Google Doodle on Friday celebrates the 166th birthday of Danish microbiologist Hans Christian Gram who is known for his development of the Gram stain.

The Google Doodle has been illustrated by Danish artist Mikkel Sommer, and it depicts Hans Christian Gram’s work on the Gram strain, according to reports.

Gram was born on this day in 1853 in Copenhagen.

Hans Christian Gram earned his MD from the University of Copenhagen in 1878 and then travelled through Europe to study bacteriology and pharmacology.

While working at a microbiologist’s lab in Berlin during this period, he noticed that treating a smear of bacteria 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.

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.

The test, however, proved to be widely applicable and his staining method continues to be used even today, more than a century later.

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