In vitro and in vivo antimicrobial activity of covalently coupled quaternary ammonium silane coatings on silicone rubber
Introduction
Infection is the most common cause of biomaterial implant failure in modern medicine [1], [2]. Adhesion and subsequent surface growth of bacteria on biomedical implants and devices causes the formation of a biofilm, in which the so-called “glycocalyx” embeds the infecting bacteria offering protection against the host immune system and antibiotic treatment [3]. Gram-positive Staphylococcus aureus, and Staphylococcus epidermidis are the predominant infecting organisms, followed by Gram-negative bacilli like E. coli and Pseudomonas aeruginosa [4], [5]. A possible approach to prevent biomaterial-centered infections is to render the biomaterial surface antimicrobial properties by functionalization with quaternary ammonium groups, which are widely known as disinfectants [6]. In this approach no antimicrobial agents are leaching from the surface, providing long term protection against bacterial colonization, and reducing the risk of developing antimicrobial resistant microbial strains, as the concentration of antimicrobial groups is constantly above the minimal inhibitory concentration [7]. As quaternary ammonium functionalized surfaces have a high positive surface charge, they exert a strong adhesive force on negatively charged bacteria, which has been proposed to physically inhibit surface growth of rod-shaped bacteria [8].
Poly(methacrylates) with methyl or ethyl quaternary ammonium chloride side groups showed antimicrobial activity [9], [10] toward Gram-negative strains, although Gram-positive staphylococci were little affected by these polymers. The antimicrobial effects of soluble quaternary ammonium compounds increase with the length of the alkyl moieties on the nitrogen atom [11], [12], [13], with an optimum chain lengths of 16–18 carbon atoms [14]. Less soluble dendrimers functionalized with dimethyldodecylammonium chloride groups were also found to have a strong antimicrobial effect towards S. aureus [15]. Cotton-polyester fabric treated with 3-(trimethoxysilyl)-propyldimethyloctadecylammonium chloride (QAS) showed significant antimicrobial activity towards most Gram-positive cocci [16]. QAS possesses a silyl group, which can be covalently bound to glass and cotton, but also to many other frequently employed biomaterials, including silicone rubber after activation of the surface with gas plasma [17], [18].
The aim of this study was to determine the antimicrobial activity of a QAS-coating on silicone rubber in vitro toward two Gram-positive and two Gram-negative strains. Subsequently, QAS-coatings have been evaluated in vivo toward a S. aureus strain.
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Animals
Eight male, 12 weeks old, specific-pathogen free Albino Oxford rats (Harlan Nederland, Horst, The Netherlands) weighing 290±30 g were used. The animals were maintained under clean conventional conditions and fed standard rat chow and water ad libitum. The animals were allowed to acclimatize to our laboratory conditions for 1 week before experiments. All animals received humane care in compliance with the “Principles of Laboratory Animal Care” (NIH Publication No. 85-23, revised 1985) and the
Results
The chemical and physical characteristics of SR and QAS-coated SR surfaces are summarized in Table 1. The presence of QAS can be clearly seen from the increases in the %N and %Cl relative to uncoated SR, indicating that the adsorbed QAS-coating has a thickness in an order of magnitude comparable with the depth of information of XPS, i.e. 5–10 nm. Equilibrium water contact angles are hardly affected by the QAS-coating, although the advancing contact angle increases upon QAS-coating whereas the
Discussion
In this study silicone rubber was coated with 3-(trimethoxysilyl) propyldodecyldimethylammonium chloride (QAS), and the antimicrobial activity of this coating was evaluated, both in vitro and in vivo. The QAS-coating showed antimicrobial activity towards all tested bacterial strains in vitro, which was confirmed in vivo for one selected S. aureus strain. At this point it is noted that, although staphylococcal killing was significant in vitro and in vivo, not all S. aureus were effectively
Conclusion
3-(trimethoxysilyl)propyldimethyloctadecylammonium chloride coating of silicone rubber yields a positively charged, highly adhesive surface with antimicrobial effects towards adhering Gram-positive and Gram-negative bacteria. The antimicrobial effects also exist under in vivo conditions, at least against S. aureus in the rat. QAS-coated biomaterial implants thus seem promising in preventing biomaterial-centered infection in man.
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