# Potential Synergistic Inhibition of _Enterococcus faecalis_ by Essential Oils and Antibiotics
## 1 Introduction  
Since their discovery, antibiotics have become essential in successfully combating infectious diseases to improve human health [1; 2]. In dentistry, antibiotics are selectively used for orofacial infections of odontogenic and non-odontogenic origins such as dental abscess, pulpal necrosis, periodontal diseases, dental caries, dental trauma, adenoiditis, optomastoiditis, and in prophylaxis [3; 4; 5; 6]. Penicillin beta-lactam and amoxicillin, along with clindamycin, are the most widely prescribed oral antibiotic agents in dentistry, prophylatically and for therapeutic use, for their broad-spectrum antimicrobial activity of the former and as an alternate to penicillin allergic responses for the latter [1; 6; 7]. Along with their health benefits, however, the overprescription of antibiotics is a concern for clinicians and for patients. An analysis of outpatient prescription data from British Columbia showed the dental antibiotic prescription rate increasing by 71.6% between the years 1996 to 2013 [6]. Similar UK and US studies also found that over 65% of antibiotic prescriptions were issued when there was no evidence of spreading infection, and 73% to 85% of antibiotic prescriptions of penicillin beta-lactam and liposomes for adult and children outpatients were either unnecessary or untimely [8; 9; 10; 11]. Microorganisms, such as _actinomyces_, _streptomycinocci_, and _enterococci_ (_E. faecalis_), that produce dental biofilms, complex communities of microorganisms composed of bacteria that adhere to surfaces forming a protective matrix of extracellular substances, often diminish the effectiveness of antibiotics, increase their resistance, and contribute to the development of endodontic and periodontal diseases, ultimately causing failures of their therapies [12; 13]. In teeth that are treated with root canal therapy, 77% to 90% of recurring infections and subsequent treatment failures are linked to _E. faecalis_[14; 15; 16]. Furthermore, _E. faecalis_ is commonly recovered in teeth that were treated in multiple visits [12; 17], likely due to its ability to form biofilms, persist in saliva, survive in nutrient-free environments, resist many antibiotics, and remain dormant as a facultative anaerobic [18; 19; 20]. In addition to root canal infections, enterococcal bacteria are also associated with endocarditis, bacteremia, urinary tract infections, intra-abdominal infections, and prostatitis [21; 22]. Increased natural defense mechanisms acquired by the microorganisms, along with overprescription and increased prophylactic use of antibiotics, have elevated global concern for the emergence of antibiotic-resistant microorganisms and increased incidences of antibiotics-related secondary health issues such as dysbiosis, clostridium difficile infection, resistant urinary tract infection, and methicillin-resistant Staphylococcus aureus infections [6; 23; 24; 25; 26]. Essential oils (EOs) and their anti-inflammatory, antifungal, antimicrobial, and antibacterial properties are well elucidated [27; 28; 29; 30; 31]. Overall, EOs display increased sensitivity to Gram-positive bacteria, and their antimicrobial responses are mediated in part by ATP and inhibition of ATPases, disruption of membrane permeability, and inhibition of biofilm synthesis in the microbes [32; 33; 34; 35]. In vitro studies report significant antibacterial effects of thymme, clove, sage, peppermit, lavender, cinnamon EOs, and their chemical components of thymmol, eugenol, thujone, menthol, linalool, and cinnamaldehyde on caries-causing bacteria such as _streptomycinocci_ and _lactobacillus_ spp. [36; 37]. Among the phenylpropanol EOs, thymmol, eugenol, menthol, and cinnamaldehyde report potent antimicrobial properties in MIC (minimum inhibitory concentration), MBC (minimum bactericidal concentration), disk diffusion, and mouth rinse tests [38; 39; 40; 41].