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

Abstract

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.

Introduction

A frequent cause of implant failure is bacterial infection. Accidental contamination during surgery can expose the implant to risks of microbial colonisation and subsequent infection. Some evidence suggests that a material surface could have an influence on the interactions with bacteria. Local inhibitory effects on the immune system due to release of active chemical substances from the implant surface could favour bacteria eluding host defense systems. It is also known that bacterial strains involved in clinically significant infections can possess adhesins receptors. These receptors bind to specific extra-cellular proteins such as collagen and fibrinogen, and can be used to selectively adhere to the proteinaceous layer adsorbed onto biomaterials, achieving a better anchoring of the bacterial cells on the material surface. Furthermore, bacteria strains often encountered in clinical situation produce a polysaccharide based matrix referred to as slime. The capability of producing slime is considered by different authors to be a virulence factor, able to provide bacteria with an additional mechanism of adhesion and, even more, with a coating protective from host defenses and somehow related to antibiotic drug therapy resistance.

Staphylococci are considered as the most important pathogens of prosthetic infections, thus contributing to the majority of hospital acquired bacteremias [1], [2]. Over the last decades, both Staphylococcus aureus and Staphylococcus epidermidis have accumulated multiple resistance determinants. The onset of antibiotic-resistance in staphylococci has been ascribed to the positive selective pressure carried on by the large clinical handling of broad-spectrum antibiotics. In periprosthesis infections, bacterial adhesion to the surface of artificial materials is considered the fundamental mechanism of virulence [3], [4]. Clinically, the available antibiotic therapies are usually ineffective, because of antibiotic resistance of the adherent biofilm bacteria [5]. Also in vitro studies have revealed an increase of the antibiotic resistance in connection with bacterial adhesion to some biomedical polymers [6], [7]. It can be conceived that the biomaterial, acting as a substratum for the bacterial adherence leads to a selection, among the whole contaminant bacterial population, of variants endowed with more marked adhesive properties as well as with increased resistance towards antibiotics.

In the present study, the susceptibility to penicillin, erythromycin, clindamycin, cefamandole, imipenem, vancomycin, ciprofloxacin, ampicillin, cefazolin, trimethoprim–sulfamethoxazole, chloramphenicol, amikacin and netilmicin has been evaluated in a methicillin-, gentamicin- and tobramycin-resistant Staphylococcus epidermidis strain from an infected prosthesis following in vitro adhesion to polymethylmethacrylate (PMMA) and to silicone elastomer. The two polymers were selected because of their recurrent use in clinical applications. PMMA finds large use in many applications particularly in the orthopaedic field as bone cement and in ophthalmology as artificial intraocular lenses [8], while silicone elastomer is employed in a wide range of surgical applications [9], i.e. as scleral buckles for retinal surgery, in bone reconstruction/correction, in plastic surgery, as tissue expander, coating of pacemakers, catheters and so on. Both polymers are known as materials prone to bacterial adhesion.

The susceptibility to antibiotics both of adherent bacteria and of bacteria which, although exposed to the same materials, had not undergone adhesion, has been investigated by means of plate antibiogram according to Kirby–Bauer method [10] and in accordance with the guidelines of the National Committee for Clinical Laboratory Standards [11]. The bacterial growth inhibition area around each antibiotic disk was measured using an image analyser, which allows optimised detection of small variations in antibiotic susceptibility.