With delicate hues of purple and pink, a lab technique called gram staining has reliably characterized bacteria for more than a century. Yet many scientists are mistaken about why the vivid method ...

Gram-positive bacteria show blue or purple after Gram-staining in a laboratory test. They have thick cell walls. Gram-negative bacteria show pink or red on staining and have thin walls.

Gram-positive bacteria have thick cell walls. A Gram stain test, which involves a chemical dye, stains the bacterium’s cell wall purple. On the other hand, gram-negative bacteria stain pink instead.

The Gram-negative bacteria turn into pink color and Gram-positive bacteria turn into violet color when stained using the Gram stain which is a bacterial staining method used since 1884.

During the test, the gram stain will turn pink if gram-negative bacteria are present. These types of bacteria can also cause infections and pneumonia.