Six Essential Oils as Potential Antibacterial Agents Against Extensively Drug-Resistant (XDR) Gram-Negative Clinical Isolates

Abstract

Abstract

Background/Objectives: Extensively drug-resistant Gram-negative pathogens severely limit antimicrobial treatment options. This study evaluated the in vitro antibacterial activity of six essential oils obtained from Romanian medicinal plants against extensively drug-resistant clinical isolates and interpreted the antibacterial findings in the context of their previously reported chemical composition. Methods: Forty-one non-duplicate clinical isolates were included: (i) Acinetobacter baumannii (n = 21); (ii) Klebsiella pneumoniae (n = 14); (iii) Pseudomonas aeruginosa (n = 4); and (iv) Providencia stuartii (n = 2). The chemical composition of the six essential oils from Thymus vulgaris, Thymus pannonicus, Satureja montana, Mentha × smithiana, Mentha × piperita, and Lavandula angustifolia had been previously characterized by gas chromatography–mass spectrometry (GC-MS), and the corresponding compositional data were used in the present study. Antibacterial activity was assessed using disk-diffusion, disk-volatilization, and broth-dilution assays to determine minimum inhibitory and minimum bactericidal concentrations. Results: Satureja montana contained 60.9% carvacrol, T. vulgaris 44.6% carvacrol, and T. pannonicus 34.9% thymol. S. montana showed the strongest overall antibacterial activity, followed by T. vulgaris and T. pannonicus. A. baumannii isolates showed the greatest overall inhibitory response; S. montana produced direct-contact inhibition zones ≥ 25 mm in all 21 isolates and vapor–phase zones of 25–29 mm in 17 of 21 isolates. K. pneumoniae showed oil-dependent inhibition patterns, while the findings for the two P. stuartii isolates were interpreted descriptively because of the limited subgroup size. The four P. aeruginosa isolates showed the lowest overall inhibitory response, particularly in the vapor phase assay. MIC and MBC findings broadly supported the diffusion-based results, whereas L. angustifolia showed the weakest overall activity. Conclusions: Essential oils rich in phenolic monoterpenes, particularly those containing carvacrol or thymol, showed promising in vitro activity against extensively drug-resistant Gram-negative clinical isolates. These findings do not establish clinical efficacy and require confirmation in larger isolate collections using standardized assays, including cytotoxicity, biofilm, formulation, and synergy studies.