Another discovery in the nineteenth century concerned the way microorganisms
interact with one another. For thousands of years, moldy cheese, meat, and bread
had been employed in folk medicine to heal wounds. Then in the 1870s, Tyndall,
Pasteur, and William Roberts, a British physician, directly observed the antagonistic
effects of one microorganism on another. Pasteur, with his characteristic foresight,
suggested that the phenomenon might have some therapeutic potential. For
the next 50 years, various microbial preparations were tried as medicines, but they
were either too toxic or inactive in live animals. This led to the pivotal moment in
microbiological history when, in 1927, Alexander Fleming discovered penicillin
(see Section 1.2 ).