## 3 Discussion
This compound was previously identified in a methanolic extract of _Pseudomonas aeruginosa_ as a chemical with anti-bacterial activity [31]. All strains displayed a similar amount of monoterpenes emitted, in a range of concentration between 0.03 to 1.89%, with the exception of _Paenibacillus_ sp. OVF10, for which monoterpenes were found in trace amounts (0.01%). The main components of the monoterpene fraction identified were \(\alpha\)-pinene, 3-carene, p-cymene, and \(\gamma\)-terpinene. Numerous _in vitro_ and _in vivo_ research studies were conducted to evaluate the potential therapeutic uses of EOs extracted from different oregano species, detecting terpenes as the main components. In particular, EOs extracted from _O. vulgare_ ssp. _vulgare_, which were found to be rich in \(\gamma\)-terpinene and p-cymene, exerted antioxidant, anti-inflammatory, antiproliferative, antitumor, and hypoglycemic activities [32]. Furthermore, 2-decenal was produced by all strains in a concentration ranging from 0.11 to 1.15%. This aldehyde is present in a wide variety of foods and it is considered to be a potent botanical nematicidal agent [33]. Among all the identified compounds, nine of them were pyrazines, which are known to have antifungal and nematicidal activities [34]. Only 2,5-dimethyl-pyrazine was produced by all strains, emitted in a range of concentrations between 0.13 and 15.30%. This substance was found to have significant antifungal activity against _Alternaria solani_ and _Botrytis cinerea_[35]. The other eight pyrazine derivatives were detected only in _Paenibacillus_ sp. OVF10, with the highest percentage for 2,3,4-trimethyl-5-propylpyrazine (83.92%). In agreement with the literature, _Paenibacillus_ is a ubiquitously occurring bacterial genus with antagonistic activity against phytopathogens, which produces pyrazines such as 2-methyl-5-(1-methylethyl)-pyrazine, 2-(2 methylpropyl)-3-(1-methylethyl)pyrazine, and 2,3,4-trimethyl-5-propylpyrazine, as previously reported [36]. Previous studies revealed that _Paenibacillus_-emitted VOCs induced systemic resistance in _Arabidopsis_ plants and enhanced plant growth at the same time [37]. Overall, alkyl-substituted pyrazines were detected as the main antimicrobial VOCs in the headspace of plant-associated _Paenibacillus_ polymyxa isolates. This plant-associated isolate exhibited a high biocontrol potential due to its pronounced antagonistic activity against devastating plant pathogens [38]. Lastly, it was seen that only _Paenibacillus_ sp. OVF10 produced 2-undecanone (0.13%). Previous SPME-GC/MS analysis conducted on _P. polymyxa_ confirmed the presence of this ketone, detected with other VOCs emitted in order to study his nematicidal and fumigant activity against _Mediologyne incognita_[39]. For further details on the biochemical mechanism of bacterial metabolites, refer to selected recent literature [40; 41; 42; 43]. In conclusion, it can be hypothesized that all the endophytes considered in the present work might be involved in host plant protection against phytopathogens. This makes sense from a plant-bacterium symbiotic relationship point of view. In this context, among all the strains, OVF10 may be the most active one, inducing plant resistance and, potentially, also plant growth (acting as a plant growth promoter, PGP, bacterium), thanks to its production of different pyrazines [40; 41]. Furthermore, the OVF10 ability to produce high levels of pyrazine could lead to other biotechnological applications considering the added value of such compounds for the food industry as flavour and fragrance [42]. Moreover, all the other seven endophytic strains (particularly the _Arthobacter_ sp. OVS8 strain) showed bactericidal and/or bacteriostatic activity against most of the Bcc strains and significantly higher production of dimethyl disulfide, dimethyl trisulfide, and monoterpenes compared to OVF10 [43]. Thus, it might be supposed that these seven endophytes could also have a role in the antibacterial properties of _O. vulgare_ ssp. _vulgare_.