# Therapeutic Potential of Gramicidin S in the Treatment of Root Canal Infections
## 1 Introduction  
Teeth with necrotic pulps and periapical radiolucencies indicate the presence of bacteria in a root canal system. They can cause apical periodontitis, an inflammatory process around the apex of a tooth root accompanied by bone resorption [1; 2]. Root canal infections are characterized by a wide variety of combinations of a few oral anaerobic bacteria, such as _Actinomyces_ species and peptostreptococci. The non-oral but environmental bacterium _Pseudomonas aeruginosa_ is frequently isolated from treatment-resistant cases because of its intrinsic multidrug resistance, including resistance to tetracycline [3]. However, more and more evidence has been provided on the prevalence of _Enterococcus faecalis_ in teeth associated with a failed endodontic treatment of root canal infections [4; 2; 5; 6; 7]. The success rate for the re-treatment of teeth with _E. faecalis_ is lower (66%) than the overall success rate of re-treatment (75%) [8]. Previous studies reported about up to 77% prevalence of _E. faecalis_ in teeth with failed endodontic treatment [7]. _E. faecalis_ can best adapt to and tolerate the conditions in the filled root canal [6]. This facultative anaerobe is able to grow at 6.5% NaCl and temperatures up to 45 \({}^{\circ}\)C and is even able to colonize dentine under alkaline pH and glucose starvation [9]. It has a remarkable ability to avoid leukocyte-dependent antibacterial mechanisms and to utilize collagen within dentin, using thisproperty for adhesion within the root canal [2]. A number of other virulence factors in _E. faecalis_, such as a plasmid-encoded surface localized adhesion protein, gelatinase (metalloendopeptidase), a serine protease, hemolysis, which is classified as a type A lantibiotic, and extracellular superoxide, permit adherence and facilitate invasion [6]. The surface adherence of biofilms is significant in persistent endodontic infections because it increases the difficulty of eliminating biofilms by conventional treatment measures [10]. Although the conventional endodontic irrigant sodium hypochlorite (1%-6%) exhibits more efficient elimination of _E. faecalis_ biofilms in comparison to chlorhexidine, calcium hydroxide, povidence iodine and ethylendiamintetraacetat [11; 12], this intracranial medication was found to have a limited action against bacterial biofilms [13].  
Currently, there are no clinically available compounds that disassemble biofilms of _E. faecalis_, although several antimicrobial peptides are in development, such as siamycin I, which affects quorum sensing, as well as protegrin, and oritavancin that can damage the bacterial membrane [14]. Recently, the use of the commercially available Ledermix(r) paste, which contains demeclocycline, or the new alternative endodontic paste Odontopaste, which contains clindamycin, was reported to be ineffective in the treatment of _E. faecalis_ infections due to the natural clindamycin and acquired tetracycline resistance of these bacteria [6; 15]. Their dominance in nosocomial infections is based on their ability to develop multidrug resistance via the acquisition of antibiotic resistance genes on plasmids or transposons from other bacteria. Notably, surface aggregation facilitates the contact between the cells, which leads to the exchange of plasmids that carry resistance. Approximately 70% of _E. faecalis_ isolates from primary endodontic infections showed a resistance to tetracycline [16]. In genomic analysis, the tetracycline resistance genes _tetM_ and _tetL_, as well as gelatinase, aggregation substance and enterococcal surface protein genes, were detected in these _E. faecalis_ isolates.  
In this paper, we report the antimicrobial and anti-biofilm activity of the cyclic decapeptide gramicidin S (GS) against tetracycline-resistant _E. faecalis_ illustrated by two cases of successful treatment of Ledermix(r)-resistant root canal infections using this peptide.