Persistent methicillin-resistant Staphylococcus aureus (MRSA) bacteremia due to a linezolid "tolerant" strain.

Persistent Staphylococcus aureus bacteremia is most frequently related to S. aureus acute bacterial endocarditis, myocardial abscess, extracardiac abscess, or a device-associated infection. Patients with continuous high-grade bacteremia who do not have acute bacterial endocarditis, an abscess, or a device-related infection should be considered to have antimicrobial "tolerance" as a possible cause. Antimicrobial "tolerance" is defined as a wide discrepancy between the minimal inhibitory concentration and the minimal bactericidal concentration of an isolate. Tolerance was first described in S. aureus and has since been described with streptococci and enterococci. Despite apparent in vitro susceptibility, infections caused by "tolerant" strains are not cured by appropriate antimicrobial therapy. The lack of bactericidal activity of the antibiotic becomes apparent when minimal bactericidal concentrations are determined for "tolerant" strains, and there is a great discrepancy between the minimal inhibitory concentration being used. Antibiotic tolerance to S. aureus has been described with a variety of antibiotics. To the best of our knowledge, this is the first case of continuous, high-grade methicillin-resistant S. aureus bacteremia due to a linezolid-tolerant strain.