agar
A complex polysaccharide made by the red marine alga Gelidium. It is used to thicken or solidify bacterial culture media as well as certain foods. It was fi rst applied for the use of culturing microorganisms by the wife of Walter Hesse, a German microbiologist in the late 1800s. The properties of agar make it well suited for use in the culturing of microorganisms. Very few microorganisms can degrade or digest it, so it remains solidifi ed in their presence. It remains solid at high enough temperatures (close to 100°C) to be able to incubate most microbes. When in a molten state, it will solidify when the temperature drops below 42°C, but it can be kept in a liquid state for long periods of time if incubated at 50°C and above. It can be poured into tubes, fl asks, petri plates, and any other support and placed in any position to solidify, such as slanted or straight to shape the surface to either maximize or minimize oxygen availablity and surface area. Most solid media are 1.5 percent agar.