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-fermenting
    • Fast fermenters
      • Escherichia coli
      • Klebsiella species
      • Enterobacter species
    • Slow fermenters
      • Citrobacter species
      • Serratia species
  • Non-lactose-fermenting
    • Oxidase-positive
      • Pseudomonas species
      • Burkholderia species
    • Oxidase-negative
      • Urease-positive
        • Helicobacter pylori
        • Proteus mirabilis
      • Urease-negative
        • Campylobacter species
        • Shigella species
        • Salmonella species
        • Yersinia species
• Cocci
  • Neisseria
    • Neisseria meningitidis
    • Neisseria gonorrheae
• Coccobacilli
  • Haemophilus species (S. aureus satellite colonies)
  • Pasteurella species
  • Brucella species
  • Bordatella pertussis
  • Francisella tularensis
  • Legionella species
• Anaerobes
  • Bacteroides species