British microbiologist Alexander Fleming (1881–1955) was the first to discover penicillin’s use as an antibacterial agent. In 1928, Fleming was researching staphylococci at St. Mary’s Hospital in London. As part of his investigation, he had spread staphylococci on several petri dishes before going on vacation; when he returned, he noticed a green-yellow mold contaminating one of the petri dishes—but the staphylococci had failed to grow near the mold. He identified the mold as being of the species Penicillium notatum. Further investigation proved that staphylococci and other organisms are killed by P. notatum, but it was not until the 1940s that British-Australian pathologist Howard Florey (1898–1968) and German-born British biochemist Ernst Boris Chain (1906–1979) rediscovered the benefits of penicillin and isolated it for medical use. In 1945 Fleming, Florey, and Chain shared the Nobel Prize in Physiology or Medicine for their work on penicillin.

Bacteria have been classified into two types based on their metabolic activity: Heterotrophs rely on organic compounds for carbon and energy needs; most bacteria are heterotrophs and must obtain organic compounds from other organisms. The majority of heterotrophs are free-living saprobes (also known as saprophytes or saprotrophs) and obtain their nourishment from dead, organic matter. Autotrophs require inorganic nutrients and carbon dioxide as their sole source of carbon and can be photosynthetic or chemosynthetic. Photosynthetic autotrophs obtain their energy from light, while chemosynthetic autotrophs obtain their energy by oxidizing inorganic chemicals. (For more about heterotrophs, see the chapter “Cellular Basics.”)