Anaerobes

Question 1

Oxygen toxicity & ability of anaerobes to cause significant disease in humans

Answer 1

• Oxygen is toxic for anaerobes
• Basis for oxygen toxicity
  
  • Direct oxidation of cellular components
  • Production of H2O2 and O2-
  • Strict anaerobes lack catalase and SOD
• Anaerobes typically associated with disease
  
  • Aerotolerant
  • May produce catalase and/or SOD (virulence factors)

Question 2

Role of anaerobic normal flora in formation of soft tissue abscesses

Answer 2

• Disease follows autoinfection (normal flora gets into sites it shouldn’t be) through trauma, aspiration, etc.
• These organisms cause abscesses (fibrin wall around purulent infection)
• Infections most often polymicrobial
• Acute stage of soft tissue abscess:
  
  • Aerobic and facultative anaerobes predominate, using up oxygen in microenvironment
  • Clinically: hypotension and fever
• Chronic stage:
  
  • Fibrin encases oxygen-depleted infection, further reducing oxygen exposure
  • Anaerobic microorganisms dominate

Question 3

Clostridium species

Answer 3

• C. tetani
• C. botulinum
• C. perfringens
• C. difficile
• All but C. botulinum are normal flora
• C. botulinum, tetani, and perfringens are found in soil & water
  
  • C. difficile colonizes fomites
• All are spore-forming, Gram-positive anaerobic rods

Question 4

C. tetani: characteristics

Answer 4

• Very widespread, common in feces and soil
• Spores are introduced into wounds –> minor infection in dead tissue
• Produces toxin that is transported retrograde up neurons into CNS
• Tetanus toxin blocks release of inhibitory NTs (GABA/glycine) onto skeletal muslce
  
  • Makes it impossible to relax the muscles
  • Seems to involve damage to axon terminals

Question 5

C. tetani: treatment and prevention

Answer 5

• Treatment
  
  • Supportive respiratory therapy
  • Debridement of wound, if it can be found (often small)
  • Antimicrobial use to stop toxin production
    
    • Metronidazole
  • Passive immunization
    
    • Doesn’t have much effect short-term, nerve terminals are still damaged, but will prevent things from getting worse = **human **tetanus Ig
• Prevention
  
  • Vaccination: humans do not acquire natural immunity to tetanus (takes more toxin to immunize than it does to kill)

Question 6

C. botulinum: characteristics

Answer 6

• Common, widespread in soil & water
• Spores can be destroyed by enough heat and pressure
  
  • Home canning generally insufficient - spores survive
  • Acidic foods don’t let spores grow
• Botulinum toxin is heat-labile (destroyed by cooking)
• Foodborne botulism affects adults with home-canned food (contains pre-formed toxin)
  
  • Infant botulism generally seen after ingesting unpasteurized honey (contains spores)
• Adult foodborne version = eating preformed toxin without active infection (spores killed in GI tract)
  
  • More dangerous (respiratory failure vs. floppy baby)
• Infant version = active infection (gut isn’t acidic enough to kill spores)

Question 7

C. perfringens

Answer 7

• Causes wound and tissue infections, including gas gangrene
  
  • Can cause food poisoning
• Grows very fast (doubling time = 8 minutes)
• Produces nasty exotoxins - pivotal one is alpha toxin:
  
  • Cleaves cell membranes (mimics phospholipase C)
  • Targets vascular endothelium, lymphocytes, platelets, muscle cells
• Spreads by killing cells, then moving into dead area
• Generally get it from traumatic injuries or seeding of normal flora
• Some species cause food poisoning, often in cooked food
• Has no trouble colonizing gut (unlike botulinum)
• Causes watery, generally self-limited diarrhea for a few days

Question 8

C. difficile

Answer 8

• Mainly involved in hospital-acquired diarrhea and pseudomembranous colitis due to broad-spectrum antibiotic use
  
  • Allows overgrowth of normal flora
• A/B toxins: transfer glucose from UDP-glucose onto variety of GTP-binding proteins and inactivates them
  
  • Screws with actin cytoskeleton and causes cell death
• Treat with metronidazole and isolation (easy to transfer difficile due to colonization of fomites)
• Growing botulinum toxin is extremely easy, extremely lethal
• Can be aerosolized (toxin, not organism)
  
  • Can then be absorbed across lung mucosa into blood
  • Would be favored route of weaponization
  • Wouldn’t necessarily kill targets but would require intensive hospitalization