Variable Tolerance of a Clinical Isolate of Staphylococcus epidermidis from an Infected Hydrocephalus Shunt for Several %beta;-Lactam Antibiotics, Vancomycin and Fosfomycin

Abstract

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 β-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 β-lactam antibiotics; however, this latter strain displayed variable tolerance for vancomycin and fosfomycin. The β-lactam tolerance of both strains proved inoculum-dependent in that tolerant variants were detected in decreasing numbers with diminishing 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 β-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.