Disk-Diffusion of Citrus aurantium dulcis, Rosmarinus officinalis, and Melaleuca alternifolia Essential Oils Against Bacteria and Yeasts ^†

Introduction: Essential oils (EOs) are volatile compounds widely recognized for their antimicrobial properties. Although sweet orange (Citrus aurantium dulcis), rosemary (Rosmarinus officinalis), and tea-tree (Melaleuca alternifolia) EO are common in aromatherapy, comparative studies on their antimicrobial potential are scarce. This study aimed to evaluate and compare the antimicrobial activity of these three essential oils. Methodology: Commercial samples of sweet orange EO (SOEO), rosemary EO (REO), and tea-tree EO (TEO) from Southeast Brazil were tested using the disk-diffusion method. The panel included Gram-positive bacteria (Staphylococcus aureus, Bacillus cereus, Enterococcus faecalis), Gram-negative bacteria (Escherichia coli, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa), and yeasts (Candida albicans, Candida krusei). Results: All EO showed relevant antifungal activity, with inhibition zones ranging from 13.81 ± 0.51 to 36.29 ± 3.50 mm for C. albicans and 8.77 ± 0.43 to 43.67 ± 0.44 mm for C. krusei. For the Gram-positive bacteria, inhibition zones varied from 9.24 ± 0.21 to 22.42 ± 1.85 mm. Among the Gram-negative bacteria, SOEO was only active against E. coli (3.33 ± 4.44 mm), whereas REO and TEO exhibited broader activity (8.88 ± 0.37 to 36.87 ± 0.78 mm). Conclusion: REO and TEO demonstrated greater antibacterial inhibition zones than SOEO, although all three were effective against yeasts. These findings suggest that REO and TEO are promising natural alternatives for treating resistant microorganisms. Further investigation into the synergistic effects could be beneficial, particularly to enhance the antibacterial activity of SOEO.