1. Introduction

Obligate aerobes: only grow in the presence of oxygen
Facultative anaerobes: can grow with or without oxygen
Obligate anaerobes: cannot grow in the presence of oxygen due to the production of reactive oxygen species (ROS)
1. Live in areas in the body with poor aeration (e.g. the colon) and outnumber facultative bacteria

2. Bacteroides

Gram-negative bacilli that inhabit the large intestine
Some species have superoxide dismutase and catalase to detoxify ROS and can therefore survive brief exposure to oxygen
B. fragilis
1. Non-toxic LPS, but has a polysaccharide capsule to evade phagocytosis
2. Encounter: normal flora in the large bowel
3. Entry: trauma to the abdomen or abdominal surgery to the bowel
4. Spread/Multiplication: abscess formation with facultative and strict anaerobes
5. Damage: abscess formation can lead to tissue necrosis, bacteremia, or metastatic infections at distant sites
6. Virulence factors
  1. non-toxic LPS
  2. capsule: prevents phagocytosis
  3. neuraminidase
  4. lipase
  5. proteases
7. Treatment: drain abscess and give antibiotics (metronidazole, clindamycin, or imipenem with gentamycin) a. resistant to many antibiotics
8. Outcome: death if left untreated and increased risk for bacteremia and septic shock
B. forsythis: oral pathogen
B. vulgatus: cause of infection in the female urino-genital tract that can lead to PID

3. Clostridium

Gram-positive spore-forming rods that normally inhabit the soil
C. Botulinum
1. Encounter: contaminated food
  1. spores are highly heat-resistant and can therefore survive food processing
2. Damage: flaccid paralysis, diplopia, dysphagia, and possible constipation
3. Virulence factors
  1. neurotoxin: blocks the release of acetylcholine from presynaptic nerve terminals
4. Treatment
  1. antitoxin: neutralize unbound neurotoxin
  2. intubation and ventilators until respiratory muscles have recovered
  3. infants are hospitalized
C. tetani
1. Encounter/Entry/Spread/Multiplication
  1. ubiquitous in GI tract and soil
  2. entry through traumatic wound
  3. prevented by immunization
2. Damage: trismus, lockjaw, and risus sardonicus
3. Virulence Factors
  1. tetanospasmin: prevents GABA and glycine release at Renshaw cell inter- neurons producing sustained tetanic contraction
4. Identification and diagnosis: endospore producing drumstick appearance
5. Treatment: penicillin G to prevent bacteria from producing more toxins
C. perfringens
1. Encounter/Entry/Spread/Multiplication
  1. GI tract and soil
  2. contaminate 20-30% of wounds of soldiers in battle
2. Damage
  1. cellulitis
  2. clostridial myonecrosis: destruction of muscle fibers that is fatal if not treated early
3. Virulence Factors
  1. alpha toxin: forms pores in host cell membranes and responsible for gas gangrene
4. Treatment
  1. hyperbaric oxygen
  2. penicillin
  3. removal of necrotic tissue
C. difficile
1. Encounter/Entry/Spread/Multiplication
  1. normal human GI flora
  2. responsible for antibiotic-associated pseudomembranous colitis
2. Damage
  1. inflammation and pussy exudates on the surface of the large intestine
  2. mucosal necrosis underneath pseudomembrane
3. Treatment
  1. discontinue initial antibiotic therapy
  2. metronidazole or vancomycin