## 2 Results
### Quantitative Analysis of VOCs Antibacterial Activity  
Endophyte antibacterial activity was quantified through the cross-streaking method in order to permit the determination of the number of viable cells of each target strain at the beginning (i.e., soon after their streaking on plates, t0) and at the end of the experiment (i.e., after 48 h incubation in the presence of the tester strain, t1). These experiments were performed according to the protocol detailed in Materials and Methods. In principle, five different scenarios can be depicted: (i) the presence of the VOCs produced by the tester strain do not interfere with the growth of the target strain (s); (ii) the tester strain might reduce the growth rate of the target strains; (iii) the tester strain synthesizes VOCs able to completely interfere with the growth of the target cells (i.e., they exhibit a bacteriostatic activity); (iv) the VOCs synthesized by the tester strain might kill the target cells (i.e., they possess a bactericidal ability); (v) some VOCs might have a bacterial growth promoting ability that cannot a priori be excluded. The whole body of data obtained is reported in Supplementary Table S1 and is schematically represented in Figures 2 and 3 and in Table 1. Overall, the quantitative analyses revealed the following:  
1. All the tester strains were able to inhibit the growth of at least one of the target strains. 2. The VOCs synthesized by the eight endophytes exhibited a different ability to interfere with the growth of target strains. 3. The most active endophytic strain was _Arthrobacter_ sp. OVS8, whose VOCs were able to strongly antagonize the growth of all the Bcc strains, showing a bactericidal activity against seven out of ten Bcc strains (see Figures 2 and 3), and a growth reducing activity on the remaining three targets. Overall, the mean number of viable cells of target strains was 0.17% in the presence of strain OVS8 with respect to the control plates (i.e., in the absence of the tester strain). 4. On the other site, _Paenibacillus_ sp. OVF10 exhibited the weakest antibacterial potential, being able to antagonize only the growth of LMG16656 and LMG21462 strains. These two targets, together with LMG24506, appeared to be the most sensitive to the VOCs produced by all the endophytes. It is also quite interesting that OVF10 is the only tester strain able to stimulate the growth of four targets (FCF23, LMG24506, LMG1222, and LMG17588). 5. The remaining six tester strains, all belonging to the _Bacillus_ genus, showed a different inhibitory effect on Bcc strains. All _Bacillus_ strains induced a bactericidal effect against at least one of the target strains, with OVS26 and OVF22 exerting the strongest antibacterial activity. \begin{table}
\begin{tabular}{c c} \hline \hline  & **Mean (\%)** \\ \hline _Arthrobacter_ sp. OVS8 & 0.17 \\ _Bacillus_ sp. OVS26 & 4.14 \\ _Bacillus_ sp. OVF22 & 5.92 \\ _Bacillus_ sp. OVI9 & 19.68 \\ _Bacillus_ sp. OVS6 & 23.83 \\ _Bacillus_ sp. OVS21 & 30.96 \\ _Bacillus_ sp. OVL24 & 82.18 \\ _Paenibacillus_ sp. OVF10 & 1492.42 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Antibacterial activity of endophytic tester strains against target bacteria belonging to the Bcc. Values represent the mean percentage of number of viable cells of target strains in the presence of tester strains with respect to the number of viable cells grown in the absence of tester bacteria. The lower the value, the higher the antibacterial activity of the VOCs emitted by the endophyte. * The clinical Bcc strains isolated from CF patients were more sensitive to the endophytic VOCs than the environmental ones. * The two _B. multivarons_ strains (one of clinical origin and the other one isolated from the environment) were less sensitive to the endophytic VOCs than the other eight Bcc strains belonging to the _B. cepacia_, _B. centocpacia_, and _B. ambifaria_ species.