• 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
  2. Enteric Bacteria
    1. Gram Negative Bacteria that colonize the Gastrointestinal Tract, some of which are pathogenic
  • 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
  • Causes
  • Gram Negative Rods
  1. Background
    1. Enteric Bacteria
      1. Gram Negative Bacteria that colonize the Gastrointestinal Tract, some of which are pathogenic
  2. Identifying Features
    1. Arrays containing dozens of media wells are typically used for Bacteria identification (with computer analysis)
    2. Cell Surface Antigens
      1. O Antigen (Outer Antigen with Oligosaccharide chains)
      2. K Antigen (Capsule Antigen)
      3. H Antigen (Found on Bacteria with flagella)
    3. Lactose Fermentation
      1. Present in most Enterobacteriaceae, but not Salmonella, Shigella or Pseudomonas aeruginosa
      2. EMB Agar (Eosine Methylene Blue)
        1. Gram Positive Bacteria are inhibited by methylene blue
        2. Lactose fermenters will appear purple to black, or in the case of E. coli, green
      3. MacConkey Agar
        1. Gram Positive Bacteria are inhibited by bile salts
        2. Lactose fermenters appear pink to purple
  3. Anaerobes
    1. Bacteroidaceae
      1. Bacteroides fragilis
      2. Bacteroides melaninogenicus
      3. Fusobacterium
  4. Facultative Anaerobes
    1. Enterobacteriaceae
      1. EKP Gram Negative Bacteria
        1. Escherichia coli (ETEC, EIEC, EAEC, EHEC, STEC)
        2. Klebsiella Pneumoniae
        3. Proteus Mirabilis
      2. ESP Gram Negative Bacteria
        1. Enterobacter
        2. Serratia Marcescens
        3. Providencia (often grouped with Proteus)
      3. SS Gram Negative Bacteria
        1. Shigella
        2. Salmonella (non-typhoid, or Typhoid Fever)
      4. Other Enterobacteriaceae
        1. Citrobacter (Escherichia freundii)
        2. Morganella
        3. Yersinia enterocolitica
        4. Yersinia pestis
        5. Erwinia
    2. Vibrionaceae
      1. Vibrio Cholerae (Cholera)
      2. Vibrio parahaemolyticus
      3. Campylobacter jejuni
      4. Helicobacter Pylori
    3. Pasteurellaceae
      1. HaemophilusInfluenzae
      2. Pasteurella multocida
      3. Actinobacillus
      4. Aggregatibacter actinomycetemcomitans (Periodontal Disease)
  5. Aerobes
    1. Pseudomonadaceae
      1. Pseudomonas aeruginosa
  • Causes
  • Gram Negative Cocci
  1. Background
    1. Neisseriaceae family includes 5 genera of aerobic Gram Negative Cocci
  2. Neisseria (diplococci)
    1. Neisseria Meningitidis
    2. Neisseria gonorrhoeae
  3. Moraxella
    1. Moraxella catarrhalis
  4. Kingella
    1. Kingella kingae (causes endocarditis)
  5. Acinetobacter
    1. Acinetobacter baumannii (opportunistic infection in ICU patients)
  6. Oligella
    1. Oligella ureolytica (Urinary Tract Infections)
  • Causes
  • Other Groupings
  1. Gram negative HACEK Bacilli (HACEK Bacteria)
    1. Background
      1. Causative organisms of Bacterial Endocarditis
    2. Haemophilus species
    3. Aggregatibacter actinomycetemcomitans
    4. Cardiobacterium hominis
    5. Eikenella corrodens
    6. Kingella kingae
  • References
  1. Davis (1990) Microbiology, Lippincott, p. 21-50
  2. Gladwin (2014) Clinical Microbiology, Medmaster, Miami, p. 1-8, 72-3