Proteobacteria
Escherichia coli
Scientific classification
Domain:	Bacteria
Phylum:	Proteobacteria Stackebrandt et al., 1988

Structure of gram-negative cell wall

Gram-positive- and negative bacteria are chiefly differentiated by their cell wall structure.

The Proteobacteria are a major phylum of bacteria.

They are gram-negative bacteria. This means they do not retain the violet dye in the Gram staining protocol.

In a Gram stain test, a counterstain (commonly safranin) is added after the crystal violet, colouring all gram-negative bacteria with a pink colour.

The test itself is useful in classifying two distinct types of bacteria based on the structural differences of their cell walls.

Proteobacteria include a wide variety of pathogens, such as Escherichia coli, Salmonella, Vibrio, Helicobacter, and many other notable genera.