Joseph Lister: first use of a bacterium as a 'model organism' to illustrate the cause of infectious disease of humans.

Joseph Lister's goal was to show that a pure culture of Bacterium lactis, normally present in milk, uniquely caused the lactic acid fermentation of milk. To demonstrate this fact he devised a procedure to obtain a pure clonal population of B. lactis, a result that had not previously been achieved for any microorganism. Lister equated the process of fermentation with infectious disease and used this bacterium as a model organism, demonstrating its role in fermentation; from this result he made the inductive inference that infectious diseases of humans are the result of the growth of specific, microscopic, living organisms in the human host.