Conventional, traditional techniques to prevent catheter infections (such as practicing good aseptic techniques, systemic administration of antibiotics, etc.) have not provided satisfactory results to date. For example, systemic antibiotic (vancomycin and gentamicin) administration alone without catheter removal is only 22-37% effective in treating tunneled catheter-associated bacteremia (Maki and Tambyah 2001). The reasons for the ineffectiveness of these traditional practices are the resistance of bacteria toward antibiotics. Placing an antibiotic solution into the catheter lumen is another method to prevent and treat catheter infections. However, this method has a problem of leading to the development of antibiotic resistant bacteria. Therefore, this method is generally not recommended by the Kidney Disease Outcomes Quality Initiative and the Centers for Disease Control and Prevention (Yahav et al. 2008).
With the number of bacteria that are resistant to antibiotic treatment alarmingly increasing, attention has been recently paid to coating or impregnating catheters with nonantibiotic antimicrobial agents (Yahav et al. 2008). Since catheters themselves are an important source of infection, and bacterial adhesion to the catheter surface is important in the pathogenesis of infection, the most promising and straightforward strategy toward decreasing infection is to fabricate catheters that resist bacteria attachment (Berra et al. 2008). However, researchers are still looking for such antibacterial coating materials that can be easily applied and are inexpensive. For example, silver and silver compounds (such as chlorhexidine-silver sulfadiazine), one of the most studied materials for antibacterial coatings on catheters or for impregnating in catheters, have yielded mixed results toward preventing infections. Some studies as early as the 1950s and later in the 1970s, and 1990s, showed that silver coatings could reduce the incidence of catheter-associated infections (Winson 1997; Akiyama and Okamoto 1979; Liedberg and Lundeberg 1990). In contrast, a large number of recent studies have demonstrated that catheters coated or impregnated with silver or silver compounds did not reduce the incidence of bacterial colonization and catheter-associated infections (Yahav et al. 2008; Kumon et al. 2001; Trerotola et al. 1998; Logghe et al. 1997) or lacked efficacy (Trerotola 2000; Dunser et al. 2005). Especially, in some studies, silver-coated catheters were shown to be ineffective against S. aureus, S. epidermidis, Enterobacter aerogenes, Klebsiella pneumoniae, P. aeruginosa, Candida albicans, and Escherichia coli (Bach et al. 1999). There is even concern about the increase in staphylococcal infections in male patients using silver oxide impregnated catheters (Bach et al. 1999; Gaonkar and Modak 2003). Coupled to this controversy of using silver towards decreasing catheter infections is their high cost (many hospitals have opted not to use silver-coated catheters due to expense, and in those few hospitals that do, only high-risk patients are receiving them). Endotracheal tubes coated with silver can cost as much as $100 (whereas noncoated endotracheal tubes are a couple of dollars).
Other examples of the ineffectiveness of current antibacterial catheters are catheters impregnated with nitrofurazone which is an antibiotic. Such catheters were shown to be ineffective against vancomycin-resistant enterococci and were predicted to be ineffective against gram-negative strains (Lawrence and Turner 2005). In addition, most of the antimicrobial catheters do not prevent infection of long-term (more than 2 weeks) catheterization (Trerotola 2000; Kumon et al. 2001).
Recently, some other materials, such as those which release nitric oxide (NO), have been investigated as antibacterial coatings for catheters. NO, a diatomic free radical, has been shown to have antibacterial properties against some gram-negative and gram-positive bacteria (such as P. aeruginosa, S. aureus, and S. epidermidis) (Ahearn et al. 2000; Nablo et al. 2005). Therefore, some research has focused on incorporating NO donor molecules (diazeniumdiolate) into polymers to make antibacterial coatings. However, such approaches require complex fabrication methods which drive up costs. For example, the most common method is incorporating diamine-containing organosilanes into a sol-gel matrix, then exposing the sol-gel to high pressure of NO gas so that the diamine coordinates two NO molecules to form a NO donor molecule.
Besides NO-releasing coatings, TiO2 (titanium dioxide) has also been studied for antibacterial coatings (Evans and Sheel 2007; Page et al. 2007). However, this approach is less applicable to coating catheters since it is difficult to coat TiO2 onto polymers because of adhesion problems between the oxide coating and the polymer substrates (Woodyard et al. 1996; Jeom Sik et al. 2005; Girshevitz et al. 2008; Navarro-Alarcon and Cabrera-Vique 2008).
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