Antibiotics

The discovery of antibiotics opened a whole new front in the war against disease. Armed with antibiotics, which act by killing bacteria or inhibiting their growth, scientists have mounted a major assault on cholera, pneumonia, tetanus, tuberculosis, and many other deadly bacterial infections that had previously struck people down relentlessly.
Some of the most important breakthroughs in science occur unexpectedly, and the discovery of penicillin—perhaps the world's most widely used antibiotic—is one such example. The British bacteriologist Sir Alexander Fleming is credited with discovering penicillin, although other scientists before him had noticed that the mold Penicillium notatum prevented the growth of some types of bacteria. So Fleming's discovery was actually a rediscovery.
In September 1928 Fleming was preparing to take a short vacation with his family, when a series of almost unbelievably lucky events occurred. Just before leaving, Fleming decided to cultivate staphylococci to study when he returned. This was the first piece of luck. He could have picked any bacterium to study, but he happened to pick one that would turn out to be susceptible to penicillin.
Fleming opened a petri dish for a few seconds to put the staphylococci inside. Ordinarily, no mold spores would have a chance to get in the dish, but two floors below his laboratory, another scientist was studying the mold Penicillium notatum. Millions of very light mold spores floated in the air, up the staircase and the elevator shaft, through the always-open doors of Fleming's laboratory, and into the open dish where, luckily, he was just putting the staphylococci.
Fleming, preoccupied with his vacation, left the petri dish on the laboratory bench instead of putting it in a warm incubator. This was lucky, too, because the bacteria and Penicillium notatum usually grow at different temperatures. Staphylococci multiply at relatively high temperatures, while Penicillium multiplies at lower temperatures. While Fleming was away the temperature turned out to be perfect for Penicillium, but not so good for the staphylococci, which grew slowly. The Penicillium mold thrived and secreted penicillin, which oozed around the dish, preventing the growth of staphylococci and leaving the Penicillium mold isolated from small bacterial colonies in the dish.
Fleming, upon his return, immediately realized what had happened, and he conducted other tests to learn what other bacteria this mysterious mold stuff could kill. He also tried to make pure penicillin, but did not succeed. Fleming believed that the mold substance, which he named penicillin, could be rubbed onto a cut or a scrape to prevent an infection. A few years later, however, Fleming gave up studying the mold.
As a consequence, penicillin was nearly forgotten until the beginning of World War II (1939-1945). Scientists at Oxford University in England showed that penicillin could prevent bacterial infections in animals and humans, and they devised a technique to mass-produce pure penicillin. The scientists encouraged companies in the United States to manufacture penicillin in vast quantities, and the new drug was credited with saving thousands of lives during the war. In 1945 Fleming and two of the Oxford scientists, Sir Howard Florey and Ernst B. Chain, received the Nobel Prize in physiology or medicine.
source: encarta encyclopedia