# Potential Synergistic Inhibition of _Enterococcus faecalis_ by Essential Oils and Antibiotics
## 3 Results
### Combined Antibacterial Effects of Antibioticstics with EOs  
There were significant increases in antimicrobial effects when 5% antibiotics (kanamycin (2.5 mg/mL), streptomycin (2.5 mg/mL), gentamicin (1.5 mg/mL), and ampicillin (5 mg/mL)) were combined with EOs (CN 5%, 2.5%, 1.25%, and CL50%). These enhanced antimicrobial effects were primarily observed when 5% antibiotics were combined with CN5%, where the combined solution produced ZOI that was larger than that of the corresponding 10% antibiotic solutions as well as their individual component solutions of 5% antibiotics and CN5% (Figure 3). For kanamycin, the ZOI of K2.5 mg + CN5% (13.00 \(\pm\) 0.07 mm; F (7, 36) = 19.44, _p_! 0.0001) was significantly larger than the ZOI of K5 mg/mL (10% antibiotic; 9.50 \(\pm\) 0.64 mm; _p_! 0.05) and CN5% (7.50 \(\pm\) 0.22 mm) and K2.5 mg/mL (5% antibiotic; 5.50 \(\pm\) 0.28 mm) individually (_p_! 0.05). For streptomycin, the ZOI of S2.5 mg + CN5% (12.38 \(\pm\) 0.56 mm; F (7, 36) = 47.67, _p_! 0.0001) was significantly larger than the ZOI of S5 mg/mL (10% antibiotic; 10.75 \(\pm\) 0.25 mm; _p_! 0.05), and CN5% (7.50 \(\pm\) 0.22 mm) and S2.5 mg/mL (5% antibiotic; 8.00 \(\pm\) 0.40 mm) individually (_p_! 0.05). For gentamicin, the ZOI of G1.5 mg + CN5% (11.75 \(\pm\) 0.49 mm; F (7, 36) = 44.14, _p_! 0.0001) was significantly larger than the ZOI of G3 mg/mL (10% antibiotic; 9.75 \(\pm\) 0.25 mm; _p_! 0.05), and CN5% (7.50 \(\pm\) 0.22 mm) and G1.5 mg/mL (5% antibiotic; 8.25 \(\pm\) 0.47 mm) individually (_p_! 0.05). For ampicillin, however, the ZOI of A5 mg + CN5% (13.00 \(\pm\) 0.39 mm; F (7, 35) = 63.19, _p_! 0.0001) was not significantly different than the ZOI of A10 mg/mL (10% antibiotic; 13.00 \(\pm\) 0.70 mm), but was significantly larger than its component solutions CN5% (7.50 \(\pm\) 0.22 mm) and A5 mg/mL (5% antibiotic; 9.75 \(\pm\) 0.25 mm) individually (_p_! 0.05).  
Figure 3: Enhanced and additive antimicrobial effects of antibiotics combined with cinnamon EO (solid red circle lines) 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 squared lines represent ZOI for different concentrations of cinnamon EO alone. The kanamycin 2.5 mg/mL (5%) with CN5% (**A**) had significantly larger ZOI than its individual components (kanamycin 2.5 mg/mL or CN5% separately; dotted line and right solid square box) and kanamycin 5 mg/mL (10%). The streptomycin 2.5 mg/mL (5%) with CN5% (**B**) solution and gentamycin 1.5 mg/mL (5%) with CN5% (**C**) solution also produced significantly larger ZOI than their corresponding 10% antibiotics, CN5%, and 5% antibiotics separately. The ampicillin 5 mg/mL (5%) with CN5% (**D**) solution showed significantly larger ZOI only to its individual components but not to ampicillin 10 mg/mL (10%). #: _p_! 0.05 vs. 5% antibiotic concentration and corresponding CN EO individually; _k_: _p_! 0.05 vs. 10% antibiotic concentration alone.  
For 5% antibiotic solutions combined with CN2.5%, all antibiotics failed to produce ZOI that was larger than their corresponding 10% antibiotic solutions on _E. faecalis_. Only streptomycin showed that the S2.5 mg + CN2.5% (10.67 \(\pm\) 0.21 mm) had significantly increased ZOI compared to its individual component solutions of S2.5 mg (8.00 \(\pm\) 0.40 mm) and CN2.5% (7.50 \(\pm\) 0.22), but not to the 10% antibiotic solution. For 5% antibiotic solutions combined with CN1.25%, all antibiotics failed to produce ZOI that was larger than one of their corresponding component solutions.  
For 5% antibiotic solutions combined with CL50% EO, there were no enhanced antimicrobial effects as the combined solutions did not produce ZOI that was significantly different from its individual component solutions nor from the 10% antibiotic solutions against _E. faecalis_ (Figure 4).