AN ASSAY OF STAPHYLOCOCCUS-EPIDERMIDIS BIOFILM RESPONSES TO THERAPEUTIC AGENTS

Implant-associated infections tend to become persistent resisting host defences and antibiotic therapy. Routine clinical laboratory testing of bacterial isolates in the fluid phase for antibiotic susceptibility may not be predictive of therapeutic outcomes and therefore a number of antibiotic regimes have been formulated empirically. The resistance of implant-associated infections has been ascribed to the production by bacteria, when adherent to the implant surface, of a shielding matr ix of polymerized carbohydrates protecting the enclosed bacteria from immune defences and antibiotics. This complex of surface, bacteria and matrix is termed a biofilm. We describe a technique to evaluate the e fficacy of antimicrobial agents directed against biofilm-forming strai ns of Staphylococcus epidermidis (a major cause of implant infections) utilizing standardized biofilm preparations formed on glass. The impa ct of the antimicrobial agents was assessed quantally using the end-po int of permanent cessation of metabolic activity (cell death) of the e ntire biofilm determined by the loss of ability to reduce 2,3,5-triphe nyltetrazolium chloride to a visible red intracellular precipitate of formazan. The relative rate of action of differing antimicrobial agent s could be determined by the minimum period of exposure of the biofilm to the agents that is required to bring about sterilization, the clin ically relevant marker. A wide range of antimicrobial substances could be evaluated, including chemical disinfectants, immunoreactive substa nces, antibiotics; singly and in combination; and the modifying effect s of interaction with non-antibacterial therapeutic agents and other e nvironmental factors. The technique is simple, inexpensive, reproducib le and readily adaptable to the clinical situation where evaluation of therapeutic regimes for individual Gases of prosthetic device-associa ted infection is required routinely with despatch and ease of interpre tation.