AAT Bioquest

How do I distinguish gram negative and gram-positive bacteria?

Posted March 1, 2023


The best way to distinguish gram-negative and gram-positive bacteria is by using the gram staining technique. This is a differential staining technique that allows you to distinguish between the two types of bacteria based on the difference of their cell wall structures and the ability of their cell walls to hold the gram stain. When stained by a counterstain such as safranin and then de-stained with an alcohol wash, gram-positive bacteria appear distinctly violet in color whereas gram-negative bacteria appear noticeably pink in color. This is because the thick peptidoglycan layer in gram-positive bacteria retains the crystal violet dye. On the other hand, the thin peptidoglycan layer in gram-negative bacteria is unable to retain the dye, hence they appear pink after staining.

Additional resources

Clinical characteristics of Gram-negative and Gram-positive bacterial infection in acute cholangitis: a retrospective observational study

Gram Staining

MycoLight™ Dyes and Kits

MycoLight™ Rapid Fluorescence Gram-Positive Bacteria Staining Kit