Biocidal Activity of Selected Essential Oils Against Bacterial and Fungal Pathogens and Their Cytotoxicity Toward Human Cell Lines

The growing global burden of antimicrobial resistance (AMR) highlights the urgency of identifying new therapeutic options or adjuvant agents capable of limiting microbial growth. This study aimed to evaluate the potential applicability of selected essential oils (EOs) as alternative antimicrobial agents by assessing their biocidal activity against clinically relevant microorganisms and determining their cytotoxicity toward human cell lines. Serial dilutions of the studied EOs were prepared to determine the minimum bactericidal (MBC), minimum fungicidal (MFC), and minimum biofilm eradication concentrations (MBEC) against Staphylococcus aureus ATCC® 25923, Escherichia coli ATCC® 25922, Candida albicans ATCC® 90028, and Nakaseomyces glabratus ATCC® 15545. Cytotoxicity towards cell lines was evaluated using the MTT assay. The tested EOs displayed significant biocidal effects against studied microorganisms in both planktonic and biofilm forms. Cinnamon bark oil and cinnamaldehyde showed the highest biocidal activity against S. aureus ATCC® 25923, E. coli ATCC® 25922, C. albicans ATCC® 90028, and N. glabratus ATCC® 15545, except for S. aureus in biofilm form. For thymol, yeast strains exhibited eight-fold lower susceptibility compared to bacterial strains. Cytotoxicity of the tested EOs towards the studied cell lines was significant. EOs proved to be effective against microorganisms but at the same time showed substantial cytotoxicity, thus potentially limiting their application. Further research on formulation strategies, selective delivery, and dose optimization is required to enhance their safety profile and clinical feasibility.