Bacillus anthracis

Introduction

Etiologic agent of anthrax. Considered 1st 'microorganism' to cause human disease. Used as 1st model in development of Koch's postulates.

Epidemiology

• found in soil
• endemic in Iran, Turkey, Pakistan, Sudan
  • natural resistance of humans is > than that of livestock Clinical characteristics:
• extracellular pathogen
• resistant to phagocytosis
• rapidly proliferating
• rapidly lethal

Pathology

Capsule:

• consists of poly-D-glutamic acid
• confers resistance to phagocytosis

Endospores:

• oxygen is required for sporulation, but not for germination
• sporulation does NOT take place in vivo
• spores can survive in dry earth for decades
• spores are destroyed by
  • boiling to 10 minutes or autoclaving
  • treatment with oxidizing agents (H2O2, permanganate)
  • dilute or vaporized formaldehyde
• spores are resistant to:
  • drying
  • heat
  • ultraviolet light
  • gamma irradiation
  • many disinfectants
• polymerase chain-reaction (PCR) can detect spores in contaminated agricultural products

Virulence factors: derived from 2 plasmids, pXO1 & pXO2

• pXO1 (184.5 kB)
  • 3 exotoxins are transcribed from genes expressed on pXO1
    • protective antigen (PA)
    • edema factor (EF)
    • lethal factor (LF)
  • the 3 exotoxins form 2 binary complexes:
    • edema toxin: edema factor & protective antigen
    • lethal toxin: lethal factor & protective antigen
• pXO2 (95.3 kB)
  • encodes 3 genes (capA, capB & capC) involved in the synthesis of the polyglutamyl capsule of B anthracis
  • the capsule inhibits phagocytosis of the vegetative form of B anthracis
  • transcription of the capsule genes is regulated by a B anthracis derived transcription factor AcpA
• expression of virulence is regulated by host-specific factors
  • elevated temperature (> 37 C)
  • carbon dioxide concentration > 5%
  • serum components
  • expression of toxin & capsule genes is mediated by transcriptional activator AtxA
• both plasmids are required for virulence*

Laboratory

• Bacillus anthracis in specimen
  • microscopic characteristics:
    • gram-positive rod with 'box-car' appearance
    • size: 1.0-1.5 by 4-10 uM (large)
    • non-motile
    • encapsulated
    • chain-forming
• Bacillus anthracis serology
• Bacillus anthracis Ag in tissue
• Bacillus anthracis DNA
• aerobic bacterial culture on blood agar
  • non hemolytic or weakly hemolytic, grayish-white, rough colonies
  • if medium contains bicarbonate in the presence of excess CO2, colonies are smooth & mucoid

Management

• see anthrax
• ciprofloxacin, levofloxacin, or doxycycline generally with a 2nd agent
• alternative agents
  • other flouroquinoline
  • beta-lactam (if susceptible)
  • rifampin
  • clindamycin
  • chloramphenicol

* Anthrax vaccines have been made by rendering virulent strains free of one or both plasmids (pX01 & pXO2).

• Pasteur: an avirulent pXO2-carrying strain that is encapsulated but does not express exotoxin
• Sterne: an attenuated pXO1-carrying strain that can synthesize extoxoin, but does not have a capsule

More general terms

• Bacillus

Additional terms

• anthrax (woolsorter's disease)
• B. anthracis edema toxin
• B. anthracis lethal factor (LF)
• B. anthracis lethal toxin
• B. anthracis protective antigen