Gram-negative

These are a group of bacteria that do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation, these bacteria appear violet when stained. They have an inner and outer membrane separated by the periplasm and display a layer of peptidoglycan which is much thicker than that of gram-positive bacteria. Examples include Escherichia coli (E. coli), Salmonella and Helicobacter.