Definition
Agar is a gel‑forming polysaccharide extracted primarily from the cell walls of red seaweeds in the genera Gelidium and Gracilaria. It consists of a mixture of agarose and agaropectin and is valued for its ability to dissolve in hot water and form a firm gel upon cooling. Because it remains solid at temperatures up to about 85 °C and is non‑nutritive to most microbes, agar serves as an ideal base for microbiological culture media.
Explanation
The main component of agar, agarose, is a neutral, repeating disaccharide of D‑galactose and 3,6‑anhydro‑L‑galactose. Agaropectin contains sulfated galactan residues and provides flexibility and porosity to the gel. When heated in water to near boiling, these polymers dissolve and align; as the solution cools, hydrogen bonding allows a three‑dimensional network to form that traps water and creates a gel. Agar melts only at high temperatures and solidifies between 32 and 40 °C, allowing media to be poured at about 50 °C without injuring heat‑sensitive additives【23652102876946†L100-L114】. Once solidified, the gel remains stable at typical incubation temperatures and does not degrade easily. Because it is not metabolized by most bacteria and fungi, it provides a supportive matrix without contributing nutrients or interfering with experimental results.
Uses and Properties
Agar is widely used to solidify culture media for bacteria, fungi, and some plant tissues. It enables the isolation of pure colonies on plates and the preparation of slants or deeps for maintenance and biochemical tests. Outside microbiology, agar’s gelling properties make it valuable in food processing as a vegetarian gelatin substitute and in pharmaceuticals and cosmetics as a stabilizer and thickener. Laboratory applications also include its use in motility assays and as a base for diffusion tests for antibiotics. Because its melting and solidifying temperatures are well separated, agar can be melted and sterilized by autoclaving and then cooled to a safe pouring temperature.
Agar’s ability to provide a firm, inert surface for microbial growth has made it indispensable in microbiology. By supplying a solid substrate without being consumed, it allows observation of colony morphology, enumeration of viable organisms, and performance of various diagnostic tests.
Related Terms: Agarose, Culture medium, Gelidium, Gracilaria, Gel electrophoresis