Low concentrations of vancomycin stimulate biofilm formation in some clinical isolates of Staphylococcus epidermidis

Aims: To examine the response of Staphylococcus epidermidis isolates from prosthetic orthopaedic infections to vancomycin concentrations below the minimum inhibitory concentration. Methods: Staphylococcal biofilms were grown in 96-well flat-bottomed cell culture plates under a variety of culture conditions and stained using an ammonium crystal violet solution. Optical density (450 nm wavelength) was recorded to estimate the biofilm density for each strain. Population analysis and time-kill studies were also performed on selected isolates. Results: A range of responses were observed, including increased biofilm density at drug concentrations approaching the minimum inhibitory concentration. This increased density was associated with the presence of a more resistant population identified on population analysis but without an apparent effect on the time-kill curves. Conclusions: The ability of some strains to show increased biofilm density could be a factor in the failure of vancomycin therapy reported in some cases. The demonstration that low concentrations of vancomycin may increase the density of newly forming S epidermidis biofilms may indicate an area of potential concern in the use of vancomycin in orthopaedic implants and intravascular catheter locks, and may partially account for some cases of treatment failure.