# Potential Synergistic Inhibition of _Enterococcus faecalis_ by Essential Oils and Antibiotics
## 4 Discussion  
The increased rate of prescription for antibiotics in recent decades, their prophylactic use, and the subsequent rise of antibiotic-resistant pathogens are changing prescription protocols for the use of antibiotics in medical and dental clinical settings and the search for alternate therapeutic medicinal compounds such as essential oils. In dentistry, endodontic diseases of periapical and intraradicular infections and their root canal treatments are becoming increasingly difficult to manage due to _E. faecalis_, a facultative aerobe that forms biofilms, survives in low-nutrient environments, and can resist antibiotics, in the root canal space. EOs, with their anti-inflammatory, antifungal, antimicrobial, and antibacterial properties, have been shown to enhance the antimicrobial effects against _E. faecalis_ when combined with antibiotics and antiseptics. Given this, the examination of synergistic effects of antibiotics with EOs becomes highly relevant. In this study, we investigated the growth-inhibiting effects of penicillin and the aminoglycosic class of antibiotics when combined with cinnamon and clove essential oils in _E. faecalis_. Figure 4: Antimicrobial effects of kanamycin(**A**), streptomycin (**B**), gentamicin (**C**) and ampicillin (**D**) combined with clove EO (solid red circles) in inhibiting the growth of _E. faecalis_ in disk diffusion test. The dotted horizontal lines and shaded blue bars represent the average ZOI for 5% and 10% concentrations of antibiotics, and the solid squares represent ZOI for clove50% EO alone. Combining 5% concentration of antibiotics with CL50% did not produce any significant increase in ZOI compared to its individual components nor to 10% concentration of antibiotics against _E. faecalis_. All concentrations of cinnamon and clove EOs were effective in inhibiting the growth of _E. faecalis_ in a dose-dependent manner. The cinnamon EO produced greater antimicrobial effects than clove EO at about 10-fold lower concentration. For clove EO, the ZOI of CL25% was 7.5 \(\pm\) 0.2 mm and increased by 139% when the concentration of clove EO was increased to CL50%. For cinnamon EO, increasing the oil concentration two-fold from 1.25% to 10% also increased the size of ZOI by about 36 \(\pm\) 1.2% for each doubling of CN EO concentration. Our data support previous reports showing 1% to 10% of cinnamon and 50% of clove EOs were effective in inhibiting the growth of _E. faecalis_ to almost 100% within 15 min of exposure, and with the cinnamon EO, the inhibitory effect was maintained for up to 10 days [48; 49; 50]. [PERSON] et.al (2020) [46] reported that cinnamon EO (MIC 1.56 \(\upmu\)g/mL) was equally effective on _E. faecalis_ embedded in biofilm, killing over 90% within 15 min and outperforming chlorhexiidine wash, which only showed 31% effectiveness. For the antibiotics examined in the present study, all antibiotics were equally effective in inhibiting the growth of _E. faecalis_ at 10% concentration with ampicillin having the largest ZOI. When the concentration was lowered to 5%, the streptomycin, gentamicin, and ampicillin still maintained strong antimicrobial effects against _E. faecalis_. Kanamycin 5% showed the least effectiveness with ZOI of 5.50 \(\pm\) 0.28 mm. Our data are in line with a previous report where the MICs for ampicillin, gentamicin, and streptomycin were in a similar range of 8-14 \(\upmu\)g/mL (MIC for kanamycin was 32 \(\upmu\)g/mL) on an antimicrobial test against _E. faecalis_, and gentamicin was much more effective than kanamycin on an in vivo _E. coli_ meningitis bacteremia test [51; 52; 53]. There were enhanced and additive synergistic antimicrobial effects against _E. faecalis_ where 5% cinnamon EO combined with 5% antibiotics produced significantly larger ZOI than 10% antibiotics alone, as well as its individual components of 5% antibiotics and CN5% separately. Such additive synergistic effects were observed only with the amino-glycoside class of antibiotics tested (kanamycin, streptomycin, and gentamicin).