Alexander Fleming
Alexander Fleming discovered Penicillin by chance. He went away on vacation and allowed his friend to use his lab at St. Mary's hospital while he was away. When he got back he was looking through all of his petri dishes to see which ones could be salvaged. He then picked up the one containing the penicillium mold and noticed something unusual. He noticed that there was a new kind of mold growing on the dish that had killed the Staphylococcus aureus bacteria that was on the dish. He tried testing it but since he was not a chemist, he could not isolate the active ingredient to do further testing. Alexander Fleming then wrote a paper on his findings. That paper did not gain any scientific interest until much later. Alexander Fleming was the original founder of Penicillin. (http://history1900s.about.com/od/medicaladvancesissues/a/penicillin.htm)
Howard Florey & Ernst Chain
Howard Florey and Ernst Chain are just as credited for the invention of Penicillin as Alexander Fleming. The Penicillium mold was discovered but not yet usable. The active ingredient was not isolated. Howard Florey and Ernst Chain isolated the ingredient, and began testing. They began a small scale manufacture of the drug and they did some testing. In 1940 they wrote a report on their findings. (http://www.nobelprize.org/nobel_prizes/medicine/laureates/1945/florey-bio.html)
Norman Heatley
Norman Heatley was responsible for figuring out how to extract the active ingredient from the penicillium mold. He thought of changing the mold's pH. He is not credited with the invention of Penicillin. ( http://news.bbc.co.uk/local/oxford/hi/people_and_places/history/newsid_8828000/8828836.stm)