{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,8,13]],"date-time":"2026-08-13T10:31:10Z","timestamp":1786617070704,"version":"build-2736575974"},"reference-count":29,"publisher":"MDPI AG","issue":"8","license":[{"start":{"date-parts":[[2026,8,13]],"date-time":"2026-08-13T00:00:00Z","timestamp":1786579200000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Antibiotics"],"abstract":"<jats:p>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 \u00d7 smithiana, Mentha \u00d7 piperita, and Lavandula angustifolia had been previously characterized by gas chromatography\u2013mass 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 \u2265 25 mm in all 21 isolates and vapor\u2013phase zones of 25\u201329 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.<\/jats:p>","DOI":"10.3390\/antibiotics15080782","type":"journal-article","created":{"date-parts":[[2026,8,13]],"date-time":"2026-08-13T09:31:09Z","timestamp":1786613469000},"page":"782","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":0,"title":["Six Essential Oils as Potential Antibacterial Agents Against Extensively Drug-Resistant (XDR) Gram-Negative Clinical Isolates"],"prefix":"10.3390","volume":"15","author":[{"given":"Bianca","family":"B\u01cedescu-Chircea","sequence":"first","affiliation":[{"name":"Doctoral School, \u201cVictor Babe\u0219\u201d University of Medicine and Pharmacy Timi\u0219oara, Eftimie Murgu Square No. 2, 300041 Timi\u0219oara, Romania"},{"name":"Department of Microbiology, Faculty of Medicine, \u201cVictor Babe\u0219\u201d University of Medicine and Pharmacy Timi\u0219oara, Eftimie Murgu Square No. 2, 300041 Timi\u0219oara, Romania"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-4866-934X","authenticated-orcid":false,"given":"Sergio","family":"Liga","sequence":"additional","affiliation":[{"name":"Faculty of Chemical Engineering, Biotechnologies and Environmental Protection, Politehnica University Timi\u0219oara, Vasile P\u00e2rvan No. 6, 300223 Timi\u0219oara, Romania"},{"name":"University Clinic of Toxicology, Drug Industry, Management and Legislation, Faculty of Pharmacy, \u201cVictor Babe\u0219\u201d University of Medicine and Pharmacy Timi\u0219oara, Eftimie Murgu Square No. 2, 300041 Timi\u0219oara, Romania"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-9245-5883","authenticated-orcid":false,"given":"Monica","family":"Licker","sequence":"additional","affiliation":[{"name":"Department of Microbiology, Faculty of Medicine, \u201cVictor Babe\u0219\u201d University of Medicine and Pharmacy Timi\u0219oara, Eftimie Murgu Square No. 2, 300041 Timi\u0219oara, Romania"},{"name":"Multidisciplinary Research Center on Antimicrobial Resistance, \u201cVictor Babe\u0219\u201d University of Medicine and Pharmacy Timi\u0219oara, Eftimie Murgu Square No. 2, 300041 Timi\u0219oara, Romania"},{"name":"Microbiology Laboratory, \u201cPius Br\u00eenzeu\u201d Emergency County Clinical Hospital, 300723 Timisoara, Romania"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Dorina","family":"Dug\u0103e\u015fescu","sequence":"additional","affiliation":[{"name":"Department of Microbiology, Faculty of Medicine, \u201cVictor Babe\u0219\u201d University of Medicine and Pharmacy Timi\u0219oara, Eftimie Murgu Square No. 2, 300041 Timi\u0219oara, Romania"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0009-0008-7487-4639","authenticated-orcid":false,"given":"Mihaela Diana","family":"Popa","sequence":"additional","affiliation":[{"name":"Department of Microbiology, Faculty of Medicine, \u201cVictor Babe\u0219\u201d University of Medicine and Pharmacy Timi\u0219oara, Eftimie Murgu Square No. 2, 300041 Timi\u0219oara, Romania"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0009-0006-9681-8289","authenticated-orcid":false,"given":"Ciprian Nicolae","family":"Pilu\u0163","sequence":"additional","affiliation":[{"name":"Department of Microbiology, Faculty of Medicine, \u201cVictor Babe\u0219\u201d University of Medicine and Pharmacy Timi\u0219oara, Eftimie Murgu Square No. 2, 300041 Timi\u0219oara, Romania"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Silvana","family":"Vulpie","sequence":"additional","affiliation":[{"name":"Department of Microbiology, Faculty of Medicine, \u201cVictor Babe\u0219\u201d University of Medicine and Pharmacy Timi\u0219oara, Eftimie Murgu Square No. 2, 300041 Timi\u0219oara, Romania"},{"name":"Multidisciplinary Research Center on Antimicrobial Resistance, \u201cVictor Babe\u0219\u201d University of Medicine and Pharmacy Timi\u0219oara, Eftimie Murgu Square No. 2, 300041 Timi\u0219oara, Romania"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-3194-2012","authenticated-orcid":false,"given":"Valentina Oana","family":"Buda","sequence":"additional","affiliation":[{"name":"Faculty of Pharmacy, \u201cVictor Babe\u0219\u201d University of Medicine and Pharmacy Timi\u0219oara, Eftimie Murgu Square No. 2, 300041 Timi\u0219oara, Romania"},{"name":"Institute of Cardiovascular Diseases Timi\u015foara, 300310 Timi\u015foara, Romania"},{"name":"Research Center for Pharmaco-Toxicological Evaluation, \u201cVictor Babe\u0219\u201d University of Medicine and Pharmacy Timi\u0219oara, Eftimie Murgu Square No. 2, 300041 Timi\u0219oara, Romania"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-9100-4530","authenticated-orcid":false,"given":"Delia","family":"Muntean","sequence":"additional","affiliation":[{"name":"Department of Microbiology, Faculty of Medicine, \u201cVictor Babe\u0219\u201d University of Medicine and Pharmacy Timi\u0219oara, Eftimie Murgu Square No. 2, 300041 Timi\u0219oara, Romania"},{"name":"Multidisciplinary Research Center on Antimicrobial Resistance, \u201cVictor Babe\u0219\u201d University of Medicine and Pharmacy Timi\u0219oara, Eftimie Murgu Square No. 2, 300041 Timi\u0219oara, Romania"},{"name":"Microbiology Laboratory, \u201cPius Br\u00eenzeu\u201d Emergency County Clinical Hospital, 300723 Timisoara, Romania"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"1968","published-online":{"date-parts":[[2026,8,13]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"1413","DOI":"10.1378\/chest.14-2171","article-title":"Antimicrobial resistance in hospital-acquired Gram-negative bacterial infections","volume":"147","author":"Mehrad","year":"2015","journal-title":"Chest"},{"key":"ref_2","doi-asserted-by":"crossref","first-page":"257","DOI":"10.1016\/S0140-6736(24)02081-6","article-title":"Multidrug-resistant Gram-negative bacterial infections","volume":"405","author":"Macesic","year":"2025","journal-title":"Lancet"},{"key":"ref_3","first-page":"271","article-title":"Pandrug-resistant Gram-negative bacteria: A systematic review of current epidemiology, prognosis and treatment options","volume":"75","author":"Karakonstantis","year":"2020","journal-title":"J. Antimicrob. Chemother."},{"key":"ref_4","doi-asserted-by":"crossref","unstructured":"World Health Organization (2026, July 22). WHO Publishes List of Bacteria for Which New Antibiotics Are Urgently Needed. Available online: https:\/\/www.who.int\/news\/item\/27-02-2017-who-publishes-list-of-bacteria-for-which-new-antibiotics-are-urgently-needed.","DOI":"10.17712\/1658-3183.2290"},{"key":"ref_5","doi-asserted-by":"crossref","first-page":"e99","DOI":"10.1016\/S1473-3099(17)30485-1","article-title":"EPI-Net COMBACTE-MAGNET Group. Surveillance for control of antimicrobial resistance","volume":"18","author":"Tacconelli","year":"2018","journal-title":"Lancet Infect. Dis."},{"key":"ref_6","doi-asserted-by":"crossref","first-page":"1033","DOI":"10.1016\/S1473-3099(25)00118-5","article-title":"WHO Bacterial Priority Pathogens List Advisory Group. The WHO Bacterial Priority Pathogens List 2024: A prioritisation study to guide research, development, and public health strategies against antimicrobial resistance","volume":"25","author":"Sati","year":"2025","journal-title":"Lancet Infect. Dis."},{"key":"ref_7","doi-asserted-by":"crossref","first-page":"268","DOI":"10.1111\/j.1469-0691.2011.03570.x","article-title":"Multidrug-resistant, extensively drug-resistant and pandrug-resistant bacteria: An international expert proposal for interim standard definitions for acquired resistance","volume":"18","author":"Magiorakos","year":"2012","journal-title":"Clin. Microbiol. Infect."},{"key":"ref_8","doi-asserted-by":"crossref","first-page":"1333","DOI":"10.1086\/429323","article-title":"Colistin: The revival of polymyxins for the management of multidrug-resistant Gram-negative bacterial infections","volume":"40","author":"Falagas","year":"2005","journal-title":"Clin. Infect. Dis."},{"key":"ref_9","doi-asserted-by":"crossref","unstructured":"Oliva, A., Costantini, S., De Angelis, M., Garzoli, S., Bo\u017eovi\u0107, M., Mascellino, M.T., Vullo, V., and Ragno, R. (2018). High potency of Melaleuca alternifolia essential oil against multi-drug resistant Gram-negative bacteria and methicillin-resistant Staphylococcus aureus. Molecules, 23.","DOI":"10.3390\/molecules23102584"},{"key":"ref_10","doi-asserted-by":"crossref","unstructured":"Geng, S., Tang, Q., and Shi, N. (2025). Antibiotic-sparing strategies for multidrug-resistant organism (MDRO) infections. Front. Pharmacol., 16.","DOI":"10.3389\/fphar.2025.1653424"},{"key":"ref_11","doi-asserted-by":"crossref","first-page":"3012462","DOI":"10.1155\/2016\/3012462","article-title":"Antimicrobial properties of plant essential oils against human pathogens and their mode of action: An updated review","volume":"2016","author":"Swamy","year":"2016","journal-title":"Evid.-Based Complement. Altern. Med."},{"key":"ref_12","doi-asserted-by":"crossref","first-page":"115189","DOI":"10.1016\/j.biopha.2023.115189","article-title":"Comparative transcriptomics analysis of multidrug-resistant Acinetobacter baumannii in response to treatment with the terpenic compounds thymol and carvacrol","volume":"165","author":"Cirino","year":"2023","journal-title":"Biomed. Pharmacother."},{"key":"ref_13","doi-asserted-by":"crossref","first-page":"43927","DOI":"10.1021\/acsomega.4c07565","article-title":"Bactericidal and synergistic effects of Lippia origanoides essential oil and its main constituents against multidrug-resistant strains of Acinetobacter baumannii","volume":"9","author":"Moura","year":"2024","journal-title":"ACS Omega"},{"key":"ref_14","doi-asserted-by":"crossref","unstructured":"Mihaylova, S., Tsvetkova, A., Stamova, S., Ermenlieva, N., Tsankova, G., Georgieva, E., Peycheva, K., Panayotova, V., and Voynikov, Y. (2025). Antibacterial effects of Bulgarian oregano and thyme essential oils alone and in combination with antibiotics against Klebsiella pneumoniae and Pseudomonas aeruginosa. Microorganisms, 13.","DOI":"10.3390\/microorganisms13040843"},{"key":"ref_15","doi-asserted-by":"crossref","first-page":"8265","DOI":"10.1038\/s41598-026-38791-2","article-title":"Thyme and cinnamon essential oils inhibit multidrug resistant Escherichia coli and Klebsiella pneumoniae and alter virulence transcripts","volume":"16","author":"Magdy","year":"2026","journal-title":"Sci. Rep."},{"key":"ref_16","doi-asserted-by":"crossref","first-page":"6939","DOI":"10.1021\/jf050709v","article-title":"Solid- and vapor-phase antimicrobial activities of six essential oils: Susceptibility of selected foodborne bacterial and fungal strains","volume":"53","author":"Batlle","year":"2005","journal-title":"J. Agric. Food Chem."},{"key":"ref_17","first-page":"772","article-title":"Chemical composition and antimicrobial activity of essential oils of lavender (Lavandula angustifolia) and lavandin (Lavandula \u00d7 intermedia) grown in Western Romania","volume":"15","author":"Jianu","year":"2013","journal-title":"Int. J. Agric. Biol."},{"key":"ref_18","first-page":"2157","article-title":"Composition and antioxidant capacity of essential oils obtained from Thymus vulgaris, Thymus pannonicus and Satureja montana grown in Western Romania","volume":"66","author":"Jianu","year":"2015","journal-title":"Rev. Chim."},{"key":"ref_19","doi-asserted-by":"crossref","unstructured":"Jianu, C., Stoin, D., Cocan, I., David, I., Pop, G., Lukinich-Gruia, A.T., Mioc, M., Mioc, A., \u0218oica, C., and Muntean, D. (2021). In silico and in vitro evaluation of the antimicrobial and antioxidant potential of Mentha \u00d7 smithiana R. GRAHAM essential oil from Western Romania. Foods, 10.","DOI":"10.3390\/foods10040815"},{"key":"ref_20","doi-asserted-by":"crossref","first-page":"2905","DOI":"10.2147\/IDR.S218141","article-title":"Evaluation of essential oil obtained from Mentha \u00d7 piperita L. against multidrug-resistant strains","volume":"12","author":"Muntean","year":"2019","journal-title":"Infect. Drug Resist."},{"key":"ref_21","doi-asserted-by":"crossref","first-page":"30","DOI":"10.1016\/j.bse.2018.04.016","article-title":"Variation of trichome morphology and essential oil composition of seven Mentha species","volume":"79","author":"Yu","year":"2018","journal-title":"Biochem. Syst. Ecol."},{"key":"ref_22","doi-asserted-by":"crossref","first-page":"222","DOI":"10.1016\/j.indcrop.2011.09.010","article-title":"Lavender and hyssop productivity, oil content, and bioactivity as a function of harvest time and drying","volume":"36","author":"Zheljazkov","year":"2012","journal-title":"Ind. Crops Prod."},{"key":"ref_23","doi-asserted-by":"crossref","first-page":"453","DOI":"10.1046\/j.1365-2672.2001.01428.x","article-title":"A study of the minimum inhibitory concentration and mode of action of oregano essential oil, thymol and carvacrol","volume":"91","author":"Lambert","year":"2001","journal-title":"J. Appl. Microbiol."},{"key":"ref_24","doi-asserted-by":"crossref","first-page":"323","DOI":"10.1016\/j.micpath.2018.11.025","article-title":"Satureja montana L. essential oil and its antimicrobial activity alone or in combination with gentamicin","volume":"126","author":"Vitanza","year":"2019","journal-title":"Microb. Pathog."},{"key":"ref_25","doi-asserted-by":"crossref","unstructured":"Cagnoli, G., Bertelloni, F., and Ebani, V.V. (2024). In vitro antibacterial activity of essential oils from Origanum vulgare, Satureja montana, Thymus vulgaris, and their blend against necrotoxigenic (NTEC), enteropathogenic (EPEC), and Shiga-toxin-producing Escherichia coli (STEC) isolates. Pathogens, 13.","DOI":"10.3390\/pathogens13121077"},{"key":"ref_26","doi-asserted-by":"crossref","unstructured":"Speranza, B., Bevilacqua, A., Campaniello, D., Altieri, C., Corbo, M.R., and Sinigaglia, M. (2023). Minimal inhibitory concentrations of thymol and carvacrol: Toward a unified statistical approach to find common trends. Microorganisms, 11.","DOI":"10.3390\/microorganisms11071774"},{"key":"ref_27","unstructured":"Clinical and Laboratory Standards Institute (2022). Performance Standards for Antimicrobial Susceptibility Testing, Clinical and Laboratory Standards Institute. [32nd ed.]. CLSI Supplement M100."},{"key":"ref_28","unstructured":"EUCAST\u2014The European Committee on Antimicrobial Susceptibility (2024). Breakpoint Tables for Interpretation of MICs and Zone Diameters, Version 14.0, EUCAST. Available online: https:\/\/www.eucast.org\/bacteria\/document-archive\/."},{"key":"ref_29","unstructured":"Clinical and Laboratory Standards Institute (2018). Methods for Dilution Antimicrobial Susceptibility Tests for Bacteria That Grow Aerobically, Clinical and Laboratory Standards Institute. [11th ed.]. M07Ed11."}],"container-title":["Antibiotics"],"original-title":[],"language":"en","link":[{"URL":"https:\/\/www.mdpi.com\/2079-6382\/15\/8\/782\/pdf","content-type":"unspecified","content-version":"vor","intended-application":"similarity-checking"}],"deposited":{"date-parts":[[2026,8,13]],"date-time":"2026-08-13T09:49:18Z","timestamp":1786614558000},"score":1,"resource":{"primary":{"URL":"https:\/\/www.mdpi.com\/2079-6382\/15\/8\/782"}},"subtitle":[],"short-title":[],"issued":{"date-parts":[[2026,8,13]]},"references-count":29,"journal-issue":{"issue":"8","published-online":{"date-parts":[[2026,8]]}},"alternative-id":["antibiotics15080782"],"URL":"https:\/\/doi.org\/10.3390\/antibiotics15080782","relation":{},"ISSN":["2079-6382"],"issn-type":[{"value":"2079-6382","type":"electronic"}],"subject":[],"published":{"date-parts":[[2026,8,13]]}}}