While microorganisms have a bad reputation for making people sick, they also play positive roles by fixing nitrogen into soil and helping cattle digest grass. Microorganisms such as the penicillin mold are antibiotics that help sick people get well.
Approximately 80 percent of Earth's atmosphere is nitrogen gas. Plants need nitrogen to survive, but they are unable to use elemental nitrogen from the atmosphere. Rhizobium, a bacterium that lives in the soil infects the rhizomes of legumes causing them to convert elemental nitrogen from the air into the type of nitrogen the infected plant and other plants that grow in the area are able to use.
Bacteria are beneficial to humans who eat red meat and dairy products. Humans are unable to digest grass, due to its high cellulose content. However, beef cattle have special sections in their stomach that help them to get the nutrients they need from high cellulose products. These bacteria break down the fiber of her food, helping the cow to receive all the nutrients she needs for to grow, reproduce and produce milk from products that pass through the digestive system of other animals unaltered.
Penicillium, a mold that grows on oranges, was the source of one of the first antibiotics. This microorganism breaks down the cell walls of bacteria that make humans ill, allowing the body's defense mechanism to kill the bacteria.