Alexander Fleming while working on _____ bacteria, observed Penicillium notatum growing in one of his unwashed culture plates around which these bacteria could not grow.

Alexander Fleming while working on Staphylococci bacteria, once observed a mould growing in one of his unwashed culture plates around which Staphylococci could not grow. He found out that it was due to a chemical produced by the mould and he named it Penicillin after the mould Penicillium notatum.