A 34-year-old man comes to the emergency department due to 2 days of fever, malaise, and myalgia. He has also had worsening shortness of breath and cough. The patient works in the mail room at a large company, does not smoke cigarettes, and drinks alcohol occasionally. Chest imaging shows lung infiltrates and prominent widening of the mediastinum. Sputum and blood cultures yield large, gram-positive rods that form colonies resembling a "Medusa head." Which of the following is the most likely bacterial virulence factor contributing to this patient's condition?

Question

A 34-year-old man comes to the emergency department due to 2 days of fever, malaise, and myalgia.  He has also had worsening shortness of breath and cough.  The patient works in the mail room at a large company, does not smoke cigarettes, and drinks alcohol occasionally.  Chest imaging shows lung infiltrates and prominent widening of the mediastinum.  Sputum and blood cultures yield large, gram-positive rods that form colonies resembling a "Medusa head."  Which of the following is the most likely bacterial virulence factor contributing to this patient's condition?

Quizplus · Accepted Answer

Bacillus anthracis is a large, nonmotile, nonhemolytic, gram-positive rod that forms colonies in culture with multiple curled extensions at the edges that resemble a "Medusa head." Because the organism proliferates in soil contaminated with manure, pasture-grazing herbivores such as sheep, cattle, and horses are at greatest risk for infection. However, transmission to humans occasionally occurs when B anthracis spores are inhaled, ingested, or inoculated into the skin. Inhalation of spores into the alveoli causes pulmonary anthrax. Alveolar macrophages phagocytose the spores and bring them to mediastinal lymph nodes, where the spores germinate into vegetative rods and multiply. The vegetative form of the bacteria contains a polypeptide capsule composed of D-glutamic acid, which is antiphagocytic. B anthracis rods also produce a potent exotoxin complex composed of edema factor, lethal factor, and protective antigen, which enter host cells and cause cell death. Initial manifestations of infection are nonspecific (eg, myalgia, fever, malaise), but proliferation of the bacteria in the mediastinum eventually causes hemorrhagic mediastinitis (often visible on chest x-ray as a widened mediastinum). Subsequent dissemination through the bloodstream causes shock, brain edema/hemorrhage, and death. Although pulmonary anthrax is quite rare, it can be seen in individuals who handle contaminated hides or animal hair ("woolsorter's disease") or those exposed to aerosolized biologic weapons (eg, opening contaminated mail). (Choice B) Many bacteria (eg, Streptococcus pneumoniae) are resistant to phagocytosis due to the presence of a polysaccharide capsule. However, B anthracis has a polypeptide, not polysaccharide, capsule. (Choice C) Staphylococcus aureus produces protein A, which binds the Fc portion of IgG. This inhibits opsonization and complement-mediated killing of the bacteria. (Choice D) Intracellular polyphosphate granules are characteristic of Corynebacterium diphtheriae; they can be visualized on microscopy after growing the pathogen on Loeffler medium and staining it with methylene blue. (Choice E) Peritrichous flagella (flagella distributed uniformly over the entire surface of a bacterial cell) are not present on the surface of B anthracis as this organism is nonmotile. Peritrichous flagella are characteristic of highly motile organisms such as Proteus mirabilis. Educational objective: Bacillus anthracis is a large, sporulating, gram-positive rod that is encased in an antiphagocytic polypeptide capsule composed of D-glutamic acid. Inhalation of B anthracis spores can cause pulmonary anthrax, which is usually characterized by nonspecific symptoms followed by hemorrhagic mediastinitis, shock, and death.

A 34-Year-Old Man Comes to the Emergency Department Due to 2