Staphylococcus epidermidis extracted slime inhibits the antimicrobial action of glycopeptide antibiotics

Certain strains of Staphylococcus epidermidis produce a mucoid slime that appears to be an important virulence factor. After crude slime was isolated from selected strains of S. epidermidis, phenol and chloroform were used to remove proteins and lipids. The remaining extract contained a polysaccharide that was seen on SDS gels stained with Stains-all. This extract was an inhibitor of the antimicrobial action of vancomycin, raising the minimum inhibitory concentration (MIC) to vancomycin in all 18 isolates of S. epidermidis. A dose-response curve was seen between the amount of extract added and the degree of resistance, as measured by both MIC and growth curves. A similar effect was noted with MICs of organisms to teicoplanin. Addition of the extract did not change the MIC to LY146032, although a modest effect on growth rate was observed. The extract did not raise the MIC to clindamycin, rifampin, and cefazolin. The extract reversed the synergism seen between vancomycin and gentamicin in the 5 strains tested in time-kill studies. The interference by slime of the antimicrobial effect of vancomycin and teicoplanin may explain why these antibiotics are sometimes ineffective in eradicating foreign body infections due to slime-producing coagulase-negative staphylococci.