## 3 Discussion  
In this work, we have determined the volatile organic compound (VOC) profile emitted by eight Gram-positive endophytic strains isolated from the medicinal plant _Origanum vulgare_ ssp. _vulgare_, in an attempt to correlate them with the antibacterial activity exhibited by each endophyte against ten representatives of the _Burkholderia cepacia_ complex (Bcc), which includes important human opportunistic pathogens able to infect immunocompromised patients, such as those affected by cystic fibrosis. In addition to this, another aim of this work was to compare the VOCs profile of each endophyte with the essential oil (EO) extracted from the same _O. vulgare_ plants from which the endophytes were isolated (for a detailed description of the isolation methodology, see [PERSON] et al. [27]) in order to check whether one (or more) compound/s was/were shared between the EO and the VOCs emitted by the endophytic bacteria. The whole body of data obtained revealed that, even though isolated from the same plant, and in some cases from the same compartment of the same plant, the eight endophytes exhibited different anti-Bcc activities, with one of them (i.e., _Arthrobacter_ sp. OVS8, isolated from the stem of _O. vulgare_) showing the highest and very strong bactericidal activity against most of the Bcc strains, and a bacteriostatic or a reducing growth ability on the other ones. On the other hand, _Paenibacillus_ strain OVF10 did not exert any antagonistic activity towards the Bcc bacteria. The two strains had very different VOC profiles, suggesting that their composition and/or the relative concentration of their compounds might represent key players for the anti-Bcc activity. The other six endophytic strains, all belonging to the genus _Bacillus_, did not exert the same anti-Bcc activity, even though all of them showed strong bactericidal activity towards the _B. cenocpacia_ clinical isolate LMG21462, representing the most sensitive target strain to the tester VOCs. In spite of this common anti-LMG21462 bactericidal activity, the six _Bacillus_ strains differentially inhibited the growth of the other target strains. Hence, the whole body of data obtained in this work revealed that all the five scenarios that were predicted, i.e., (i) no activity vs Bcc strains, (ii) reducing growth, (iii) bacteriostatic activity, (iv) bactericidal activity, and (v) promoting growth, actually occurred. In order to check whether the different anti-Bcc activities might be related to a different VOC profile and/or to different concentrations of the compounds, the VOCs produced by the eight endophytes were determined. Based on the relative peak area, dimethyl disulfide was the most abundant volatile compound for all the endophytic strains, followed by 2-methyl-2-butene. Among sulfides, dimethyl trisulfide was also detected, which is a metabolite common to all strains. It was demonstrated that dimethyl disulfide and dimethyl trisulfide cause significant growth inhibition on different pathogens such as _Rhizoctonia solani_ and _Pythium ultimum_ and many Gram-negative and Gram-positive bacteria [29; 30]. Therefore, the highest percentage of dimethyl disulfide (84.55%) detected in _Arthrobacter_ sp. OVS8 is consistent with the observation that this endophytic strain is the most active one. This finding is also in agreement with previous data on VOCs synthesized by Antarctic bacteria exhibiting strong antibacterial activity against Bcc strains [14; 15]. Indeed, some of the compounds belonging to the VOC profile of the endophytes tested in this work were also synthesized by Antarctic bacteria (i.e., styrene, dimethyl trisulfide, and dimethyl disulfide, as well as oxime-methoxy-phenyl). Moreover, this is also in agreement with the behaviour of _Paenibacillus_ sp. OVF10, which showed the weakest antibacterial potential,producing the lowest concentration of dimethyl disulfide (0.28%) and dimethyl trisulfide (0.03%) of all strains. Oxime-, methoxy-phenyl- was present in all strains' VOC profiles, in a concentration ranging from 0.04 to 8.29%.