A 36-year-old man is admitted to the hospital with severe left leg pain after suffering a minor laceration to the leg 2 days earlier. Temperature is 39.5 C (103.1 F) and blood pressure is 85/45 mm Hg. On physical examination, the left leg is swollen and firm with erythema of the overlying skin. There is a small wound draining blood-tinged exudate on the left calf. The patient is taken to the operating room immediately, and a large area of necrotic tissue is excised. Gram stain of the necrotic tissue shows gram-positive cocci, and culture of the bacteria performed on a blood agar plate grows small colonies with a wide zone of surrounding hemolysis. Coagulase and catalase tests are negative, and pyrrolidonyl arylamidase (PYR) test is positive. Which of the following pathogens is most likely responsible for this patient's condition?

Question

A 36-year-old man is admitted to the hospital with severe left leg pain after suffering a minor laceration to the leg 2 days earlier.  Temperature is 39.5 C (103.1 F) and blood pressure is 85/45 mm Hg.  On physical examination, the left leg is swollen and firm with erythema of the overlying skin.  There is a small wound draining blood-tinged exudate on the left calf.  The patient is taken to the operating room immediately, and a large area of necrotic tissue is excised.  Gram stain of the necrotic tissue shows gram-positive cocci, and culture of the bacteria performed on a blood agar plate grows small colonies with a wide zone of surrounding hemolysis.  Coagulase and catalase tests are negative, and pyrrolidonyl arylamidase (PYR) test is positive.  Which of the following pathogens is most likely responsible for this patient's condition?

Quizplus · Accepted Answer

11ec160b_384a_56f8_9249_a7896d7b2f35_MD0001_00 This patient has necrotizing fasciitis ("flesh-eating disease"), a severe soft tissue infection characterized by fulminant tissue necrosis and high mortality. The condition is often polymicrobial, but monomicrobial infection can occur with Streptococcus pyogenes, Staphylococcus aureus, and Clostridium perfringens. S pyogenes (group A Streptococcus) is a gram-positive cocci that is coagulase negative, catalase negative, and pyrrolidonyl arylamidase (PYR) positive. It also forms small colonies with a wide zone of beta-hemolysis and is sensitive to bacitracin, which has been widely replaced by the PYR test. S pyogenes can be transmitted through wounds and spreads rapidly to the deep layers of the skin and fascia due to production of hyaluronidase and streptokinase (converts plasminogen to plasmin). M protein is expressed on the surface and allows the bacteria to evade phagocytosis by preventing activation of the alternative complement pathway. The bacterium also secretes hemolysins O and S (cytotoxins that cause hemolysis) and pyrogenic exotoxins (superantigens that result in tissue injury and septic shock). Sudden-onset, severe pain and swelling are typically present at a site of trauma or recent surgery. Patients quickly become hypotensive and develop septic shock. Necrotizing fasciitis is treated with aggressive surgical debridement of all necrotic tissue plus empiric broad-spectrum antibiotics due to the high incidence of polymicrobial infection. (Choice A) A hydrophila is an oxidase-positive, non-lactose-fermenting, gram-negative rod. It can cause gastroenteritis, wound infections, and bacteremia following exposure to contaminated water. (Choice B) C perfringens is a large, spore-forming, anaerobic, gram-positive rod that is catalase and coagulase negative. It is implicated in food poisoning, clostridial myonecrosis (gas gangrene), and bacteremia. On blood agar, it produces a double zone of beta-hemolysis. (Choice C) S aureus is a gram-positive coccus that grows in clusters. It is catalase positive, coagulase positive, and PYR negative. S aureus produces large colonies and demonstrates beta-hemolysis. Like S pyogenes, S aureus can also cause monomicrobial necrotizing fasciitis. (Choice D) S agalactiae (group B Streptococcus) is a coagulase- and catalase-negative, PYR negative, gram-positive coccus that grows in chains. Colonies produce a narrow zone of beta-hemolysis that enhances when plated perpendicular to S aureus (positive CAMP test). S agalactiae can cause skin and soft tissue infections, as well as newborn sepsis and meningitis. Educational objective: Necrotizing fasciitis is a severe infection of the subcutaneous tissue and deep fascia and is a surgical emergency. The infection is often polymicrobial, but monomicrobial cases due to Streptococcus pyogenes (group A Streptococcus) can also occur. S pyogenes is a PYR-positive, beta-hemolytic, gram-positive coccus that grows in chains.

A 36-Year-Old Man Is Admitted to the Hospital with Severe