Gram-negative bacteria

• Bacteria that have a thin peptidoglycan layer in the cell wall and therefore stain pink or red on Gram stain
• Outer membrane contains lipopolysaccharide (LPS, which consists of lipid A, core polysaccharide, and O antigen)

Organisms

Bacilli

Lactose		Oxidase	Urease
fermenter	fast			Escherichia coli, Klebsiella species, Enterobacter species
	slow			Citrobacter species, Serratia species
non-fermenter		positive		Pseudomonas species, Burkholderia species
		negative	positive	Helicobacter pylori, Proteus mirabilis
		negative	negative	Campylobacter species, Shigella species, Salmonella species, Yersinia species

Cocci

• Neisseria species
  • Neisseria meningitidis
  • Neisseria gonorrheae

Coccobacilli

• Haemophilus species (S. aureus satellite colonies)
• Pasteurella species
• Brucella species
• Bordatella pertussis
• Francisella tularensis
• Legionella species

Anaerobes

• Bacteroides species
• Fusobacterium species