Scientists have a variety of methods at their disposal when they need to cultivate microorganisms such as bacteria. Two of those methods involve growing the bacteria in special plates called Petri dishes. Scientists fill these Petri dishes with a special kind of food that the bacteria need to live and to multiply. The two types of special food used are nutrient agar and blood agar.
In and of itself agar provides no nutria support for bacteria. Agar is a complex polysaccharide that scientists derive from marine alga. It possesses several unique properties that make it valuable to microbiologists. First, few microbes can degrade agar, so it remains solid. Second, it will not liquefy until it reaches a temperature of 100° Celsius, and once liquefied it will remain so until brought down to 40° Celsius. Its ability to remain solid at high temperatures make it an ideal medium for growing thermophilic (heat-loving) bacteria.
Since agar is only a solidifying agent, it carries no value for the bacteria grown on it. Bacteria need nutrients to live and reproduce. One solution involves the mixing of agar with a nutrient broth, containing peptone and beef extract, to create nutrient agar. Carbohydrates, vitamins, salts, and trace amounts of organic nitrogen make up the beef extract. The principle source of organic nitrogen, amino acids, and long-chained peptides is the peptone.
For practical purposes, nutrient agar works well for growing most types of non-fastidious heterotrophic bacteria. "Fastidious" means selective, and "heterotrophic" means the bacteria cannot make their own food. Non-fastidious heterotrophic bacteria, therefore, need their food supplied to them, and they are not fussy about from where it comes. Since many pathogenic (disease-causing) bacteria fall into the non-fastidious heterotrophic category, a complex media consisting of various nutrients such as peptones and beef extracts is the ideal choice for bacterial growth and cultivation.
Blood agar is almost identical to nutrient agar except that it contains five to ten percent sheep, rabbit, or horse blood. Blood agar consists of beef extract, for nitrogen, blood, for nitrogen, amino acids and carbon, sodium chloride, for maintaining osmotic balance, and agar. Microbiologists use blood agar to identify fastidious pathogenic bacteria by studying the hemolytic (blood cell destroying) reactions they cause.
Microbiologist use differential media to identify and isolate specific bacteria. An example of this is the bacteria Streptococcus pyogenes, which causes strep throat. You can grow these bacteria on a complex media such as nutrient agar, but if other bacteria are also growing on that agar, it is very difficult to distinguish one bacterial colony from another without the use of microscopic examination and special staining techniques. If you grow it on blood agar, though, it will destroy the red blood cells in a process called beta-hemolysis, and other cells will not cause this reaction, which makes identifying Streptococcus pyogenes much easier.
