A 35-year-old woman presents to clinic with acute onset of fever and chills that come and go throughout the day. On physical examination, there are notable track marks on her arms. She also has a grade III/VI regurgitant systolic ejection murmur that is best heard at the left sternal border. No splinter hemorrhages are noted.

What is the most likely diagnosis?

Acute bacterial endocarditis, most likely of the tricuspid valve and secondary to Staphylococcus aureus infection.

What are the characteristics of the causative organism?

S. aureus is a gram-positive coccus that is usually paired or in irregular grape-like clusters. It is non-motile, non-spore forming, and catalase- and coagulase-positive. Carriage of this bacterium in humans is commonly in the axillary or inguinal regions or in the anterior nares.

What are the characteristics of the infection in this patient and how is it diagnosed?

Endocarditis in intravenous drug users commonly results from S. aureus infection and presents with spiking fevers and chills. Often, infection is of the right side of the heart, because injection is into the venous system, which returns to the right side of the heart. Typical hallmarks of endocarditis like splinter hemorrhages, Roth’s spots, and Janeway’s lesions are often not found in these patients. A mnemonic for endocarditis is FROM JANE: Fever, Roth’s spots, Osler’s nodes, Murmur, Janeway’s lesions, Anemia, Nailbed hemorrhage, and Emboli. S. aureus can cause infection of previously healthy valves. This makes it different from both the endocarditis caused by viridans streptococci, which affects pre-damaged valves, and the endocarditis caused by Staphylococcus epidermidis, which affects synthetic valves. Diagnosis is made by repeat blood cultures and by echocardiogram to look for valvular vegetations.

What other infections are associated with this bacterium?

S. aureus is also known for causing nosocomial pneumonia and sepsis. It can also cause septic arthritis. In the skin, it can cause impetigo, folliculitis, furuncles, and carbuncles. Enterotoxin produced by S. aureus can result in food poisoning, commonly seen in those eating salads with mayonnaise or undercooked red meat. The exfoliative toxin produced by S. aureus can also cause scalded skin syndrome, which is commonly seen in babies when the umbilical cord is not cut in a sterile manner. The exotoxin toxic shock syndrome toxin-1 causes toxic shock syndrome, classically described in women using tampons.

How is infection with this bacterium commonly treated?

Infection by S. aureus is treated with antibiotics. S. aureus is not susceptible to penicillin, so the penicillinase-resistant penicillins and cephalosporins are commonly used. Commonly one of these classes is combined with clindamycin or an aminoglycoside to achieve a synergistic effect (ie, oxacillin plus clindamycin). However, methicillin-resistant S. aureus (MRSA) is resistant to the cephalosporins and penicillinase-resistant penicillins via an altered penicillin binding protein that is encoded by the mec A gene. Thus for MRSA, a combination of vancomycin and an aminoglycoside is commonly used. For food poisoning, antibiotics are not usually given because the illness is self-limited and due to toxin secretion.

By Christina L. Shenvi, PhD, class of 2009, Yale University School of Medicine; in association with Le TT, Takiar V, eds: First Aid Cases for the USMLE Step 1. New York: McGraw-Hill, 2009.