The Staphylococcus aureus FnBPA and FnBPB proteins promote acid-induced biofilm accumulation. Meticillin-resistant S. aureus (MRSA) isolates from device-related infections with both fnbA and fnbB produced significantly more biofilm than isolates with either gene alone. Under mildly acidic growth conditions, FnBP-mediated biofilm and fnbA and fnbB transcript levels were substantially higher during growth at 37 degrees C than at 30 degrees C. Thus, in addition to a lowered pH, carriage of both fnbA and fnbB and growth at 37 degrees C promote MRSA biofilm development, further supporting a role for the FnBPA and FnBPB surface proteins in the pathogenesis of MRSA device-related infections.