103-108 Supp A-19

Many different techniques have been used to study the effects of antibiotics on staphylococci in biofilms. Methods can be divided broadly into two groups, static and dynamic. In the former, biofilms are formed on various biomaterials that may be encountered in medical practice (stainless steel, polymethylmethacrylate, polyethylene, segments of catheters made from Teflon, polyurethane or polyvinyl chloride), or may be artificial substrates such as glass beads, polystyrene tissue culture plates or cellulose membrane filters. Biofilms are then exposed to various concentrations of antibiotics. This type of approach has been reported by many workers (e.g., references 1–8). The dynamic approach involves perfusion of biofilms formed on tubing or flat surfaces in a continuous culture apparatus, or a modified Robbins device. Each method has associated advantages and disadvantages, and bears differing degrees of verisimilitude to the situation in vivo, as discussed by Brown & Gilbert. The formation of bacterial biofilm on in-dwelling lines gives rise to major problems, which remain unsolved despite much research. Organisms within biofilms appear to be in a metabolically altered state, such that they are phenotypically resistant or tolerant to antibiotics, despite being genotypically sensitive. Results of conventional tests for antibiotic susceptibility may not be helpful in suggesting effective therapy, and often infected vascular lines have to be removed. Strategies to prevent biofilms from forming, e.g. by incorporating biocides into catheter material or by using electric current, have either not yet reached the clinic or have proven unsatisfactory in practice despite encouraging findings in vitro. The commonest organism involved in infections of in-dwelling lines is Staphylococcus epidermidis. This species offers two further obstacles to successful treatment: it is often intrinsically resistant to antibiotics, and it often produces slime. The latter characteristic enhances the ability of bacteria to adhere to plastic surfaces and inhibits phagocytic cells. In view of the above, we now consider testing for activity against sessile bacteria an essential part of the evaluation of any new antimicrobial agent. Here, we report the results of a comparative bactericidal study of ciprofloxacin and the semi-synthetic streptogramin quinupristin/dalfopristin acting on slime-positive and slime-negative strains of S. epidermidis, in a model of biofilms on silicone rubber.