- Gram Negative Bacteria
- Bacteria that do not retain crystal violet stain, and stain pink on microscopy with gram's method
- Like Gram Positive Bacteria, a peptidoglycan cell wall surrounds the plasma membrane of Gram Negative Bacteria
- Gram Negative Bacteria have an additional outer membrane of lipopolysaccharides protecting the cell wall
- Enteric Bacteria
- Gram Negative Bacteria that colonize the Gastrointestinal Tract, some of which are pathogenic
- Gram Negative Bacteria do NOT retain crystal violet stain on microscopy
- Instead, Gram Negative Bacteria stain pink with the counter stain Safranin
- Gram Negative Bacteria have only a thin Peptidoglycan cell layer
- Does not retain the crystal violet stain
- Contrast with the thick peptidoglycan layer of Gram Positive Bacteria that does retain crystal violet
- Gram Negative Bacteria have 3 cell wall layers
- Outer membrane (unique to Gram Negative Bacteria)
- Bound to the underlying peptidoglycan layer with murein Lipoproteins (helical)
- Membrane contains porin Proteins that allow for nutrient passage
- Outer membrane is highly effective at blocking entry of bactericidal drugs that act at the peptidoglycan layer
- Contrast with Gram Positive organisms that are susceptible to these agents (e.g. Penicillin)
- Unique phospholipid bilayer
- Inner facing phospholipids are typical
- Outer facing structure is composed of 3-part lipopolysaccharides (LPS)
- Oligosaccharide chains with up to 50 saccharides (O-Antigen)
- Core Polysaccharide (water soluble)
- Lipid A (Gram NegativeEndotoxin)
- Disaccharide with mutiple Fatty Acid tails
- Results in Endotoxin reactions when Bacterial cells are lysed
- Thin peptidoglycan cell layer (thinner and more simple than with Gram Positive Bacteria)
- Periplasmic space (unique to Gram Negative Bacteria)
- Gel filled space containing Proteins and enzymes
- Inner cytoplasmic cell membrane
- Phospholipid bilayer with embedded Proteins
- Outer membrane (unique to Gram Negative Bacteria)
- Gram Negative Rods
- Background
- Enteric Bacteria
- Gram Negative Bacteria that colonize the Gastrointestinal Tract, some of which are pathogenic
- Enteric Bacteria
- Identifying Features
- Arrays containing dozens of media wells are typically used for Bacteria identification (with computer analysis)
- Cell Surface Antigens
- Lactose Fermentation
- Present in most Enterobacteriaceae, but not Salmonella, Shigella or Pseudomonas aeruginosa
- EMB Agar (Eosine Methylene Blue)
- Gram Positive Bacteria are inhibited by methylene blue
- Lactose fermenters will appear purple to black, or in the case of E. coli, green
- MacConkey Agar
- Gram Positive Bacteria are inhibited by bile salts
- Lactose fermenters appear pink to purple
- Anaerobes
- Facultative Anaerobes
- Enterobacteriaceae
- EKP Gram Negative Bacteria
- ESP Gram Negative Bacteria
- Enterobacter
- Serratia Marcescens
- Providencia (often grouped with Proteus)
- SS Gram Negative Bacteria
- Shigella
- Salmonella (non-typhoid, or Typhoid Fever)
- Other Enterobacteriaceae
- Vibrionaceae
- Pasteurellaceae
- HaemophilusInfluenzae
- Pasteurella multocida
- Actinobacillus
- Aggregatibacter actinomycetemcomitans (Periodontal Disease)
- Enterobacteriaceae
- Aerobes
- Pseudomonadaceae
- Pseudomonas aeruginosa
- Pseudomonadaceae
- Gram Negative Cocci
- Background
- Neisseriaceae family includes 5 genera of aerobic Gram Negative Cocci
- Neisseria (diplococci)
- Moraxella
- Kingella
- Kingella kingae (causes endocarditis)
-
Acinetobacter
- Acinetobacter baumannii (opportunistic infection in ICU patients)
- Oligella
- Oligella ureolytica (Urinary Tract Infections)
- Gram Negative Obligate Intracellular Parasites
- Rickettsiae (e.g. Rickettsia)
- Other Groupings
- Gram negative HACEK Bacilli (HACEK Bacteria)
- Background
- Causative organisms of Bacterial Endocarditis
- Haemophilus species
- Aggregatibacter actinomycetemcomitans
- Cardiobacterium hominis
- Eikenella corrodens
- Kingella kingae
- Background
- Davis (1990) Microbiology, Lippincott, p. 21-50
- Gladwin (2014) Clinical Microbiology, Medmaster, Miami, p. 1-8, 72-3