Diagrammatic representation of the cell wall, cell membrane, and a flagellum of a Gram-negative bacterium. Gram-negative bacteria have an outer lipopolysaccharide layer (above, yellow), as an outer membrane to the cell wall. Beneath it is the peptidoglycan cell wall, rigid & protective, made of sheets of glycan (sugar, white bricks) with peptide cross-links. The true cell membrane is at bottom (yellow, phospholipid). Pathogens attach to both membranes, molecules diffuse through, & anti- gens are found on them. The rotor mechanism of a flagellum is portrayed (red and blue); it rotates the flagellum like a propeller for movement.