What type of gram stain is Escherichia coli?

Escherichia coli is classified as a gram-negative bacterium, which is why the correct answer is gram negative. In the Gram staining procedure, gram-negative bacteria do not retain the crystal violet stain used in the process. Instead, they take up the counterstain, usually safranin, resulting in a pink or red appearance under a microscope. This distinction arises from the structural differences in their cell walls; gram-negative bacteria possess a thinner peptidoglycan layer and an outer membrane, which prevents the retention of the crystal violet dye.

The other classifications do not apply to E. coli. Gram-positive bacteria, characterized by a thick peptidoglycan layer, retain the crystal violet and appear purple after staining. Gram-variable bacteria can show a mixed reaction to the gram stain, which is not typical for E. coli. Non-staining indicates organisms that do not retain stains effectively or are not stained at all, which does not apply here either, as E. coli can be clearly identified using the Gram stain method.