Antibiotic tolerance of Staphylococcus epidermidis

The phenomenon of antibiotic tolerance was studied in 50 strains of Staphylococcus epidermidis isolated from blood cultures. The antibiotics used for study were methicillin, nafcillin, oxacillin, cephalothin, cefazolin and vancomycin. The MICs and MBCs were determined in Mueller Hinton Broth (MHB) and Brain Heart Infusion broth (BHI) after incubation at 24 and 48 h at 37 degrees C. Tolerance was defined as the MBC/MIC ratio of greater than or equal to 16. About one third of the strains were tolerant to vancomycin and another one third were tolerant to multiple betalactam antibiotics. However, cross tolerance between vancomycin and betalactam antibiotics occurred very infrequently (4 strains). Tolerance was demonstrated for some strains in both MHB and BHI. Other strains were tolerant only in MHB or BHI. Similarly, some strains were tolerant at both 24 and 48 h. However, many strains were tolerant only at 24 and not at 48 h.