Arrowsmith by Sinclair Lewis

For a week Martin's life had all the regularity of an escaped soldier in the enemy's country, with the same agitation and the same desire to prowl at night. He was always sterilizing flasks, preparing media of various hydrogen-ion concentrations, copying his old notes into a new book lovingly labeled "X Principle, Staph," and adding to it further observations. He tried, elaborately, with many flasks and many reseedings, to determine whether the X Principle would perpetuate itself indefinitely, whether when it was transmitted from tube to new tube of bacteria it would reappear, whether, growing by cell-division automatically, it was veritably a germ, a sub-germ infecting germs.

During the week Gottlieb occasionally peered over his shoulder, but Martin was unwilling to report until he should have proof, and one good night's sleep, and perhaps even a shave.