The relationship between inhibition of bacterial adhesion to a solid surface by sub-MICs of antibiotics and subsequent development of a biofilm

Many studies have demonstrated that subminimal inhibitory concentrations (sub-MICs) of antibiotics can inhibit initial microbial adherence to medical device surfaces. It has been suggested that, by inhibiting initial adhesion, biofilm formation might be prevented. However, since initial adherence and subsequent biofilm formation may be two distinct phenomena, conclusions regarding the effects of sub-MIC antibiotics on initial adhesion cannot be extrapolated to biofilm formation. In this study, we evaluated the adherence of several clinical isolates of coagulase-negative staphylococci (CoNS) to acrylic and the effect of sub-MICs of vancomycin, cefazolin, dicloxacillin and combinations of these antibiotics on adherence and biofilm formation. Most of the antibiotics used resulted in effective reduction of bacterial adherence to acrylic, in some cases reaching over 70% inhibition of adherence. When strains with a high biofilm-forming capacity were grown in sub-MICs of those antibiotics, there existed combinations of the drugs that significantly inhibited biofilm formation. However, most of the antibiotic combinations that inhibited adherence did not have a profound effect on biofilm formation. When comparing the results of the effect of sub-MIC amounts of antibiotics in inhibiting adherence with their effect on the inhibition of biofilm formation, significant differences were found, mainly when using combinations of antibiotics. In general, the effect on the inhibition of adherence was greater than the effect on inhibiting biofilm formation. These results demonstrate that assays evaluating the inhibition of initial adherence to medical surfaces cannot fully predict the effect on inhibition of biofilm formation.