In Berlin, in 1884, Gram developed a method for distinguishing between two major classes of bacteria.

[1] This technique, known as Gram staining, continues to be a standard procedure of medical microbiology.

Gram was a modest man, and in his initial publication he remarked, "I have therefore published the method, although I am aware that as yet it is very defective and imperfect; but it is hoped that also in the hands of other investigators it will turn out to be useful."

Gram's initial work concerned the study of human red blood cells.

[2] On 13 September 2019, Google commemorated the anniversary of his birth with a Doodle for Canada, Peru, Argentina, Australia, New Zealand, Israel, India and some European countries.