Bacterial endocarditis with a tolerant staphylococcal organism.

An eight-year-old boy developed staphylococcal endocarditis. He was treated initially with nafcillin and gentamicin, but he remained febrile. The minimum bactericidal concentration (MBC) of nafcillin was 130 times the minimum inhibitory concentration (MIC). Following the addition of rifampin to the antibiotic regimen, the patient became afebrile and recovered. The discrepancy between the MBC and MIC indicated tolerance to nafcillin. Tolerance is a mode of antibiotic resistance that must be considered in treating life-threatening illness.