Updated: 6/9/2020

Bacillus anthracis

Review Topic
  • A 35-year-old woman presents to the emergency room for an ulcer on her arm. She reports that she recently completed a travel program in Africa, where she worked on the farms in exchange for room and board. She reports coming into contact with farm animals every day. She said she accidentally cut her left forearm on some wooden post a few days ago. Yesterday, she noticed a painless but pruritic lesion. On physical exam, there is a 4-mm papule with a dusky-looking central vesicle and surrounding edema. There is also axillary lymphadenopathy. She is started on antibiotics.
  • Classification
    • Bacillus anthracis
      • spore- and exotoxin-forming gram + rod
      • capsule protects against phagocytosis
        • the only bacteria with a polypeptide capsule (poly-D-glutamate)
    • transmission
      • inhalation of spores
      • introduction of spores into a skin break
      • ingestion of spores
  • Epidemiology
    • incidence
      • more common in areas where animal vaccination rates are low
      • bioterrorism
    • risk factors
      • intravenous drug use (e.g., heroin)
      • occupational exposure to unvaccinated animals
      • occupational exposure to animal hides
  • Pathogenesis
    • anthrax toxin composed of 3 components
      • protective antigen
        • binds cell surface and mediates entry of edema and lethal factor
      • edema factor 
        • binds calmodulin and performs the same function as adenylate cyclase, ↑ cAMP and resulting in
          • black eschar with edematous borders
          • vasodilation and hypotension
      • lethal factor
        • a protease that cleaves the amino terminus of mitogen-activated protein kinase kinases (MAPKK), inhibiting this signalling pathway, and resulting in macrophage apoptosis
    • infection may spread via lymphatics
  • Associated conditions
    • cutaneous anthrax
      • most common
    • pulmonary anthrax
      • “woolsorter’s disease”
    • gastrointestinal anthrax
  • Prevention
    • post-exposure prophylaxis
      • 3 doses of anthrax vaccine
      • 60 days of a single antibiotic
        • ciprofloxacin or doxycycline are first line
  • Prognosis
    • biphasic nature of pulmonary anthrax
      • prodromal symptoms
      • fulminant bacteremic phase
        • often leads to death within days
  • Symptoms
    • pulmonary anthrax
      • flu-like syndrome with non-productive cough
      • nausea and vomiting
      • hemoptysis
      • chest pain
    • gastrointestinal anthrax
      • nausea and vomiting
      • dysentery
      • abdominal pain
  • Physical exam
    • cutaneous anthrax  
      • initial lesion is a painless and pruritic papule with a central vesicle or bulla
      • this progresses to painless and necrotic black eschar
        • surrounded by edema
      • eschar sloughs off at day 14
    • pulmonary anthrax
      • mediastinitis
      • shock
      • hypoxia
      • dyspnea
    • lymphadenopathy
  • Chest radiography
    • indication 
      • pulmonary anthrax
    • findings
      • pleural effusion
      • pulmonary consolidation
      • widened mediastinum
  • Labs
    • multiple methods of detection
      • “medusa head” appearance on microscopy
        • halo of projections
      • culture of blood, pleural fluid, or eschar
      • positive Gram stain of affected tissue
      • polymerase chain reaction
      • anti-protective antigen immunoglobulin G on enzyme-linked immunosorbent assay
      • biopsy with immunohistochemistry staining
    • marked hemoconcentration
  • Making the diagnosis
    • most cases are diagnosed clinically and confirmed with
      • positive culture, serology, or immunohistochemistry
      • detection of Bacillus anthracis DNA in tissue
  • Community-acquired pneumonia
    • distinguishing factor
      • less likely to have nausea, vomiting, pallor, or unexplained mediastinal widening on chest radiography
  • Management approach
    • antibiotics should be given in the prodromal phase of the disease
    • cutaneous anthrax can be treated with 1 antibiotic
    • systemic anthrax can be treated with 2 antibiotics
  • Medical
    • ciprofloxacin or doxycycline
      • indication
        • all patients
    • protein synthesis inhibitor
      • indications
        • systemic anthrax
        • combination therapy with fluoroquinolone or doxycycline
      • mechanism
        • reduces toxin production
      • drugs
        • clindamycin
        • linezolid
    • antitoxins
      • indication
        • all patients
      • drugs
        • monoclonal antibodies
          • raxibacumab
        • anthrax immunoglobulin
  • Bacteremia from cutaneous anthrax
  • Death

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(M1.MC.17.4754) A 61-year-old male presents to the emergency department complaining of malaise. He reports a two-day history of rapidly progressive fatigue, malaise, fever, and dyspnea. The patient immigrated from Finland five years ago and works on a large farm. His past medical history is notable for diabetes mellitus and emphysema. He takes metformin, salmeterol, and tiotropium. He has a 40 pack-year smoking history. His temperature is 102.3°F (39.1°C), blood pressure is 90/50 mmHg, pulse is 130/min, respirations are 30/min, and oxygen saturation is 92% on room air. Physical examination is notable for rales at the bilateral lung bases. Chest radiography reveals a widened mediastinum and bilateral pulmonary infiltrates. He is subsequently admitted and started on broad-spectrum antibiotics and fluid resuscitation. However, his fever rapidly progresses and he perishes the following day. Results from a blood culture taken on admission are shown in Figure A. The pathogen responsible for this patient’s condition produces a toxin with which of the following mechanisms of action?

QID: 108945

Directly activates adenylyl cyclase



Mimics adenylyl cyclase



Inhibits 60S ribosomal subunit



Binds Fc portion of immunoglobulin



Depolymerizes actin cytoskeleton



M 1 C

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(M1.MC.15.75) A 65-year-old farmer presents for evaluation of a lesion on his arm. The lesion originally started as a painless pruritic papule, but enlarged over the last several days. The current appearance of the lesion is shown in Figure A. Which of the following is the most likely cause of the lesion?

QID: 106723

Autoimmune disorder against hemidesmosomes



S. aureus infection



B. anthracis infection



Squamous cell carcinoma



Basal cell carcinoma



M 2 E

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