Gram-Positive Bacilli

• Spore-forming gram-positive bacilli


  • Bacillus
  • Clostridium
• Bacillus and Clostridium


  • Ubiquitous
  • Spore-forming
  • Survive in the environment for years
• Bacillus species

  • B. antracis
  • B. cereus
  • B. thuringiensis
• Clostridium species

  • C. tetani
  • C. botulinum
  • C. perfringens
  • C. difficile
• Bacillus species

  • Large, aerobic, gram-positive rods occurring in chains
  • Closely-related but differ phenotypically
  • Pathogenic species possess virulence plasmids
• Bacillus species

  • Mostly saprophytic
  • Some are insect pathogens
  • Some cause food poisoning
• Bacillus subtilis


  Mostly saprophytic
• Bacillus thuringiensis


  Some are insect pathogens
• Bacillus cereus


  Some cause food poisoning
• Bacillus species are large, aerobic, gram-positive rods occurring in chains
• Bacillus species are closely-related but differ phenotypically
• Pathogenic Bacillus species possess virulence plasmids
• Bacillus species are mostly saprophytic, some are insect pathogens, and some cause food poisoning
• Bacillus colonies

  • Round
  • "Cut glass" appearance in transmitted light
  • Hemolysis (uncommon with B anthracis, common with B cereus and saprophytes)
  • Gelatin liquefied, growth ~ inverted fir tree
• Bacillus growth characteristics

  • Saprophytic bacilli use a simple source of nitrogen and carbon for energy and growth
  • Spores are resistant to environmental changes and withstand dry heat and some chemical disinfectants
  • Spores persist for years in dry earth
  • Animal products contaminated with anthrax spores can be sterilized by autoclaving

  
• Bacillus anthracis

  • Affects herbivores
  • Endemic among agrarian societies in Africa, the Middle East, and Central America
• Pathogenesis of anthrax

  1. Spores from contaminated soil find easy access when ingested with spiny or irritating vegetation

  2. Spores germinate in the tissue at the site of entry

  3. Growth of vegetative organisms leads to gelatinous edema and congestion

  4. Bacilli spread via lymphatics to bloodstream and multiply freely

  
• Bacillus anthracis


  • Nonencapsulated isolates are nonvirulent
  • Poly-y-D-glutamic acid capsule is antiphagocytic
  • Capsule gene is present on plasmid pXO2
  • Anthrax toxins are encoded on plasmid pXO1

  
• Anthrax toxins


  • Protective antigen (PA)
  • Edema factor (EF)
  • Lethal factor (LF)

  
• Protective antigen (PA)


  Binds to specific cell receptors, undergoes proteolytic activation, forms a membrane channel, mediates entry of EF and LF into the cell
• Edema factor (EF)


  Adenylate cyclase that forms edema toxin with PA, causes cell and tissue edema
• Lethal factor (LF)


  Forms lethal toxin with PA, major virulence factor that impairs both innate and adaptive immunity
• In susceptible animals


  Organisms proliferate at site of entry, capsules remain intact, rapid dissemination to the bloodstream
• In resistant animals


  Organisms proliferate for a few hours, capsules gradually disintegrate and disappear, organisms remain localized
• Types of anthrax


  • Cutaneous
  • Inhalation
  • Gastrointestinal
  • Injection

  
• Cutaneous anthrax


  • Occurs on exposed surfaces of the arms or hands, predominantly in the face then neck
  • Pruritic papule develops 1-7 days following entry, rapidly changes into a vesicle or small ring of vesicles that coalesce
  • Necrotic ulcer develops, lesions are 1-3 cm in diameter
  • Characteristic black eschar associated with marked edema
  • Eschar fully develops in 7-10 days, dries, loosens, separates, healing by granulation and scar formation

  
• Cutaneous anthrax


  20% of cases lead to sepsis, complications include meningitis and death

  
• Inhalation anthrax


  • Incubation period of 6 weeks
  • Early: marked hemorrhagic necrosis and edema of mediastinum, substernal pain, mediastinal widening, pleural involvement with hemorrhagic pleural effusion, cough due to effects on trachea
  • Sepsis, hematogenous spread to GIT and meninges

  
• Gastrointestinal anthrax is extremely uncommon, acquired through ingestion of spores and spread of organisms through the GIT

  
• Injection anthrax


  • Extensive, painless, subcutaneous edema, notable absence of eschar of cutaneous anthrax, may lead to septicemia and hemodynamic instability

  
• Diagnostic specimens


  • Fluid or pus from local lesion
  • Blood, pleural fluid and CSF for inhalational
  • Stool for GI anthrax

  
• Bacillus anthracis identification


  • Stained smears show chains of large gram-positive rods
  • May use immunofluorescence staining
  • Blood agar: nonhemolytic gray to white, tenacious colonies with rough texture and ground glass appearance, comma-shaped outgrowths, "medusa head" and "curled hair" appearance
  • Demonstration of capsule requires growth on bicarbonate-containing medium in 5-7% CO2
  • Gram stain shows large gram-positive rods
  • Carbohydrate fermentation not useful, nonmotile on semisolid medium
  • Definitive ID requires lysis by specific anthrax y-bacteriophage, detection of capsule by fluorescent antibody, identification of toxin genes by PCR
• Anthrax vaccination


  • Live attenuated bacilli, spore suspensions, or PA from culture filtrates
  • Animals in known anthrax districts should be immunized annually
  • US vaccine made from supernatant of a cell-free culture of an unencapsulated but toxigenic strain of B anthracis containing PS adsorbed to AlOH, gives only short-lived immunity

  
• Anthrax treatment


  • Ciprofloxacin
  • Penicillin G
  • Doxycycline
  • Erythromycin
  • Vancomycin
  • Raxibacumab (recombinant human monoclonal antibody that prevents binding of PA to host cell receptors)

  
• In the setting of potential exposure to B anthracis, prophylaxis with ciprofloxacin or doxycycline should be given for 60 days plus 3 doses of vaccine
• Anthrax prevention and control

  • Disposal of animal carcasses by burning or deep burial in lime pits
  • Decontamination usually by autoclaving of animal products
  • Protective clothing and gloves for handling potentially infected materials
  • Active immunization of domestic animals with live attenuated vaccines

  
• Persons with high occupational risk should be immunized against anthrax

  
• Spores of Bacillus anthracis remain viable for decades, and can germinate in soil at pH 6.5 at proper temperature
• Grazing animals in areas with anthrax spores serve to perpetuate the chain of infection

  
• Bacillus cereus food poisoning


  Toxic-mediated illness, emetic type caused by preformed cyclic peptide toxin, diarrheal type caused by enterotoxins secreted by vegetative cells