JJBS Volume 14, Number 1, March 2021 ISSN 1995-6673 Pages 41 - 49 Jordan Journal of Biological Sciences Antimicrobial Activities of Natural Volatiles Organic Compounds Extracted from Dittrichia viscosa (L.) by Hydrodistillation Nafila Zouaghi 1 , Nour El Houda Bensiradj 2 , Carlos Cavaleiro 3 , Boubekeur Nadjemi 1 and Ahmad Telfah 4,5,* 1 Laboratoire d’étude et de développement des techniques de traitement et d’épuration des eaux et de gestion environnementale. Ecole Normale Supérieure, Kouba, Algeria, 2 Laboratoire de Chimie Théorique Computationnelle et Photonique Faculté de chimie USTHB Algeirs, Algeria , 3 Centro de Estudos Farmacêuticos / Faculdade de Farmácia, Universidade de Coimbra, Portugal, 4 Leibniz-Institut für Analytische Wissenschaften - ISAS - e.V. 44139 Dortmund, Germany, 5 Hamdi Mango Center for Scientific Research (HMCSR), The University of Jordan, Amman 11942, Jordan Received Oct 18, 2019; Revised May 1, 2020; Accepted June 26, 2020 Abstract The chemical characterization and antimicrobial activities of Dittrichia viscosa (L.) essential oils extracted by hydrodistillation technique from leaves and stems are reported in this work. Dittrichia viscosa (L.) samples were collected from the Bainem forest in the northwest part of Algiers (the capital). Gas chromatography-mass spectrometry (GC-MS) analytical method was employed to identify the oil’s chemical composition. It was found that leaves are mainly composed of three major abundant composites, specifically, caryophyllene oxide (10.4%), fokienol (9.6%) and trans-nerolidol (7%). Moreover, the oil isolated from the stems was found to be chiefly composed of trans-totarol (18.1%), α-cedrol (16.7%), and ferruginol (16.6%). Additionally, antimicrobial activity tests were performed on the isolated essential oils using the zone of inhibition (agar disk-diffusion method) to determine the minimum inhibitory concentration (MIC) of four bacteria strains, mainly, Bacillus subtilis, Staphylococcus aureus, Pseudomonas aeruginosa, and Escherichia coli. The MIC values of leaves are found to range between 15 μg and 100 μg, whereas stems are found to exhibit MIC values ranging between 20 μg and 300 μg. Furthermore, antifungal susceptibility tests, which become important tools to dictate the treatment of fungal diseases, are conducted on two yeast strains: Saccharomyces cerevisiae and Candida albicans. The obtained antimicrobial results are correlated with the chemical composition findings of the essential oils from leaves and stem to determine the roles of the chemical composites on the antibacterial activity. Interestingly, the oil obtained from the leaves displayed a better inhibitory effect on (bacteria and yeast strains) in comparison with oil (stems). This difference in inhibitory effect can be attributed to the dominant existence of the oxygenated sesquiterpenes and trans-nerolidol compounds in leaves. Keywords: Dittrichia viscosa (L.); Volatile Organic Compounds; Chemical Reactivity; Antibacterial Activity. * Corresponding author e-mail: telfah.ahmad@isas.de. 1. Introduction Finding new antibiotics has gained extensive attention, taking into consideration that several existing drugs have become inefficient due to the timewise increase in antimicrobial resistance (AMR) (Prestinaci et al., 2015). Nowadays, natural products extracted from plants have become one of the major sources of new drugs. Plants can provide a wide range of complex and structurally diverse compounds. It is well known that plant products constitute a significant sector of the existing antimicrobial compounds (Berdy, 2005). Many researchers have geared their investigations toward taking advantage of plant and microbial extracts such as essential oils as potential candidates for antimicrobial agents (Runyoro et al., 2006; Nazzaro et al., 2013). The comparison of the results deduced from different articles on the in vitro antimicrobial activities of natural products is of prime importance. However, the comparison is extremely difficult due to the non- standardization of the methodology and the insufficiency of plant products database. Antibiotics are influential drugs used to fight diseases at large commercial scales. Any powerful antibiotic has a wide range of side effects, consequently, patients taking antibiotics, are subjected to side effects ranging from mild to severe depending on the type of antibiotic, the targeted microbes and the patient himself (Slama et al., 2005). Due to their rich and diverse chemical composition, plants could be very efficient against microbes and thus provide precious sources of natural antimicrobial agents. The elements isolated from plants are important alternatives to many synthetic antimicrobial drugs because of their weak or no side effects and their better bioavailability (Roosita et al., 2008). Nevertheless, ensuring minimum toxicity, specific concentrations of the natural products should be considered. Antiquity plants were used as antimicrobial due to their healing and antiseptic properties. Dittrichia viscosa (L.) (D. viscosa) is a perennial herbaceous plant described as a flowering plant with viscous and glandular leaves (Quezel and Santa, 1963). It is a native of the Mediterranean basin and is