Essential Oils and their Active Constituents Effective against Non-growing Mycobacterium intracellulare

Background Mycobacterium intracellulare ( M. intracellulare ) is a common, slow-growing opportunistic pathogen that can cause chronic pulmonary and extrapulmonary infections. Despite its rising incidence, standard treatments are often ineffective in eradicating M. intracellulare , leading to prolonged treatment and high recurrence rates, likely due to persistence of non-growing bacteria. Although essential oils are known for their antimicrobial properties, their effects on M. intracellulare , particularly in its non-growing phase, have not been well studied. Methods We screened 151 essential oils to assess their antimicrobial activity against stationary-phase non-growing M. intracellulare . Essential oils with significant activity were further evaluated at different concentrations by MIC and drug exposure tests. Results Thirty-four essential oils were found to have activity at 5000 µg/mL, with 18 showing effectiveness at 1250 µg/mL. Six essential oils, Ajwain, Oregano, Palmarosa, Thyme, Mountain Savory, and Litsea Cubeba had the highest activity, achieving 100% bacterial clearance after one day exposure. Carvacrol, the key active component of Ajwain, Oregano, Thyme, Mountain Savory, eradicated stationary-phase bacteria at 310 µg/mL concentration within one day, while citronellol, the active component of Palmarosa, at 630 µg/mL achieved complete clearance after three day exposure. Conclusions We have newly identified several essential oils, including Ajwain, Oregano, Thyme, Mountain Savory, Palmarosa, and Litsea Cubeba and their active components such as carvacrol and citronellol, to have promising activity against M. intracellulare, and these findings may have implications for developing improved treatments for M. intracellulare infections.