In the 1920's, researchers came upon the idea of isolating microbes which can perform a single activity. Streptomycin was isolated in 1943 and found to be antagonistic to the tuberculosis microbe. It was later administered to guinea pigs infected with tuberculosis and found to be effective. Streptomycin was shown to be effective against tuberculosis in humans in 1945. Also in the early 20th century, British researcher Alexander Fleming discovered penicillin from an old culture of the deadly bacterium staphylococcus in which mold had grown.
Molds are simple, non-flowering plants belonging to the phylum called fungi.
They form minute reproductive particles called spores that float about in the air and often find their way into bacterial cultures where they then grow and multiply. This pioneering work led other scientists to investigate penicillin. As a result, improved methods were developed for growing the penicillium mold, as well as for harvesting the penicillin. Pure penicillin was eventually isolated and tested in healthy mice, rats, rabbits and cats, none of whom showed ill effects. A method had to be found to manufacture it.