What did Alexander Fleming observe in 1928?

Alexander Fleming, a Scottish biologist, made a serendipitous discovery in 1928 that would revolutionize the field of medicine: the antibacterial properties of the mold Penicillium notatum. While working at St. Mary's Hospital in London, Fleming was conducting experiments on the bacterium Staphylococcus aureus when he noticed something unusual. One of his culture plates had been accidentally contaminated with a fungal mold. To his surprise, the growth of the bacteria was inhibited in the area surrounding the mold. He isolated and cultivated the mold, identifying it as Penicillium notatum. Through further research, Fleming found that the mold produced a substance that was able to kill bacteria, which he named "penicillin." This pioneering observation laid the foundation for the development of penicillin as the first antibiotic drug, leading to a breakthrough in the treatment of bacterial infections and saving countless lives. Fleming's discovery earned him the Nobel Prize in Physiology or Medicine in 1945, shared with Howard Florey and Ernst Chain, who played crucial roles in developing penicillin for therapeutic use.