Gram Negative Bacteria

Aka: Gram Negative Bacteria, Gram-Negative Bacteria, Gram Stain Negative, Gram Negative Rod, Gram Negative Bacilli, Gram Negative Coccobacilli, Gram Negative Cocci

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II. Definitions

  1. Gram Negative Bacteria
    1. Bacteria that do not retain crystal violet stain, and stain pink on microscopy with gram's method
    2. Like Gram Positive Bacteria, a peptidoglycan cell wall surrounds the plasma membrane of Gram Negative Bacteria
    3. Gram Negative Bacteria have an additional outer membrane of lipopolysaccharides protecting the cell wall

III. Pathophysiology

  1. Gram Negative Bacteria do NOT retain crystal violet stain on microscopy
    1. Instead, Gram Negative Bacteria stain pink with the counter stain Safranin
    2. Gram Negative Bacteria have only a thin Peptidoglycan cell layer
      1. Does not retain the crystal violet stain
      2. Contrast with the thick peptidoglycan layer of Gram Positive Bacteria that does retain crystal violet
  2. Gram Negative Bacteria have 3 cell wall layers
    1. Outer membrane (unique to Gram Negative Bacteria)
      1. Bound to the underlying peptidoglycan layer with murein Lipoproteins (helical)
      2. Membrane contains porin Proteins that allow for nutrient passage
      3. Outer membrane is highly effective at blocking entry of bactericidal drugs that act at the peptidoglycan layer
        1. Contrast with Gram Positive organisms that are susceptible to these agents (e.g. Penicillin)
      4. Unique phospholipid bilayer
        1. Inner facing phospholipids are typical
        2. Outer facing structure is composed of 3-part lipopolysaccharides (LPS)
          1. Oligosaccharide chains with up to 50 saccharides (O-Antigen)
          2. Core Polysaccharide (water soluble)
          3. Lipid A (Gram NegativeEndotoxin)
            1. Disaccharide with mutiple Fatty Acid tails
            2. Results in Endotoxin reactions when Bacterial cells are lysed
    2. Thin peptidoglycan cell layer (thinner and more simple than with Gram Positive Bacteria)
    3. Periplasmic space (unique to Gram Negative Bacteria)
      1. Gel filled space containing Proteins and enzymes
    4. Inner cytoplasmic cell membrane
      1. Phospholipid bilayer with embedded Proteins

IV. Causes: Gram Negative Rods

  1. Anaerobes
    1. Bacteroidaceae (e.g. Bacteroides fragilis)
  2. Facultative Anaerobes
    1. Enterobacteriaceae (enteric Bacteria such as Escherichia coli)
    2. Vibrionaceae (e.g. Vibrio Cholerae)
    3. Pasteurellae (e.g. HaemophilusInfluenzae)
  3. Aerobes
    1. Pseudomonadaceae (e.g. Pseudomonas aeruginosa)

V. Causes: Gram Negative Cocci

  1. Aerobes
    1. Neisseriaceae (e.g. Neisseria Meningitidis, Neisseria gonorrhoeae)
    2. Moraxella catarrhalis

VI. Causes: Gram Negative Obligate Intracellular Parasites

  1. Rickettsiae (e.g. Rickettsia)

VII. References

  1. Davis (1990) Microbiology, Lippincott, p. 21-50
  2. Gladwin (2014) Clinical Microbiology, Medmaster, Miami, p. 1 to 8

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