# Antimicrobial Air Filter Coating with Plant Extracts Against Airborne Microbes
## 3 Results and Discussion
### Inactivation of Microorganisms on Plant Extract-Coated Air Filters  
The antimicrobial function of the plant extract-coated air filters was investigated by measuring the inactivation rate of _M. luteus_. _M. luteus_ was recommended as a testing microorganism in air filter antimicrobial tests by the Korea Air Cleaning Association. Figure S5 presents the colony-forming units of _M. luteus_ detached from the air filter immediately after exposure to bioaerosols. The tea-tree oil-coated air filter inactivation of _M. luteus_ is presented in Figure S5a. When coated with 0.57 mg/cm\({}^{2}\) of tea-tree oil, the colony-forming unit decreased from \(5.23\times 10^{5}\) to \(2.47\times 10^{5}\) CFU/mL, with a 52.87% inactivation rate. Other tea-tree oil-coated air filters exhibited a cell inactivation between 40 and 55%. However, neither rosemary nor garlic extract-coated air filters exhibited inactivation of _M. luteus_ at any extract concentration (Figure S5b,c). As shown in Table 1, tea-tree oil showed the highest cell inactivation among the three plant extracts tested, and similar results were found in plant extract-coated air filters. Although the tea-tree oil-coated air filter initiated the inactivation of _M. luteus_, its antimicrobial functions were not severe after the initial exposure to bioaerosols (Figure S5a). The reduced initial antimicrobial activity of the plant extracts in air filters could be attributed to their mass transfer limitation in a silicate polymer matrix. However, significant cell inactivations were found after 48 h of cultivation (Figure 3a). The bioaerosol formed \(4.73\times 10^{6}\) CFU/mL in non-coated air filters when it was incubated for 48 h, but the tea-tree oil-coated air filters formed only \(1.63\times 10^{4}\) and \(1.33\times 10^{3}\) CFU/mL for 0.57 and 2.89 mg/cm\({}^{2}\) coated air filters, respectively, which induced a 99.65 and 99.97% cell inactivation (Figure 3a). No bacteria colonies formed for tea-tree oil concentrations in excess of 2.83 mg/cm\({}^{2}\). Rosemary coated air filters exhibited a 93.3%, 99.0%, and 98.7% inactivation of _M. luteus_ for 4.47, 6.73, and 13.40 mg/cm\({}^{2}\) solution concentrations, respectively (Figure 3b). Garlic extract-coated air filters also showed a 99.9%, 99.7%, and 99.8% cell inactivation for 11.51, 16.98, and 33.96 mg/cm\({}^{2}\) treated filters, respectively (Figure 3c). Typically, cell inactivation rates exhibited a dose-dependent increase pattern, indicating that the inactivation was due solely to the antimicrobial activity of the plant extracts. An air filter allows clean air to pass and rejects fine particles including bioaerosols. Even though air filters capture bioaerosols, they are active on the surface of the filters and can cause a variety of diseases. In contrast to common air filters, which can serve as microorganism reservoirs and pose the risk of secondary infections, our results clearly show that the plant extract-coated air filters fabricated in this study reduce the risk of secondary infections. \begin{table}
\begin{tabular}{c c c c c} \hline  & **Control** & **Tea-tree oil** & **Rosemary** & **Garlic** \\ \hline Pressure drop (mmAq) & 1.97 \(\pm\) 0.06 & 2.17 \(\pm\) 0.17 & 2.13 \(\pm\) 0.22 & 2.16 \(\pm\) 0.15 \\ \hline \end{tabular}
\end{table} 
Table 3: Pressure drop of the control and plant extract-coated air filters. To investigate the anti-pathogen function of the plant extract-coated air filters, we evaluated the inactivation of _Klebsiella pneumoniae_ and _Staphylococcus aureus_. In this experiment, three air filter samples were prepared by coating the filters with 11.32 mg/cm\({}^{2}\) tea-tree oil, 13.40 mg/cm\({}^{2}\) rosemary, and 33.96 mg/cm\({}^{2}\) garlic extract, respectively. Strains of _K. pneumoniae_ and _S. aureus_ are usually found  
Figure 3: _M. luteus_ colony-forming units 48 h after bioaerosol exposure vs. weight of plant extract per unit area. (**a**) tea-tree oil, (**b**) rosemary, and (**c**) garlic. in air filters and cause microbial contamination.