Gentamicin promotes staphylococcus aureus biofilms on silk suture

Background: Communities of bacteria, termed biofilms, develop on biotic and abiotic surfaces, including medical devices and surgical suture. Biofilm-associated bacteria are typically recalcitrant to antibiotic therapy, and the effects of antibiotics on microbial biofilms are not clearly understood. There is emerging evidence that under specific conditions, aminoglycosides may actually promote biofilm development. Experiments were designed to study the effects of gentamicin on suture-associated Staphylococcus aureus biofilms. Materials and Methods: S. aureus biofilms were formed after 24 h incubation of bacteria with silk suture. Susceptibility of planktonic S. aureus (from broth culture) to gentamicin was compared with the susceptibility of cells from mechanically dispersed S. aureus biofilms. Subinhibitory and inhibitory concentrations of gentamicin were subsequently incubated with intact suture-associated biofilms. S. aureus viability and metabolic capacity were assessed, and biofilm biomass was quantified with crystal violet (binds negatively charged surface molecules) and with the nucleic acid stain Syto 9. Scanning electron microscopy was used to assess the effect of gentamicin on the ultrastructure of suture-associated S. aureus biofilms. Results: Planktonic cells and S. aureus cells from mechanically dispersed biofilms had similar susceptibility to gentamicin. However, after incubation of high concentrations of gentamicin with intact biofilms, high numbers of S. aureus remained both viable and metabolically active; biofilm biomass was increased and biofilm ultrastructure showed staphylococcal cells within copious amounts of extracellular material. Conclusion: Gentamicin does not effectively kill S. aureus within intact suture-associated biofilms, and gentamicin also promotes the biomass of S. aureus biofilms.