## 5 Summary and Conclusions  
Previous studies have extensively explored the antagonistic effects of _Bacillus_ species and their VOCs against _Fusarium oxysporum_, including GC-MS metabolite profiling and various biocontrol assays. However, to the best of our knowledge, no study has focusedspecifically on the VOCs produced during the active interaction between _Bacillus_ spp. and _F. oxysporum_. Our research addressed this gap by comparing volatile emissions from monocultures of _B. amyloliquefaciens_ and _F. oxysporum_ to those released during their co-cultivation. We discovered that the VOC profile from the interaction was not a mere sum of the individual microbial emissions. Instead, it included novel compounds absent in either monoculture, such as \(\alpha\)-cubebene, linalool, undecan-2-ol, decan-2-one and 2,6-dichloroanisole. Additionally, several VOCs were enriched during co-cultivation, indicating the activation of specific metabolic pathways through microbial crosstalk. This finding offers a new perspective: Bacillus bacteria may dynamically alter their VOC emission profile in response to fungal presence, potentially as a targeted defence strategy. Without this approach, such interaction-induced compounds would remain unidentified and untested for antifungal activity. To build on these results, we are planning in vitro bioassays using standards of key VOCs identified in the interaction, in planta infection studies to validate biocontrol efficacy and transcriptomic analyses to investigate fungal responses to selected VOCs at the molecular level. The following summarises the main results:  
* Volatile organic compounds produced by three tested species of _Bacillus_ bacteria (_B. amyloliquefaciens_, _B. subtilis_ and _B. thuringiensis_) inhibited the growth of the fungus _F. oxysporum_. * The highest inhibition of _F. oxysporum_'s growth (70%) was observed with _B. amyloliquefaciens_ after 24 h of the antagonism test, while the inhibition decreased to 45% after 96 h. With the growth of _F. oxysporum_, the quantity of VOCs produced by _B. amyloliquefaciens_ decreased in relation to the surface area of the fungus. This could be the reason for the decrease in inhibition over time. * The quantitative and qualitative compositions of the VOCs emitted by the studied _Bacillus_ bacteria were different. _B. amyloliquefaciens_ was characterised by a higher content of carbonyl compounds (46.83 \(\pm\) 8.41%) than _B. subtilis_ (41.50 \(\pm\) 6.45%) and _B. thuringiensis_ (34.62 \(\pm\) 4.77%). 3-Hydroxybutan-2-one, undecan-2-one, dodecan-5-one and tetradecan-5-one were only identified in the volatile emissions of _B. amyloliquefaciens_. * The main components of the volatile emission of _F. oxysporum_ were chlorinated benzaldehyde derivatives (e.g., 3,5-dichloro-4-methoxybenzaldehyde) and chlorinated benzene derivatives (e.g., 1,4-dichloro-2,5-dimethoxybenzene) as well as carbonyl compounds (e.g., benzaldehyde) and alcohols (e.g., benzyl alcohol). * During the interaction of _B. amyloliquefaciens_ and _F. oxysporum_, new compounds (e.g., \(\alpha\)-cubebene, linalool, undecan-2-ol, decan-2-one and 2,6-dichloroanisole) were found that were not present in the volatile emissions of the separated bacteria and fungal strains. On the other hand, some compounds found in the emissions of _B. amyloliquefaciens_ (including limonene, nonan-2-ol, phenethyl alcohol, heptan-2-one and tridecan-2-one) or _F. oxysporum_ (including propan-1-ol, 2-methylbutan-1-ol, isopentanal, 3-chloro-4-(methyloxy)benzaldehyde and 3,5-dichloro-4-methoxybenzaldehyde) were not found in the interaction of the microorganisms. The explanation for this could be that the two microorganisms interact via volatile compounds, thereby stimulating some metabolic pathways and inhibiting others. * Standards of chemical compounds identified in the volatile emission of _B. amyloliquefaciens_ or the interaction of the microorganisms can be used for further testing of the inhibition of _F. oxysporum_.