A young butcher cuts his forearm with a knife. Over the next week, he notices swelling, redness, and warmth at the site. Four days later, he presents to the emergency depament with fever, shaking chills and severe lower back pain. Physical examination reveals a temperature of 39.4 C (102.9 F), swelling in his forearm with an area of central softness, and tenderness to pressure over his lower spine. Laboratory data show a leukocyte count of 14,000/mm3 with 81% polymorphonuclear leukocytes. Blood cultures grew a gram positive cocci in clusters on blood agar; colonies show a yellow pigment, and the organism is positive on mannitol/salt agar. The organism is catalase and coagulase positive. Which of the following is the most likely pathogen?
This organism is a gram positive coccus organized into grape-like clusters. It produces a yellow pigment and the positive mannitol salt agar test showed that it could ferment mannitol and had a high salt tolerance. This organism is catalase positive and coagulase positive, two critical tests for the correct identification of Staphylococcus aureus. Bacteroides fragilis is a gram-negative rod that grows anaerobically. It does cause abscesses, but does not have the characteristics described for the organism in this case. Clostridium perfringens is a gram-positive spore forming organism that causes gangrene. It would not grow aerobically on blood agar. The organism can ceainly spread in the skin when introduced by trauma, but would not demonstrate the biochemical profile described. Escherichia coli is a gram-negative rod that would grow on the blood agar but would not produce yellow pigment. Growth on MacConkey's agar after it was gram-stained would indicate lactose fermentation, and other biochemical tests would be used to specifically identify the organism.
Bacteroides fragilis
Clostridium perfringens
Escherichia coli
Staphylococcus aureus