A 20-year-old man presents to the clinic with a 3-day history of “boils” on his right leg and 24 hours of fever and malaise. The patient is a swimmer, and he mentions that he borrowed a teammate's razor to shave his legs before a swim meet. Physical examination shows four raised, warm, erythematous lesions with a central pustule, one of which is draining purulent discharge. A Gram stain and culture if the pus reveals that the causative organism is a catalase-positive, gram-positive coccus. The organism also tests positive for the MecA gene.
Which of the following is the most appropriate oral antibiotic to treat this organism?
A) Amoxicillin
B) Aztreonam
C) Linezolid
D) Nafcillin
E) Vancomycin
C) Linezolid
This question tests understanding of dermatologic findings of bacterial infections, of how to use tests to work across the spectrum of various bacteria to pinpoint a diagnosis, and of the wide array of antibiotics to fight bacterial infections. The causative organism in this case is Staphylococcus aureus which is a gram-positive coccus and is the only one that is both catalase and coagulase positive.
A positive MecA gene means that this S. aureus has a mutant peptidoglycan transpeptidase (also known as penicillin-binding protein) that does not bind to most beta-lactams, such penicillins and cephalosporins. Nafcillin and amoxicillin are both beta-lactams to which this organism would have resistance.
Aztreonam is also a beta-lactam and is resistant to some beta-lactamases. It has strong activity against susceptible gram-negative bacteria but has no useful activity against gram-positive bacteria or anaerobes and is only given intravenously or intramuscularly, not orally.
Both vancomycin and linezolid use a mechanism that does not target the peptidoglycan transpeptidase of the bacterial cell wall. Vancomycin works by inhibiting the synthesis/polymerization of peptidoglycan for the bacterial cell wall, and linezolid binds to the 50S subunit of the bacterial ribosome and prevents the initiation complex. However, vancomycin requires intravenous administration for systemic effects so is not an appropriate choice for an oral antibiotic. Linezolid would be the most appropriate choice in this case.
Key Learning Point
Linezolid binds to the 50S subunit of the bacterial ribosome and prevents the initiation complex and is useful for treating methicillin-resistant Staphylococcus aureus.
