to the inability of the bacteria to make their food. Therefore, heterotrophic non-fastidious bacteria require external nutrients to grow and do not have any particular nutrient requirements to follow. 

Several pathogenic bacteria fall in the category of non-fastidious heterotrophic and therefore a complex media with peptone and beef extract makes the ideal media choice for their growth and culture. If any particular research demands the culture of genetically modified bacteria, microbiologists have the option to tweak the media composition of nutrient agar to suit the experiment’s purpose.

Blood Agar Media

Although similar to the idea of nutrient agar media, Blood agar media contains five to ten percent sheep or rabbit blood. Blood agar composition consists of Beef extract (nitrogen source), Blood (nitrogen, amino acids, and carbon source), Sodium chloride (maintains osmotic balance), agar (solidifying agent).

Blood Agar- Differential Media

Nutrient agar plates are commonly used for non-fastidious bacteria whereas blood agar plates are used by microbiologists for culturing fastidious pathogenic bacteria. These bacteria populations are selected by studying their hemolytic ability by which they destroy blood cells.

To define an example, let us take Streptococcus pyogenes. These microorganisms can be grown on nutrient agar plates but if other bacteria are also growing, it gets difficult to distinguish this specific microbe. In this scenario, blood agar media can be used. Upon growing the bacteria on blood agar plates, this strain will destroy the red blood cells by the beta-hemolysis process, thus making detection easier. 

Therefore, although microorganisms can be grown both in blood agar and nutrient agar media plates, blood agar serves a more specific purpose for microbiologists.


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