Hans Christian Joachim Gram (1853-1938) was a Danish physician working in a laboratory at the morgue of the City Hospital in Berlin, microscopically examining the lungs of patients who had died of pneumonia. He was working under the direction of Dr. Carl Friedlander who was trying to identify the cause of pneumonia by studying patient who had died of it. Gram’s task was to stain the infected lung tissue to make the bacteria easier to see under the microscope. Strangely, one of the methods he developed did not stain all bacteria equally; some types retained the first dye applied in this multi-step procedure whereas others did not. Gram’s staining method revealed that two different kinds of bacteria were causing pneumonia, and that each of these retained the dye differently. We now recognize that this important staining method, called the Gram stain, efficiently identifies two large, distinct groups of bacteria – Gram-positive and Gram-negative. The variation in the staining outcome of these two groups reflects a fundamental difference in the structure and chemistry of their cell walls.

For a long time, historians thought that Gram did not appreciate the significance of his discovery. In more recent years, however, several letters show that Gram did not want to offend the famous Dr, Friedlander under whom he worked, therefore he played down the importance of his staining method. In fact, the Gram stain has been used as a key test in the initial identification of bacterial species ever since the late 1880’s.