Google celebrated the 166th birth anniversary of Danish microbiologist Hans Christian Joachim Gram with a doodle on Friday who is known for his development of the Gram stain.

Today’s Google doodle is illustrated by Danish artist Mikkel Sommer and it depicts Hans Christian Gram’s work on the Gram stain.

Gram was born on September 13, 1853, in Denmark’s Copenhagen. Gram is best known for pioneering the method of the Gram stain to identify bacteria.

The Danish microbiologist earned an M.D. from the University of Copenhagen in 1878. He then travelled through Europe between 1883 to 1885 studying pharmacology and bacteriology.

He published his findings in a journal in 1884 which led to the coining of the terms “Gram-positive” and “Gram-negative”. Gram-positive bacteria appear purple under a microscope because their cell walls are so thick that the solvent cannot penetrate them, while Gram-negative bacteria have thinner cell walls that allow the solvent to wash away the stain.

“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,” Hans Christian Gram had notably added as a modest disclaimer in his publication.

More than a century later, Gram’s simple staining method, named after its inventor, is still widely used.

Gram showed a keen interest in the clinical education of students throughout his career. He retired from his medical practice in 1923 and died at the age of 85 in 1938.