Agar is extracted from several species of red-purple algae such as Pterocladia and Gelidium. Other varieties of algae such as Gelidiella and Gracilaria also produce agar, but these varieties have higher gelling temperatures which makes them unsuitable for microbial culture media. The process of preparing bacteriological grade agar from sea weeds is a stringently monitored process that requires specialized equipment and strict control of the manufacturing environment.
Agar is a carbohydrate polymer chain comprising units of the sugar galactose. When you dissolve agar in boiling water and subsequently allow it to cool, it jells to form a gelatinous mass. It jells at a temperature between 34 and 36 degrees Celsius and remains firm even at 65 degrees Celsius. Agar also withstands shear forces, so the gels produced retain their firmness under ordinary laboratory conditions. Although gelatin has similar jelling properties, it is not resistant to attack by bacterial enzymes. Agar is unaffected by such enzymes; therefore, you can prepare a culture medium without the use of bacterial inhibitors, which may interfere with the growth of the organism growing in the medium.
Most culture media contain agar in a concentration ranging between one to two percent of the culture volume. This concentration permits sufficient solidification of the medium without posing any problem to the growth of organisms. Lower concentrations between 0.05 and 0.5 percent are used in culture media intended for growing microaerophiles and anaerobes and motility experiments. Agar is a component of most culture media used for clinical microbiological studies, auxotrophic studies, bacterial genetic studies and bacterial transformation studies.
What varieties of agar media you use depends on the specific purpose of the microbial culture medium. Nutrient agar medium is useful for growing most bacteria and fungi. Non-nutrient agar is generally used to grow organisms other than bacteria. Blood agar contains blood cells and supports growth of most bacteria. Sabouraud agar medium is useful to selectively grow fungi.