What Are Antibiotics? The first effective antibacterial agents, the sulfonamides, were developed in the 1930s and are still used today. These drugs only killed a limited number of bacterial species and had no effect on many important diseasecausing microbes. The use of penicillin and other antibiotics, beginning in the 1940s, revolutionized the practice of medicine, because these drugs were able to stop many otherwise deadly pathogens (organisms that cause disease) in their tracks. THE DISCOVERY OF PENICILLIN Sir Alexander Fleming was a microbiologist at St.Mary’s Hospital in London (Figure 1.1). During the late 1920s he was studying an important human pathogen, Staphylococcus aureus. During the summer of 1928, Fleming went on an extended vacation, and he left some cultures of S. aureus in petri plates on his lab bench.When Fleming returned from his vacation, he briefly checked the plates that had been sitting out. Initially, he discarded the now-famous plate into a tray of disinfectant. Fortunately, he had such a large pile of discarded plates that they didn’t all get submerged in the antibacterial liquid. A colleague then appeared in the lab, and to make a point, Fleming picked up the famous plate and happened to notice something unusual. On that plate a mold (now called Penicillium notatum) had grown. Surrounding this fungus was a clear zone, where the S. aureus had been killed (Figure 1.2). Fleming had previously done work with other substances that killed bacteria, and he quickly recognized the significance of his observation.2 Following his discovery, Fleming made some crude extracts of the material from the fungus and found that this material killed manydifferent types of important pathogenic bacteria. He also found he could inject this crude penicillin extract into rabbits, and the rabbits weren’t harmed.However, Fleming wasn’t able to extract and purify a significant amount of penicillin, so his discovery languished for the next 10 years. In 1938, three other English Scientists—Howard Florey, Ernst Chain, and Norman Heatley—started working on producing large amounts of penicillin. Despite the outbreak ofWorld War II they had made substantial progress by 1941. By then, however, German planes were regularly attacking England, and there was concern that work on penicillin in Britain would be difficult under wartime conditions. Consequently, Florey and Heatley traveled to the United States to gain support for the large-scale production of penicillin and to assist with the process. A series of incremental improvements involving new fungal strains, better growth media, and otherdevelopments finally led to the production of enough penicillin to treat soldiers wounded in the D-Day invasion in 1944. Antibiotics are truly wonder drugs, and today they cure thousands of infected people each day. But what are they? Antibiotics are small molecules usually produced by bacteria or fungi that kill bacteria without harming the person or animal being treated. Chemically synthesized compounds are sometimes called antibacterial agents, but in this book, both synthetic and naturally produced antibacterial compounds will be called antibiotics. Antibiotics are distinguished fromantiseptics (antibacterial chemical agents, such as detergents, only suitable for application to the skin) and disinfectants (strong antibacterial chemicals, such as bleach, only suitable for treating inanimate objects). Antibiotics are also distinguished from antifungal agents, which specifically kill molds and yeasts, and antiviral agents, which prevent viruses from replicating. Antibiotics are only effective on bacteria; they do not kill viruses such as HIV or fungi, like those that cause athlete’s foot.