Effects on antibiotic resistance of Staphylococcus epidermidis following adhesion to polymethylmethacrylate and to silicone surfaces

A number of studies appears to give emphasis to the role of prosthetic materials in determining microbial adherence and resistance to host defence and drug therapy. Aim of this study was to explore whether the direct contact with biomaterial substrata of different chemical nature could influence bacterial behaviour, determining possible changes in the bacteria population as far as antibiotic resistance is concerned. To this end, susceptibility to penicillin, erythromycin, clindamycin, cefamandole, imipenem, vancomycin, ciprofloxacin, ampicillin, cefazolin, trimethoprim–sulfamethoxazole, chloramphenicol, amikacin and netilmicin was evaluated in a methicillin-, gentamicin- and tobramycin-resistant Staphylococcus epidermidis strain, after in vitro adhesion to polymethylmethacrylate (PMMA) and to silicone elastomer. The susceptibility to antibiotics of both adherent bacteria and bacteria which, although exposed to the materials, had not undergone adhesion was measured as bacterial growth inhibition area onto a plate antibiogram, according to Kirby–Bauer and using an image analyser system. The results obtained suggest that the two test materials considered in this study were capable to condition bacterial behaviour. In particular, the adhesion onto PMMA surfaces induced a marked and significant decrease in susceptibility to the following β-lactam antibiotics: cefamandole (32%), cefazolin (23%), imipenem (27%), ampicillin (31%). Moreover, PMMA caused a lower but significant reduction in resistance to vancomycin (15%), chloramphenicol (16%), amikacin (13%), netilmicin (13%), erythromycin (11%), trimethoprim–sulfamethoxazole (13%). In contrast, the adhesion onto silicone elastomer appeared to influence bacterial changes to a lesser extent and elicited a significant decrease in susceptibility only to cefazolin (10%) and amikacin (11%). Further studies are required to thoroughly investigate the mechanisms of these variations, even though, also according to other authors, one of the best conceivable conclusions is that some material substrata can lead to selection of variant adhesive bacteria with increased antibiotic resistance.