What is the Gram stain reaction of Escherichia coli?

The Gram stain reaction of Escherichia coli is Gram-negative. In the standard Gram-staining procedure, E. coli does not retain the crystal-violet–iodine complex after alcohol or acetone decolorization, so it takes up the counterstain and appears pink or red under a microscope.

The result reflects E. coli’s cell-envelope structure. Its thin peptidoglycan layer lies between an inner membrane and an outer membrane. That outer membrane contains lipopolysaccharide, whose lipid A component can act as an endotoxin during certain infections. Gram-positive bacteria instead have a much thicker peptidoglycan wall and usually remain purple after staining.

The method was developed by Danish bacteriologist Hans Christian Gram in 1884 as a way to distinguish bacterial groups. A common mistake is to interpret “Gram-negative” as meaning “dangerous”: many E. coli strains live harmlessly in the intestine, while particular pathogenic strains cause disease. “Not stained” is also inaccurate; Gram-negative cells are stained, but they appear the counterstain’s color.