Inefficacy of vancomycin and teicoplanin in eradicating and killing Staphylococcus epidermidis biofilms in vitro

Biofilm-associated bacteria display a decreased susceptibility towards antibiotics. Routine assessment of antibiotic susceptibility of planktonic bacteria therefore offers an insufficient prediction of the biofilm response. In this study, in vitro biofilms of eight clinical Staphylococcus epidermidis strains were subjected to treatment with vancomycin, teicoplanin, oxacillin, rifampicin and gentamicin. In addition, the biofilms were subjected to combinations of an antibiotic with rifampicin. The effects on the biofilms were assessed by crystal violet staining to determine the total biofilm biomass, staining with XTT to determine bacterial cell viability, and microscopy. Combining these methods showed that treatment of S. epidermidis biofilms with glycopeptides increased the total biofilm biomass and that these antibiotics were not effective in killing bacteria embedded in biofilms. The decreased killing efficacy was more pronounced in biofilms produced by strains that were classified as 'strong' biofilm producers. Rifampicin, oxacillin and gentamicin effectively killed biofilm-associated bacteria of all tested strains. Combining antibiotics with rifampicin increased the killing efficacy without influencing the total biofilm biomass. When vancomycin or teicoplanin were combined with rifampicin, the increase in biofilm biomass was neutralised and also the killing efficacy was influenced in a positive way. We conclude that the combined methodology used in this study showed that glycopeptides were not effective in eradicating S. epidermidis biofilms but that combination with rifampicin improved the killing efficacy in vitro.