A 52-year-old man is hospitalized due to 2 weeks of low-grade fever, malaise, anorexia, and fatigue. The patient has a history of bicuspid aortic valve and underwent aortic valve replacement a year ago. Physical examination reveals a new regurgitation murmur. Blood cultures repeatedly grow gram-positive cocci in clusters, which are identified as Staphylococcus epidermidis. This pathogen most likely demonstrates which of the following characteristics?

Question

A 52-year-old man is hospitalized due to 2 weeks of low-grade fever, malaise, anorexia, and fatigue.  The patient has a history of bicuspid aortic valve and underwent aortic valve replacement a year ago.  Physical examination reveals a new regurgitation murmur.  Blood cultures repeatedly grow gram-positive cocci in clusters, which are identified as Staphylococcus epidermidis.  This pathogen most likely demonstrates which of the following characteristics?

Quizplus · Accepted Answer

Staphylococcus epidermidis is a low-virulence skin commensal that rarely causes infection in healthy patients. However, those with prosthetic devices (eg, indwelling catheters, artificial heart valves, artificial joints) are at risk because the pathogen produces adhesion and biofilm proteins that allow it to grow on artificial surfaces. S epidermidis is one of the most common contaminants of blood cultures, but infection should be suspected when multiple blood cultures grow the bacteria and/or the patient has symptoms of clinical infection such as fever, malaise, and leukocytosis. Staphylococcus species are gram-positive cocci that grow in grape-like clusters. They are differentiated from streptococci by the catalase test: Streptococci are catalase-negative whereas staphylococci are catalase-positive (Choice C). Further speciation of staphylococci leverages the coagulase test (the ability to clot blood plasma), which differentiates S aureus from coagulase-negative staphylococci such as S epidermidis, S haemolyticus, and S saprophyticus. (Choice A) Alpha hemolysis (partial, green hemolysis when plated on blood agar) is a feature of viridans streptococci, a common cause of infective endocarditis. Staphylococci species are either beta-hemolytic (complete hemolysis) or gamma-hemolytic (no hemolysis). S epidermidis is gamma-hemolytic. (Choice B) S aureus and S haemolyticus can ferment mannitol, but S epidermidis cannot. (Choice E) Novobiocin is an antibiotic that can be applied to plated colonies to differentiate S saprophyticus from other coagulase-negative Staphylococcus species: S saprophyticus is novobiocin-resistant whereas others, such as S epidermidis, are novobiocin-sensitive. (Choice F) S aureus produces a golden-yellow surface pigment, which accounts for its name (aureus means gold in Latin). S aureus is a common cause of infective endocarditis and osteomyelitis, particularly in intravenous drug users. Educational objective: Staphylococcus epidermidis, a gram-positive coccus that grows in clusters, is a skin commensal that is a common cause of infection in patients with prosthetic devices such as artificial joints or heart valves. Unlike S aureus, S epidermidis is coagulase-negative. Unlike S saprophyticus (another coagulase-negative staphylococci species), S epidermidis is susceptible to novobiocin.

A 52-Year-Old Man Is Hospitalized Due to 2 Weeks of Low-Grade