Tolerance ofStaphylococcus epidermidis grown from indwelling vascular catheters to antimicrobial agents

Abstract

During a prospective study of indwelling vascular catheter-related infections, 134 isolates ofStaphylococcus epidermidis were grown from 700 catheter tips.In vitro antimicrobial susceptibility testing of these isolates to oxacillin, vancomycin and ofloxacin was performed using the standard broth microdilution technique. These results were compared to those for the same organisms grown in biofilm before the addition of antimicrobial agents. In 96-well flat bottom microtiter plates, 10⁴–10⁵ colony forming units ofS. epidermidis in 0.1 ml broth were grown for 18 h at 37°C, at which time a biofilm was observed for all isolates. Different concentrations of antimicrobial agents (0.1 ml) were then added to the plates. The plates were incubated for 18 h at 37°C. Since MICs could not be estimated in these plates, all the wells were subcultured after mixing the biofilm with the broth. Minimum bactericidal concentrations (MBCs) were defined as 99.9% reduction in colony forming units. For organisms grown in suspension, 100% of the isolates were susceptible to vancomycin, 81% to ofloxacin and 40% to oxacillin. MBCs of susceptible isolates were within four-fold differences for vancomycin (53%), oxacillin (50%), and ofloxacin (51%). When grown as a biofilm, 78%, 93% and 71% of isolates had MBCs of ≥2048 μg ml⁻¹ of oxacillin, vancomycin and ofloxacin respectively. These data demonstrate the reduced bactericidal activity of antimicrobial agents againstS. epidermidis in a biofilm and a simple method for its detection in the microbiology laboratory.