## 3 Results and Discussion  
### Identification of Test Microorganisms  
To obtain bacterial strains for analysis of the antibacterial activity of the _C. myrrha_ (T.Nees) Engl. resin extracts, airborne bacteria were collected and isolated. The isolated bacterial strains showed high homology to _Paenibacillus psadenensis_ NBRC 161214 (98% homology), _Microoccus yunanenensis_ YIM 65004 (99% homology), _Pseudomonas azototrans_ NBRC 12693 (99% homology), _Rhodococcus qingshengii_ djl-6-2 (99% homology), _Staphylococcus capitis_ JCM 2420 (99% homology), _Staphylococcus epidermidis_ NBRC 100911 (98% homology), and _Deinococcus radiodurans_ R1 (99% homology). The isolated strains were mostly Gram-positive, except for _Pseudomonas azototrans_ NBRC12693. These strains, which are found in soil, water, sewage, and human skin, have various characteristics (e.g., pollutant degradation, infection of cereal grains, carbendazim degradation, and radiation resistance) (Table 1) [44; 45; 46; 47; 48; 49]. Hence, these airborne bacterial strains could be used as potential samples for testing antibacterial activity. Screening of Optimal Solvent for Antibacterialativity of C. myrrha (T.Nees) Engl. Resin Extracts on Airborne Bacterium  
The different extraction yields, antioxidant activities, and antibacterial activities depend on the type of solvent because the characteristics of the extraction solvent influence the sorting and activity of the compounds extracted from the raw materials. Extracts of _C. papaya, A. dealbata, O. europaea_, and _P. betle_ Linn. using various solvents, such as ethanol, methanol, hexane, and ethyl acetate, showed different antibacterial activities depending on the physical properties of the extraction solvent [18,19,22]. Thus, the antibacterial effects of _C. myrrha_ (T.Nees) Engl. resin extracts using various solvents (hot water, hexane, DMSO, methanol, and ethanol) were evaluated to determine the optimal solvent for acquiring the extract with effective antibacterial properties. The extracts were tested against isolated airborne bacteria assayed in agar plates using the disk diffusion method. _C. myrrha_ (T.Nees) Engl. resin extracted using methanol and ethanol exerted antibacterial effects against _Paenibacillus psandenensis_ NBRC 161214, _Microoccus yunmanensis_ YIM 65004, and _Deinococcus radiodurans_ R1 (Table 2). Additionally, an antibacterial effect of _C. myrrha_ resin extracts obtained using only methanol was observed against _Rhodococcus qingshengii_ dj1-6-2. However, no antibacterial effects were observed in _C. myrrha_ (T.Nees) Engl. resin extracted using hot water, DMSO (except _Microoccus yunmanensis_ YIM65004), and hexane. Hence, the various extraction solvents used in obtaining _C. myrrha_ (T.Nees) Engl. resin extracts are likely to influence their antibacterial effects due to differences in their effective components. In general, the antibacterial effect may be related to the cell wall structure. However, no correlation between the antibacterial effect and the cell wall structure (e.g., Gram-positive and Gram-negative strains) was observed in this study. These phenomena may be due to the action and resistance mechanisms (e.g., cell wall synthesis, nucleic acid synthesis, protein synthesis, and folic acid metabolism) of the effective components in _C. myrrha_ (T.Nees) Engl. resin extracts [50]. Therefore, selecting an effective extraction solvent is an important parameter. The polarity indices of the solvents used for extraction were in the following order: water (10.2) > DMSO (7.2) > ethanol (5.2) > methanol (5.1) > hexane (0.1), whereas the dielectric constants of the solvents were in the following order: water (78.54) > DMSO (47) >