He left a lasting mark on microbiology thanks to the pioneering staining technique he invented, known as the Gram stain, which is why he's being celebrated today.
The Google doodle that honours Gram on his birth anniversary today was illustrated by Danish guest artist Mikkel Sommer.
Hans Christian Gram earned his MD from the University of Copenhagen in 1878 and then travelled through Europe to study bacteriology and pharmacology.
He is credited with devising a staining technique to identify and classify different types of bacteria.
Born on September 13, 1853, in the Danish capital of Copenhagen, Gram began his medical career by working as a physician in the local civic hospital.
The Google doodle shows Gram carrying out an experiment, a close up of the bacteriologist, a microscope and bacteria particles.
In 1884, he developed "Gram stain" - a method for distinguishing between two major classes of bacteria, in the Berlin laboratory of microbiologist Karl Friedländer.
Gram noticed that treating a heat-fixed smear of bacterial cells with a crystal violet stain, followed by an iodine solution and an organic solvent, revealed differences in the structure and biochemical function of various samples. Bacteria that possess a thick cell wall remain purple and are called Gram-positive while bacteria with a thinner cell wall fail to retain the stain and are classified as Gram-negative. The Gram staining technique has single-handedly helped diagnose countless bacterial infections - notably, Streptococcus pneumococci, a Gram-positive bacterial pathogen that causes a variety of life-threatening illnesses.
On publishing the method, he modestly noted that "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", according to Google. Terms like "Gram-positive" and "Gram-negative" were coined after him.
This simple test, however, proved widely applicable. Gram stain is a scientific method used to identify and characterise bacteria.