A 36-year-old man is hospitalized due to worsening fever, productive cough, and shortness of breath. He has no prior medical conditions but experienced an influenza-like illness during the previous week. Temperature is 38.3 C (102 F), blood pressure is 100/50 mm Hg, pulse is 122/min, and respirations are 26/min. Pulse oxymetry is 86% on room air. On physical examination, the patient is ill-appearing and lethargic. There are crackles over the right lower lung. Leukocyte count is 20,000 cells/mm³. Chest imaging reveals extensive parenchymal consolidation and cavitations suggestive of abscesses in the right middle and lower lobes. The patient is endotracheally intubated for mechanical ventilation due to worsening respiratory failure. Culture of the tracheal aspirate grows gram-positive cocci in clusters. Which of the following bacterial virulence factors most likely contributed to this patient's necrotizing infection?

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Young, healthy individuals with recent influenza infection are at risk for rapidly progressive, necrotizing pneumonia (eg, pulmonary cavitations/abscesses) due to infection with Staphylococcus aureus, a gram-positive cocci that grows in grapelike clusters. S aureus colonizes the nares and skin of up to 30% of individuals in the community setting. Although it typically behaves as a commensal organism, breakdowns in the cutaneous or mucosal barrier (eg, respiratory epithelial damage due to influenza) can create a portal of entry for the bacteria and lead to skin or soft-tissue infection (eg, boil) or deeper, life-threatening disease (eg, pneumonia, osteomyelitis). The most virulent strains of S aureus have been infected with a bacteriophage that transmits genes for Panton-Valentine leukocidin (PVL), a cytotoxin that destroys leukocytes and causes tissue necrosis. PVL is primarily seen in community-acquired strains of S aureus that are methicillin-resistant (methicillin-resistance is conferred by a different mobile genetic element [mecA]). (Choice B) Lipooligosaccharide (a type of endotoxin) is part of the outer membrane of Neisseria meningitidis. Release of endotoxin into the bloodstream induces a strong proinflammatory response, which can result in rapid-onset sepsis and shock. Endotoxin is found in gram-negative, not gram-positive, organisms. (Choice C) M protein coats the surface of Streptococcus pyogenes. It prevents phagocytosis, inhibits complement binding, and aids in epithelial attachment. (Choice D) The virulence of Streptococcus pneumoniae is directly related to the presence of a polysaccharide capsule that protects the organism from phagocytosis (vaccines for S pneumoniae generate protective antibodies against capsular antigens). Although many strains of S aureus are encapsulated, PVL is a far more potent virulence factor in this organism. Educational objective: Staphylococcus aureus strains that express Panton-Valentine leukocidin (PVL), a protease that kills leukocytes and causes necrosis, are most likely to cause skin or soft-tissue abscess and invasive disease (eg, necrotizing pneumonia). PVL expression is most common in community-acquired, methicillin-resistant strains.

A 36-Year-Old Man Is Hospitalized Due to Worsening Fever, Productive