Variable tolerance of a clinical isolate of Staphylococcus epidermidis from an infected hydrocephalus shunt for several beta-lactam antibiotics, vancomycin and fosfomycin.

A strain of Staphylococcus epidermidis isolated from an infected hydrocephalus shunt with associated ventriculitis in a 3-month-old infant proved variably tolerant for the beta-lactam antibiotics ampicillin, cephalothin, cefotaxime, cefoxitin, oxacillin, penicillin G, as well as vancomycin and fosfomycin. In contrast, Staphylococcus aureus control strain ATCC 25923 was regularly tolerant to the above stated beta-lactam antibiotics; however, this latter strain displayed variable tolerance for vancomycin and fosfomycin. The beta-lactam tolerance of both strains proved inoculum sizes. Penicillin G-tolerant variants were detected more readily among cells of the S. epidermidis strain in the stationary than in the logarithmic growth phase; in contrast, the yield of penicillin G-tolerant variants from S. aureus ATCC 25923 was independent of the growth phase. Tolerant variants of both staphylococcal strains failed to survive more than three successive passages in Mueller-Hinton broth with added beta-lactam antibiotics at concentrations identical to those which had originally served to select these tolerant variants. Gentamicin combined with vancomycin displayed additive bactericidal activity against both staphylococcal strains; conversely, the combination of rifampin plus vancomycin showed inadequate bactericidal activity against both strains.