Forms large colonies, which can grow all over the agar plate. The colour is grayish in the middle and greenish gray-blue closer to the edge and then whitish at the edge. The surface of the colony is soft and velvety
P. rubens was first described in 1923 and it is the species from which Alexander Fleming discovered the antibioticum penicillin. For this discovery he was awarded the Nobel Prize in Physiology and Medicine in 1945.