Through careful and thorough research, scientists Howard Florey and Ernst Boris Chain were later able to use penicillin to develop the first antibiotics—drugs that are used to this day to fight serious infections like pneumonia. Thus, although the initial discovery of penicillin occurred by accident, it was ultimately Fleming's determination to find a cure for bacterial infections and Florey and Chain's assiduous labor that were responsible for the development of these life-saving drugs.
What is the meaning of assiduous as used in the passage?