Antimicrobial Air Filters Using Natural Euscaphis japonica Nanoparticles

Controlling bioaerosols has become more important with increasing participation in indoor activities. Treatments using natural-product nanomaterials are a promising technique because of their relatively low toxicity compared to inorganic nanomaterials such as silver nanoparticles or carbon nanotubes. In this study, antimicrobial filters were fabricated from natural Euscaphis japonica nanoparticles, which were produced by nebulizing E. japonica extract. The coated filters were assessed in terms of pressure drop, antimicrobial activity, filtration efficiency, major chemical components, and cytotoxicity. Pressure drop and antimicrobial activity increased as a function of nanoparticle deposition time (590, 855, and 1150 µg/cm2filter at 3-, 6-, and 9-min depositions, respectively). In filter tests, the antimicrobial efficacy was greater against Staphylococcus epidermidis than Micrococcus luteus; ~61, ~73, and ~82% of M. luteus cells were inactivated on filters that had been coated for 3, 6, and 9 min, respectively, while the corresponding values were ~78, ~88, and ~94% with S. epidermidis. Although statistically significant differences in filtration performance were not observed between samples as a function of deposition time, the average filtration efficacy was slightly higher for S. epidermidis aerosols (~97%) than for M. luteus aerosols (~95%). High-performance liquid chromatography (HPLC) and electrospray ionization-tandem mass spectrometry (ESI/MS) analyses confirmed that the major chemical compounds in the E. japonica extract were 1(ß)-O-galloyl pedunculagin, quercetin-3-O-glucuronide, and kaempferol-3-O-glucoside. In vitro cytotoxicity and disk diffusion tests showed that E. japonica nanoparticles were less toxic and exhibited stronger antimicrobial activity toward some bacterial strains than a reference soluble nickel compound, which is classified as a human carcinogen. This study provides valuable information for the development of a bioaerosol control system that is environmental friendly and suitable for use in indoor environments.


Introduction
Bioaerosols, which are aerosols of biological origin, may include intact microorganisms and/or parts or products of organisms [1]. Among them, airborne viruses, bacteria, and fungi have been investigated actively because airborne pathogens are readily transmitted by airflow and can cause a variety of diseases, including allergic rhinitis, asthma, chronic obstructive pulmonary disease (COPD), influenza, and severe acute respiratory syndrome (SARS) [2][3][4][5].
Over the last several decades, much effort has been devoted to develop efficient bioaerosol control methods and devices, including thermal methods [6][7][8], ultraviolet irradiation [9][10][11], antimicrobial filters [12,13], and titanium dioxide catalysis [14,15]. Among these, antimicrobial air filtration technologies are considered promising because they are easily applied to conventional air-conditioning systems. Previous studies have shown that air filtration technologies employing antimicrobial inorganic nanoparticles are effective in controlling bacterial aerosols. The antimicrobial efficacies of such systems depend on the exposure time, particle size, and concentration [16,17]. In particular, silver (Ag) nanoparticles are antimicrobial agents with a broad antimicrobial spectrum. Ag nanoparticles damage bacterial cell membranes and induce metabolic changes by decreasing enzyme activity [18,19]. Due to the outstanding antimicrobial activity of these materials, they have been extensively studied and applied in a variety of fields including indoor air quality (IAQ) and human health, air filtration, clothing manufacturing, electronics, food processing, cosmetics, and medical devices [20,21]. Similarly, copper (Cu) nanoparticles are widely known as antimicrobial substances. Previous studies showed that Staphylococcus aureus, Escherichia coli, Bacillus subtilis, Klebsiella pneumoniae, and Pseudomonas aeruginosa are sensitive to Cu nanoparticles [22]. Carbon nanotubes (CNTs) have also been applied to the control of water quality and IAQ. In their aquatic dispersion, CNTs showed strong antimicrobial activities as the reduction in bacterial viability reached a maximum of~6 log, and in combination with Ag nanoparticles enhanced the antimicrobial activity of air filters. CNTs in direct contact with bacterial cells induce membrane damage and subsequent cell death. Single-walled CNTs are more toxic to bacteria than multi-walled CNTs [23][24][25].
Despite these advantages, inorganic nanoparticles [26,27] can exert adverse effects on health [28][29][30]. Previous studies have indicated that Ag nanoparticles are toxic to mammalian cells and certain organs because of transcutaneous penetration of the particles. Copper oxide nanoparticles induce DNA damage and oxidative stress in cells [31][32][33][34]. Various toxicity mechanisms for CNTs have been reported, including the interruption of transmembrane electron transfer, penetration of the cell envelope, and oxidation of cell components [35,36]. Moreover, long-term exposure or inhalation of these nanoparticles can lead to a reduction in respiratory functions [37,38].
To overcome these disadvantages, alternative air filtration technologies employing natural antimicrobial materials have been proposed [39][40][41]. Natural products, such as plant extracts, are typically less toxic relative to inorganic antimicrobial materials [42]. Natural-product nanoparticles consist of multiple compounds with various chemical properties. Depending on the extract and the nature of the material, these properties can include antibiotic activities such as anti-inflammatory, antiviral, and/or antimicrobial effects [43][44][45][46]. Many natural antimicrobial products have been discovered, including extracts from Ratibida latipalearis, Teloxys graveolens, Dodonaea viscosa, Hyptis albida, Melaleuca alternifolia (tea tree oil), and Sophora flavescens [47][48][49][50]. In addition, various chemical compounds contained in natural products have been shown to control bacterial metabolism. Recently, reports detailing the control of bacterial aerosols using natural-product nanoparticles have shown that extracted essential oils can reduce bacterial loads when applied to contaminated ventilation systems [51]. Air filters coated with tea tree oil inactivated~99% of bacteria on their surface within 2-8 min. Filters coated with S. flavescens nanoparticles inactivated >91% of bacteria within 2 min [12,41,51].
In this study, a Euscaphis japonica extract was used to produce natural-product nanoparticles that were deposited onto air filters. Extracts of E. japonica, a tree grown in Northeastern Asia, are an ingredient in traditional herbal medicines used primarily to treat inflammation [52]. Several antibiotic activities of E. japonica have been reported, including anti-fibrotic [53], antiproliferative, and antimutagenic activities [54]. However, the antimicrobial activity of E. japonica when used in an air filtration system has not been reported. E. japonica nanoparticles were produced by a nebulization-thermal drying process [55,56]. The characteristics of E. japonica nanoparticle-coated filters were evaluated in terms of filtration efficiency, pressure drop, and antimicrobial activity under various particle deposition conditions. Additionally, the major components of the extract were analyzed using high-performance liquid chromatography-electrospray ionization tandem mass spectrometry (HPLC-ESI/MS/MS) and the cytotoxicity of E. japonica was compared with that of a soluble nickel compound (SNC), a known human carcinogen.

Materials and Methods
Preparation of Euscaphis japonica methanolic extract powder E. japonica was purchased from a plant extract bank at the Korea Research Institute of Bioscience & Biotechnology (KRIBB), dried at room temperature, and pulverized in a blender. Pulverized E. japonica was extracted with methanol (106,009; Merck KGaA, Darmstadt, Germany) and sonicated for 3 days. Sonication was performed for 15 min, 10 times per day. The extracts were passed through nonfluorescent cotton filters (0.45 μm pore size, 13 mm filter diameter; Smartpor GHP syringe filter, Woongki Science, Seoul, Republic of Korea) and concentrated in vacuo to yield the methanol extract. After concentration, the extract was lyophilized for 24 h and the resulting E. japonica extract powder was stored at 5°C.

Preparation of E. japonica nanoparticle-coated filters
Antimicrobial filters were coated with E. japonica nanoparticles. Then 0.25 g of E. japonica extract powder was dissolved in 40 mL of methanol and filtered through a 0.45-μm cellulose acetate membrane filter (National Scientific Co., Rockwood, TN, USA) to remove insoluble residues. Fig 1 (A) shows the experimental setup for the fabrication of nanoparticle-coated filters. Twenty milliliters of the above E. japonica solution was poured into a one-jet Collison nebulizer (BGI Inc., Waltham, MA, USA). The nebulizer was supplied with 1 L/min of HEPAfiltered air under 1 psig. The resulting E. japonica aerosol was passed through an activated carbon absorber, mixed with 9 L/min of clean and dry air, and passed through a thermal glass quartz tube heater to remove methanol. The size and number concentration of the fabricated natural-product nanoparticles were measured using a wide-range particle spectrometer (WPS-100XP; MSP Co., Minneapolis, MN, USA) calibrated to a size range of 10-10,000 nm. The particle morphology was examined using a scanning electron microscope (200 NANO SEM; FEI Co., Hillsboro, OR, USA). The fabricated natural-product nanoparticles were continuously deposited onto polyurethane resin fiber filters (Clean & Science Co., Ltd., Seoul, Republic of Korea) (fiber diameter: 10-20 μm; thickness: 0.3 mm; and packing density: 33%).

Test bacteria
Staphylococcus epidermidis (Korean Collection for Type Cultures KCTC 1917; Biological Resource Center, Republic of Korea) and Micrococcus luteus (KCTC 9856) were used as the test bacteria in this study. Gram-positive bacteria are widely used in bioaerosol research [57] and are common in indoor environments and on human skin [58,59]. The bacteria were incubated in a nutrient broth medium (Becton Dickinson, Franklin Lakes, NJ, USA) at 37°C. When the optical density of the bacterial suspension reached~0.8 at 600 nm, the bacteria were harvested by centrifugation and washed three times with distilled water. The concentration of the resulting suspension was~10 8 colony forming units (CFU)/mL. One milliliter of the bacterial suspension was mixed with 19 mL of distilled water and loaded into a six-jet Collison nebulizer (BGI Inc.). Fig 1 (B) shows a schematic diagram of the apparatus used to measure the filtration efficiency, pressure drop, and antimicrobial activity of the coated filters. Droplets containing test bacteria were sprayed using a six-jet Collison nebulizer supplied with a 5 L/min airflow under 1 psig. Moisture was removed from the droplets by passing through a diffusion dryer, and the bacterial aerosols were introduced onto the surface of the filter medium. The particle size and concentration of bacterial aerosols were measured with an aerodynamic particle sizer (APS model 3321; TSI, Inc., Shoreview, MN, USA) at both the inlet and outlet of the filter holder. Filtration efficiencies of the coated filters were calculated using the following equation:

Filtration efficiency and pressure drop
where C inlet and C outlet represent the particle concentrations (particles/cm 3 air ) of the bacterial aerosol at the inlet and outlet of the filter, respectively. The pressure drops of the coated filters were measured using a micromanometer (FC012; Furness Control, Ltd., Bexhill, UK).

Antimicrobial tests
The bacterial aerosols were deposited onto the filters for~3 min. After a contact time of 10 min, the filters were placed in 5 mL (V extraction ) of phosphate-buffered saline (PBS) containing 0.01% Tween 80 and sonicated for 10 min to transfer the bacteria from the filters to the PBS solution. The resulting bacterial suspension was serially diluted onto plates of nutrient agar (Becton Dickinson) and incubated at 37°C for 24 h. The colonies that grew on the plates were counted. Bacterial inactivation efficiency was calculated as follows: Ratio of bacterial inactivation where ABCF is the active proportion of bacteria from the control filter and ABAF is the active proportion of bacteria from the antimicrobial filter; CFU control and CFU antimicrobial are the concentrations (CFU/mL) of active bacterial suspensions produced from the control and antimicrobial filters, respectively; and N is the concentration of bacteria (particles/mL) in the extraction suspension that was plated onto agar. Q sampling is the total airflow sampling volume and z extraction is the physical extraction efficiency of the filter for bacteria, which is defined as the ratio of the number of particles transferred from the filter to the extraction liquid to the number of particles removed from the airflow using the filter. In this study, we assumed that the physical extraction efficiency for bacterial particles from all filters was identical.

Chemical analysis of the E. japonica extract
Identification of the major chemical compounds in the E. japonica extract was performed using an HPLC-ESI/MS device (Thermo Fisher Scientific Inc., San Jose, CA) equipped with an ACCELA photodiode array detector (PDA), an autosampler, a quaternary pump, and an LCQ FLEET ion trap with an electrospray ionization source. The Thermo Xcalibur software (version 2.1) was used for data acquisition and processing. The mobile phase consisted of 0.1% formic acid in water (solvent A) and 0.1% formic acid in acetonitrile (solvent B In vitro cytotoxicity of E. japonica nanoparticles A549 human lung adenocarcinoma cancer cells and HEL 299 human lung fibroblasts were obtained from the American Type Culture Collection (ATCC, Rockville, MD, USA) and the cells were maintained in a 5% CO 2 humidified atmosphere at 37°C. Eagle's minimum essential medium (EMEM; ATCC) was used to support the HEL 299 cells and RPMI-1640 (Hyclone, Logan, UT, USA) was used for A549 cultivation. These cells were supplemented with 10% (v/v) fetal bovine serum (FBS), 100 U/mL penicillin and 100 μg/mL streptomycin. Cell viability upon exposure to E. japonica extract was evaluated using an EZ-Cytox cell viability assay kit (Daeil Lab Service, Ltd., Seoul, Republic of Korea) according to the manufacturer's instructions. Cells (1 × 10 4 cells/well) were plated in 96-well plates, incubated at 37°C for 24 h, and given a fresh change of medium containing E. japonica extract at the indicated concentration for 48 h. At the end of the incubation, 10 μL of EZ-Cytox solution was added to the well and incubated for at least 1 more hour. The absorbance at 450 nm was measured using a Synergy HT Multi-microplate reader (BioTek Instruments, Winooski, VT, USA). Data were expressed as cell growth percentages relative to the controls (cells treated with dimethyl sulfoxide (DMSO) only) for each extract concentration.

Statistical analyses
Correlation coefficients, linear regressions, and t-statistics of experimental data were calculated using the SPSS statistical software, version 12.0 (SPSS, Inc., Chicago, IL, USA).

Results and Discussion
As shown in Fig 2 (A), nebulization and thermal drying produced natural-product nanoparticles with a wide size distribution, ranging from a few to several hundred nanometers in diameter. The distribution is best represented by a monomodal curve with a peak diameter of 75.16 nm, a geometric mean diameter (GMD) of 75.82 nm, and geometric standard deviation (GMD) of 1.217. The particles generated from methanol showed a broad size distribution, and compared to size distribution and concentration of natural-product nanoparticles, the particles generated from methanol were trivial (<3%). The deposition efficiency of these particles is shown in Fig 2 (B). More than 98% of particles with diameters less than 43 nm and greater than 277 nm were deposited onto the control filter. The deposition efficiency of particles between 43 and 277 nm was lower. The lowest deposition efficiency on the control filter was 85.3% for particles 92 nm in diameter. The relatively low deposition efficiency observed for particles between 43 and 277 nm results from the mechanism of particle filtration. Particles with midrange diameters were too large for diffusion filtration and too small for impaction and interception mechanisms [1]. SEM micrographs show spherical and polydisperse naturalproduct nanoparticles on the filter fibers (Fig 2 (B)). The quantity of deposited material was determined by weighing the filters before and after particle deposition using a microbalance (Mettler MT5; Mettler-Toledo International, Inc., Seoul, Republic of Korea). The weight of the particles deposited on the filter surface ranged from~590 to 1150 μg/cm 2 filter (590, 855, and 1150 μg/cm 2 filter correspond to deposition times of 3, 6, and 9 min, respectively). Fig 3 shows that pressure drop across the filter increased linearly from 1.1 to 13.4 mmH 2 O as the quantity of deposited nanoparticles increased (y = 1.37x + 1.94, r 2 = 0.9857, p <0.05) [60]. Filtration efficiencies were tested using S. epidermidis and M. luteus aerosols. As shown in Table 1, the test bacteria concentrations were~278 (S. epidermidis) and~234 particle/cm 3 (M. luteus), respectively. The distribution of the S. epidermidis aerosol was a monomodal curve with a peak diameter of~0.84 μm, a GMD of~0.81 μm, and a GSD of~1.22 (S1 Fig, Table 1). Although the shape of size distribution curve of the M. luteus aerosol was similar to that of S. epidermidis aerosol, the M. luteus particles were larger than those of S. epidermidis (S1 Fig). No statistically significant difference was observed between the filtration efficiencies of filters that  had been coated for 3 and 9 min (p >0.05). However, because of the dissimilar size distributions of the two bacterial aerosols, filtration efficiencies of the two species were slightly different (p <0.05):~97% for S. epidermidis and 95% for M. luteus. Also, note that flat panel-type filters were used in these tests to estimate the effects of natural-product nanoparticles. Real-world applications would typically employ folded filters to increase the filtration area and decrease the pressure drop.
As shown in Fig 4, the inactivation efficiencies of antimicrobial filters increased for both bacteria with increasing nanoparticle deposition time (M. luteus: r 2 = 0.9612, S. epidermidis: r 2 = 0.9959, p <0.05). Approximately 61, 73, and 82% of the M. luteus aerosols were inactivated on filters that had been coated for 3, 6 and 9 min, respectively, while the corresponding values were~78,~88, and~94% with S. epidermidis. M. luteus was more resistant to the E. japonica extract than S. epidermidis under all conditions (p <0.05), with a maximum difference of 17%. Although both M. luteus and S. epidermidis are Gram-positive bacteria, M. luteus is more suited to survive in extreme and/or nutrient-poor environments over extended periods of time [61]. Similarly, in extreme environments, Micrococcus strains have been shown to exhibit greater resistance to novobiocin than Staphylococcus strains [62].
The major chemical constituents of E. japonica extract were investigated using HPLC. The results showed that the chromatogram detected at 280 nm contains three major peaks   To assess the toxicity of E. japonica, in vitro cytotoxicity tests were conducted using EZ-Cytox cell viability assay kits, the results of which were compared with those of SNCs, which are known to produce genotoxic effects in cells. The SNC used in this study is classified as a human carcinogen by the U.S. National Toxicology Program (NTP) and Beraterkreis Toxikologie in Germany [68] although it is a weaker carcinogen than other, insoluble nickel compounds. Fig 5 shows the concentration of E. japonica and SNC required to attain 50% inhibition of A549 cancer cells and HEL 299 cells. For both cell types, the inhibitory concentration of E. japonica was statistically confirmed to be higher than that of the SNC following 48-h exposures (p <0.05). The required E. japonica concentrations were 137 and 256 μg/mL, >57% higher than those of the SNC.
In addition, the antimicrobial activities of E. japonica and the SNC were casually evaluated using the disk diffusion method. Approximately 50 mg of E. japonica extract and SNC powders were dissolved in 1 mL of DMSO and distilled water, respectively, and 10 μL of each suspension was used to soak Whatman filter papers (11-mm diameter; GE Healthcare Life Sciences, Pittsburgh, PA, USA). Fig 6 shows the results against S. aureus, Enterococcus hirae, M. luteus, and S. epidermidis. The antimicrobial activities of E. japonica were similar to those of the SNC against E. hirae and M. luteus (p >0.05), while it was considerably more effective against S. aureus and S. epidermidis with inhibition zones extending to more than twice the radius of those of the SNC (p <0.01) The results in Figs 5 and 6 confirm that E. japonica is less toxic and exhibits stronger antimicrobial activity on some bacteria than the SNC.

Conclusion
Filters coated with natural E. japonica extract nanoparticles are effective in inactivating bioaerosols. The pressure drop and antimicrobial activity of nanoparticle-coated filters increased with increasing nanoparticle deposition time. HPLC and ESI/MS analyses showed that the major chemical compounds in the E. japonica extract were 1(ß)-O-galloyl pedunculagin, quercetin-3-O-glucuronide, and kaempferol-3-O-glucoside. The latter two compounds likely play important roles in the inactivation of bacterial aerosols. In vitro cytotoxicity and disk diffusion tests showed that E. japonica nanoparticles were less toxic and had stronger antimicrobial activity on some bacterial strains than the SNC, which is classified as a human carcinogen. Note that the amount of nanoparticles deposited on a given filter must be optimized for the type of bacterial aerosol. The antimicrobial performance of the E. japonica nanoparticle-coated filters depended both on the amount of nanoparticles deposited and the nature of the airborne bacteria. Additionally, previous studies showed that major chemical components in a natural product were naturally degraded over time at room temperature [69] and antimicrobial activity and morphologies of natural-product nanoparticles were affected by a variety of environmental factors such as humidity and temperature [40,70]. Thus, the long-term stability and effects of humidity and thermal energy on E. japonica nanoparticles need to be evaluated to estimate the efficiency of nanoparticle-coated filters for real-world applications. Although the in vitro tests showed that E. japonica is less toxic than the carcinogen, it does not mean that the natural product is harmless to human health. Thus, additional studies such as in vivo experiments are required. In a biological setting, many experimental values should be considered. Thus, field experiments are required to confirm the applicability of the natural-product nanoparticle-coated filter in a real environment. We are considering a field experiment of our filters in a future study. This study provides valuable information for the development of an environmentally friendly bioaerosol control system that is suitable for use in indoor environments.