Antibiotic penicillin led to renewed interest in the search for antibiotic compounds with similar efficacy and safety. For their successful development of penicillin, which Fleming had accidentally discovered but could not develop himself, as a therapeutic drug, Chain and Florey shared the 1945 Nobel Prize in Medicine with Fleming. Florey credited Rene Dubos with pioneering the approach of deliberately and systematically searching for antibacterial compounds, which had led to the discovery of gramicidin and had revived Florey's research in penicillin. In 1939, coinciding with the start of World War II, Dubos had reported the discovery of the first naturally derived antibiotic,
Antibiotic use in livestock growth in the animals so treated. It is theorized that sub-therapeutic dosages kills some, but not all, of the bacterial organisms in the animal – likely leaving those that are naturally antibiotic-resistant. The actual mechanism by which sub-therapeutic antibiotic feed additives serve as growth promoters is thus unclear. Some people have speculated that animals and fowl may have sub-clinical infections, which would be cured by low levels of antibiotics in feed, thereby allowing the creatures to thrive. Humans can be exposed to antibiotic-resistant bacteria by ingesting them through the food supply. Dairy products, ground beef, and poultry are the most