Lipids, including fats, oils, sterols, and other substances, are found in phospholipid cell membranes and in nutrient storage compartments of plants and animals. The breakdown of lipids provides energy for microbes, and this degradation is made possible by hydrolytic lipases, enzymes which are able to engage in lipid hydrolysis. Large lipids are not able to pass through the cell membrane of microbes, but after hydrolysis smaller units pass through the membrane and are transformed into a supply of carbon and energy. Once an unknown bacterium is isolated and diluted in liquified agar medium, lipase production can be ascertained. Using aseptic technique, a needle inocculated with bacterium is used to scratch an egg yolk agar plate. After a 48 hour incubation period, if there is a clearing around growth, the bacteria has the ability to break down lipids for energy. This is a positive lipase reaction. If there is no growth, or there is growth without clear surrounding, the bacteria was unable to utilize the energy potential of lipids. This is a negative lipase reaction.