For top agar and motility assays use about 7 grams of agar per liter. For details on fractionation of agar, manufacture and special uses see page b041a.htm . This in blue is proposed new text. Move the black to b041a. add more to this blue text. There are several kinds of agar which are long chain polymers made by certain marine alga.
Commercial agar is often a mixture of molecules having some what differing chemical structures.
Agar from different algae have different structures, and, therefore, have differing properties. While some marine bacteria can use agar as food and form pits when spread on a plate of agar, most bacteria can't damage agar. Some agars gel around 30C and are used when 40 or 45C would be harmful. For example, one often wants to mix bacteria or antiserum with melted agar and then pour this into individual wells of well plates, test tubes, or petri plants for special assays. Since agar contains many different molecules and impurities, some companies fractionate agar and sell special products for use in foods and laboratories.