Comparative study of volatile oil constituents, anti-microbial properties, and antibiofilm activities in <i>Eucalyptus camaldulensis</i> and <i>Eucalyptus globulus</i>: insights from central Saudi Arabia

<i>Eucalyptus</i> is a fragrant plant with a highly volatile oil content grown worldwide. This study investigated the phytochemical and biological features of two <i>Eucalyptus</i> species, <i>E. camaldulensis</i> and <i>E. globulus</i>, cultivated in Central Saudi Arabia. Phytochemical analysis revealed 17 compounds in <i>E. camaldulensis</i> and 13 in <i>E. globulus</i>, with α-pinene and 1,8-cineole being both plants’ most abundant volatile components. The antimicrobial activity revealed that <i>Salmonella typhimurium</i> ATCC 13311 and <i>Candida albicans</i> ATCC 10231 are the most susceptible to <i>E. camaldulensis</i>, with the zone of inhibition diameters (ZIDs) of 17.0 ± 0.2 mm and 22.0 ± 0.3 mm, respectively. In contrast, for <i>E. globulus</i>, methicillin-resistant <i>Staphylococcus aureus</i> and <i>Candida albicans</i> ATCC 10231 were the most susceptible, with ZIDs of 13.0 ± 0.2 mm and 20.0 ± 0.2 mm, respectively. <i>E. camaldulensis</i> had minimum inhibitory concentration (MIC) range from 1.56 to 12.5 μL/mL, minimum biocidal concentration (MBC) range from 3.125 to 25.0 μL/mL, minimum biofilm inhibitory concentration (MBIC) range from 3.125 to 25.0 μL/mL, and minimum biofilm eradication concentration (MBEC) from 6.25 to 50.0 μL/mL, while <i>E. globulus</i> had MIC range from 3.125 to 12.5 μL/mL, MBC range from 6.25 to 25.0 μL/mL, MBIC range from 6.25 to 25.0 μL/mL, and MBEC range from 12.5 to 50.0 μL/mL, for the tested organisms. <i>E. camaldulensis</i> demonstrated better antimicrobial activity than <i>E. globulus</i> against the tested organisms, making it a promising antimicrobial drug candidate. Further research is required to determine its effectiveness in treating different human infections.