| Summary: | Background: <i>Artemisia sieberi</i> (mugwort) is a member of the daisy family Asteraceae and is widely propagated in Saudi Arabia. <i>A. sieberi</i> has historical medical importance in traditional societies. The current study aimed to assess the antibacterial and antifungal characteristics of the aqueous and ethanolic extracts of <i>A. sieberi</i>. In addition, the study investigated the effect of silver nanoparticles (AgNPs) synthesized from the <i>A. sieberi</i> extract. Methods: The ethanolic and aqueous extracts and AgNPs were prepared from the shoots of <i>A. sieberi</i>. The characteristics of AgNPs were assessed by UV–visible spectroscopy, transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), and dynamic light scattering (DLS). The antibacterial experiments were performed against <i>Staphylococcus aureus</i>, <i>Bacillus subtilis</i>, <i>Escherichia coli</i>, and <i>Pseudomonas aeruginosa</i>. The fungal species used were <i>Candida parapsilosis</i>, <i>Candida krusei</i>, <i>Candida famata</i>, <i>Candida rhodotorula</i>, and <i>Candida albicans</i>. The antibacterial and antifungal characteristics were evaluated by measuring the diameter of growing organisms in Petri dishes treated with different concentrations of either extracts or AgNPs compared to the untreated controls. Furthermore, TEM imaging was used to investigate any ultrastructure changes in the microbes treated with crude extracts and AgNO<sub>3</sub>. Results: The ethanolic and aqueous extracts significantly decreased the growth of <i>E. coli</i>, <i>S. aureus</i>, and <i>B. subtilis</i> (<i>p</i> < 0.001), while <i>P. aeruginosa</i> was not affected. Unlike crude extracts, AgNPs had more substantial antibacterial effects against all species. In addition, the mycelial growth of <i>C. famata</i> was reduced by the treatment of both extracts. <i>C. krusei</i> mycelial growth was decreased by the aqueous extract, while the growth of <i>C. parapsilosis</i> was affected by the ethanolic extract and AgNPs (<i>p</i> < 0.001). None of the treatments affected the growth of <i>C. albicans</i> or <i>C. rhodotorula</i>. TEM analysis showed cellular ultrastructure changes in the treated <i>S. aureus</i> and <i>C. famata</i> compared to the control. Conclusion: The biosynthesized AgNPs and extracts of <i>A. sieberi</i> have a potential antimicrobial characteristic against pathogenic bacterial and fungal strains and nullified resistance behavior.
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