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, Enteric Bacteria, Gram negative HACEK Bacilli, HACEK Bacteria

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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
  2. Enteric Bacteria
    1. Gram Negative Bacteria that colonize the Gastrointestinal Tract, some of which are pathogenic

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. 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. Erwinia
    2. Vibrionaceae
      1. Vibrio Cholerae (Cholera)
      2. Vibrio parahaemolyticus
      3. Campylobacter jejuni
      4. Helicobacter Pylori
    3. Pasteurellaceae
      1. Haemophilus Influenzae
      2. Pasteurella Multocida
      3. Actinobacillus
      4. Aggregatibacter actinomycetemcomitans (Periodontal Disease)
    4. Yersiniaceae (previously categorized as Enterobacteriaceae until 2016)
      1. Yersinia enterocolitica
      2. Yersinia Pestis (Plague)
  5. Aerobes
    1. See Aerobic Gram Negative Rod
    2. Acetobacteraceae
      1. Acetobacter
    3. Alcaligenaceae
      1. Achromobacter xylosoxidans
      2. Alcaligenes faecalis
      3. Bordetella (coccobacillus)
        1. Bordetella Pertussis
        2. Bordetella Parapertussis
    4. Bartonellaceae
      1. Bartonella Bacilliformis (acute in Oroya Fever, chronic in Peruvian Wart)
      2. Bartonella Quintana (Trench Fever)
      3. Bartonella Henselae (Cat Scratch Disease)
    5. Brucellaceae
      1. See Brucellosis
      2. Brucella melitensis (acquired from goats)
      3. Brucella abortus (acquired from cows)
      4. Brucella suis (acquired froim pigs)
      5. Brucella canis (acquired from dogs)
    6. Burkholderiaceae (organisms previously categorized under Pseudomonadaceae)
      1. Burkholderia Cepacia (Nosocomial Infection)
      2. Burkholderia Mallei (Glanders, Bioterrorism)
      3. Burkholderia pseudomallei (melioidosis, occurs in southeast asia)
    7. Flavobacteriaceae
      1. Flavobacterium meningosepticum
    8. Francisellaceae (coccobacilli)
      1. Francisella Tularensis (Tularemia)
    9. Legionellaceae
      1. Legionella pneumophila
    10. Moraxellaceae
      1. Acinetobacter Baumannii (Nosocomial Infection)
    11. Pseudomonadaceae
      1. Pseudomonas Aeruginosa
    12. Xanthomonadaceae
      1. Stenotrophomonas Maltophilia (Nosocomial Infection)

V. 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. Chlamydiaceae (tiny obligate Intracellular Organisms)
    1. Chlamydia Trachomatis
    2. Chlamydophila psittaci
    3. Chlamydophila pneumoniae (TWAR)
  4. Moraxella
    1. Moraxella catarrhalis
  5. Kingella
    1. Kingella kingae (causes endocarditis)
  6. Acinetobacter
    1. Acinetobacter Baumannii (opportunistic infection in ICU patients)
  7. Oligella
    1. Oligella ureolytica (Urinary Tract Infections)

VI. Causes: Gram Negative Intracellular Bacteria

  1. See Intracellular Parasites
  2. Facultative Intracellular Parasites
    1. Brucella (Brucellosis)
    2. Bartonella Henselae
    3. Francisella Tularensis
    4. Salmonella typhi and other species
    5. Legionella
    6. Yersinia Pestis (Plague)
  3. Obligate Intracellular Parasites
    1. Chlamydiaceae
      1. Chlamydia Trachomatis (Sexually Transmitted Infection and vertical neonatal transmission)
      2. Chlamydophila psittaci (bird borne Zoonosis)
      3. Chlamydophila pneumoniae or TWAR (uncommon Atypical Pneumonia)
    2. Coxiellaceae
      1. Coxiella Burnetii (Q Fever)
    3. Rickettsiae (e.g. Rickettsia, except Rochalimaea)
      1. Rickettsia
        1. Spotted Fever Group
          1. Rocky Mountain Spotted Fever (Rickettsia rickettsii)
          2. Other species in the spotted fever group are found worldwide
        2. Typhus Group
          1. Rickettsia prowazekii (Louse-Borne Typhus)
          2. Rickettsia typhi (Murine Typhus)
      2. Orientia
        1. Orientia tsutsugamushi (Scrub Typhus)
      3. Ehrlichia
        1. Human Monocytic Ehrlichiosis (Ehrlichia chaffeensis, Ehrlichia ewingii)
        2. Human Granulocytic Anaplasmosis (Ehrlichia equi, Ehrlichia phagocytophila)

VII. Causes: Spirochetes

  1. Treponema
    1. Treponema pallidum (Syphilis)
    2. Treponema Pertenue (Yaws)
    3. Treponema Carateum (pinta)
  2. Borrelia
    1. Borrelia Recurrentis (Louse-Borne Relapsing Fever)
    2. Borrelia Hermsii, Borrelia Miyamotoi, Borrelia tunicate (Tick-Borne Relapsing Fever in North America)
    3. Borrelia Burgdorferi (Lyme Disease)
  3. Leptospira
    1. Leptospira interrogans: Weil's Disease, Leptospirosis

VIII. Causes: Other Groupings

  1. Gram negative HACEK Bacilli (HACEK Bacteria)
    1. See Endocarditis
    2. Background
      1. Represent up to 5% of the causative organisms of Bacterial Endocarditis
    3. Haemophilus species
      1. Haemophilus parainfluenzae
      2. Haemophilus aphrophilus
      3. Haemophilus paraphrophilus
    4. Aggregatibacter actinomycetemcomitans
    5. Cardiobacterium hominis
    6. Eikenella corrodens
    7. Kingella kingae

IX. References

  1. Davis (1990) Microbiology, Lippincott, p. 21-50
  2. Gladwin (2014) Clinical Microbiology, Medmaster, Miami, p. 1-8, 72-3

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