Autotrophic bacteria can grow on powdered sulfur and other inorganic substances and get carbon from carbon dioxide in the air. Sugars, amino acids, and vitamins may be used for other kinds of bacteria. However, there is no limit to the varieties of food that support bacteria. The identification of bacteria is often difficult. Many known kinds of bacteria have been studied very little, and undoubtedly there are hundreds yet to be discovered.
Bacteria are versatile helpers in the laboratory.
Many of them use the same vitamins that human beings or animals require. The amount of vitamin in a given medium can be measured, under carefully controlled conditions, by the extent of the multiplication of a particular kind of bacterium. Amino acids, the building blocks of proteins, can be similarly measured. Antibiotics are measured by the number of bacteria that are prevented from multiplying. A few dozen test tubes of growing bacteria can measure in several days what would require many weeks and hundreds of animals to accomplish.