Penicillin the life-saving antibiotic ( Penicillin-the-systemic) was more of a threat than an opportunity to Alexander Fleming's main activity, which was making money from dodgy vaccines not making new science , which is why he always preferred to promote penicillin (weakly) as an antiseptic and (strongly) as an aid to vaccine production.
He did consciously (briefly) study penicillin as a possible systemic life saver and threat to replace his profitable vaccine business, but I argue that subconsciously, his heart really wasn't into a through attempt to make penicillin work as a systemic.
Fleming hardly had a personal lab , because he was hardly ever there : spending his days running a large medical business , just as his other brothers did.
His brothers' eye glass business is more a model to his behavior than the Lives of the Saints, despite the best efforts of the Jim-Savile loving British Establishment to turn Fleming into a saint.
Fleming's saving grace was that he carefully saved and stored his accidental penicillin visitor (as well as investigating it to a certain extent and published that work) giving it out freely to all who asked.
Because some who asked for his unique fungus, had more of a heart of the dying and for systemic medicines than Fleming and they brought it into general use as the world's best known, best loved, medication - ever.
If his attitude had been more like the evil (I use the word carefully) penicillin czar Dr Richards in the USA, penicillin would have died on the vine : so we still have a lot to thank him for....