Bacteria can only grow if they have nutrition, just like you have to eat to grow. The role of agar is simply to shape. Agar is the best solid agent in solid culture. Its main function is to solidify the liquid culture medium. Some reports mentioned that filter paper, glass balls, glass wool, quartz sand, vermiculite, gelatin, silica gel, acrylamide, foam, even pumpkin and potato were used to replace agar. However, until now, no more convenient and better support than agar has been found. By the way, agar itself does not provide any nutrition. It is a high molecular carbohydrate, which is extracted from algae such as red algae, only dissolved in hot water to become a sol, and solidified into a solid gel after cooling (below 40°C). The so-called "boiling" culture medium is to dissolve agar in hot water (above 90°C).