## 2 Materials and Methods  
### Bacillus spp. and F. oxysporum Strains  
Strains of three species of _Bacillus_ bacteria (_B. amyloliquefaciens_ (ex Fukumoto) [PERSON], _B. subtilis_ ([PERSON]) [PERSON] and _B. thuringiensis_ [PERSON]) and the fungus _F. oxysporum_ came from the collection of the Forest Research Institute in Sekocin Stary (Poland). Casein-Soy Broth TSB (Tryptic Soy Broth) (Cat. No. P-0120, BTL, Lodz, Poland) was used for the culture of _Bacillus_. Potato Dextrose Agar (PDA) (Cat. No. P-0134, BTL, Lodz, Poland) was used for _F. oxysporum_. The media were prepared according to the manufacturer's instructions and then sterilised in an autoclave at 121 \({}^{\circ}\)C for 20 min. The _Bacillus_ bacteria were stored in liquid culture with TSB medium at 4 \({}^{\circ}\)C until analysed. The _F. oxysporum_ fungus strain was stored in Petri dishes with PDA medium at 4 \({}^{\circ}\)C until analysis. Test for Inhibition of F. oxysporum Growth by Volatile Organic Compounds Produced by Bacillus Bacteria  
The inhibitory effects of volatile organic compounds (VOCs) produced by _Bacillus_ strains in the antagonism assay were measured using the sealed-plate method described by [PERSON] et al. [25], adjusting the composition of the culture medium and the incubation temperature. A 200 \(\mu\)Lcell suspension (\(1\times 10^{8}\) cfu/mL) of bacteria was spread on a TSB solid medium in a Petri dish (Figure 1) and incubated at 28 \({}^{\circ}\)C for 24 h. A 6 mm diameter mycelial disc of _F. oxysporum_ was cut from an actively growing culture using a cork board and placed in the centre of a second Petri dish containing PDA medium (Figure 1). The lids were removed from both plates and the plate inoculated with _Bacillus_ was inverted and placed on a plate with _Fusarium_ mycelium (Figure 1). The two plates were connected with parafilm to create a sealed two-plate chamber. A two-plate chamber without bacterial strains was used as a control. The average distance between the surface of the TSB agar and the PDA was 2 cm. The two-plate chamber was incubated at 25 \({}^{\circ}\)C for 96 h and the mycelial diameter was measured every 24 h. The degree of inhibition of mycelial growth (I%), expressed as a percentage, was then determined according to the formula [6]:  
\[I\%=(A-B)/A\times 100,\]  
with I%--inhibition of mycelium growth (%), A--mycelium diameter in the control sample (mm) and B--mycelium diameter in the test sample (mm). Figure 1: Scheme of the double-plate antagonism test. Reprinted with permission from Ref. [26]. A—Petri dishes with _Bacillus_ bacteria, B—Petri dishes with _F. oxysporum_, C—double-plate chamber with _Bacillus_ bacteria on the top and _F. oxysporum_ on the bottom plate. The experiment was repeated three times for each variant; therefore, the mycelium diameters A and B are the average values of the three measurements. Analysis of Volatile Organic Compounds Produced by Bacillus Bacteria and F. oxysporum Using Headspace Solid-Phase Microextraction and Gas Chromatography with Mass Spectrometry (HS-SPME/GC-MS)  
Petri dishes with _Bacillus_ bacteria and _F. oxysporum_ fungi were closed and sealed with parafilm. In one side wall of each Petri dish there was a previously prepared hole with a diameter of 2 mm, which was also sealed with the parafilm. The volatile organic compounds emitted by _Bacillus_ bacteria and _F. oxysporum_ fungus were analysed 96 h after sealing the Petri dishes using headspace solid-phase microextraction and gas chromatography with mass spectrometry (HS-SPME/GC-MS). A needle of the SPME equipment was inserted into the Petri dish containing the microorganism (_Bacillus_ bacterium or _F. oxysporum_, or bacterium and fungus in interaction) and the adsorption fibre with divinylbenzene/carboxyles/polydimethylsiloxane (DVB/CAR/PDMS) stationary phase was introduced. The sorption of volatiles was performed in the headspace for 30 min at 25 \({}^{\circ}\)C in a laboratory thermostat (Figure 2). Analyses were performed for 3 types of _Bacillus_ bacteria, _F.