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

Antibiotic "tolerance" is a rare cause of antibiotic failure. Antibiotic "tolerance" is defined as an minimal bactericidal concentration (MBC) 32x the minimal inhibitory concentration (MIC) of the isolate. Although susceptibility testing based on the MIC suggests susceptibility of "tolerant" strains, bactericidal concentrations are often beyond achievable serum levels and therapeutic failure may result. We present a case of persistent methicillin-sensitive S. aureus (MSSA) bacteremia due to a linezolid "tolerant" strain unresponsive to daptomycin therapy. We believe this is the first report of persistent MSSA bacteremia due to a linezolid "tolerant" strain.