## 4 Discussion
### Chemical Compounds Detected by GC-Ms  
Based on HS-SPME/GC-MS analyses, it was found that the volatile emissions of the three investigated _Bacillus_ species, i.e., _B. amyloliquefaciens_, _B. subtilis_ and _B. thuringiensis_, contained different groups of chemical compounds, including carbonyl compounds, alcohols and alkanes, as well as pyrazine derivatives, amides, sulphides and monoter. The presence of the above-mentioned groups of volatile compounds in the emissions of _Bacillus_ cultures is also evident from the results of studies published by [PERSON] et al. [40],[PERSON] et al. [25], [PERSON] et al. [43] and [PERSON] et al. [23]. Bacteria of the genus _Bacillus_ produce a range of different carbonyl compounds, among which ketones predominate and aldehydes are less abundant. [PERSON] et al. [40] identified heptan-2-one, nonan-2-one, tridecan-2-one and acetophenone in the VOC composition emitted by _B. amyloliquefaciens_. The presence of these ketones in _B. amyloliquefaciens_ was also confirmed by the results of our work (Table 1). [PERSON] et al. [25] also found the presence of 3-hydroxybutan-2-one (acetoin) and undecan-2-one in _B. amyloliquefaciens_, which was also confirmed by the results of our analyses (Table 1). In addition, [PERSON] et al. [43] and [PERSON] et al. [23] detected acetone and butan-2-one in _B. amyloliquefaciens_ and _B. subtilis_ cultures, which is also consistent with the chemical results we obtained (Table 1). [PERSON] et al. [23] found that benzaldehyde was present in the volatile emissions of _B. subtilis_ and not present in _B. amyloliquefaciens_. This correlation was also confirmed by our results (Table 1). [PERSON] et al. [43] also indicated the presence of benzaldehyde in _B. subtilis_. In contrast, [PERSON] et al. [47] identified heptano-2-one, 5-methylheptano-2-one and 6-methylheptano-2-one in _B. subtilis_, and the presence of the three mentioned ketones in _B. subtilis_ was also confirmed by the presented results (Table 1). In addition, [PERSON] et al. found [41] butan-2-one, 6-methylheptan-2-one, acetophenone and benzaldehyde in the chemical composition of the VOCs produced by _B. thuringiensis_, and ketones were also detected in _B. thuringiensis_ cultures in our study (Table 1). [PERSON] et al. [48] detected acetone in the composition produced by _Bacillus_ spp. and [PERSON] et al. [49] identified butan-2-one, 5-methylheptan-2-one, 6-methylheptan-2-one and nonan-2-one, while the results of our analyses confirmed the presence of these ketones in all analysed _Bacillus_ species, i.e., _B. amyloliquefaciens_, _B. subtilis_ and _B. thuringiensis_ (Table 1). [PERSON] et al. [30] reported an acetoin content of 1.56 \(\pm\) 0.09% in cultures of _Bacillus_ sp. LPPC170, while the results of our study showed that the content of this ketone in _B. amyloliquefaciens_ was 3.12 \(\pm\) 0.72% and it was not present in the other two _Bacillus_ species analysed, namely, _B. subtilis_ and _B. thuringiensis_ (Table 1). In addition, [PERSON] et al. [50] and [PERSON] et al. [49] identified tridecan-2-one in cultures of _Bacillus_ sp., which was also confirmed by our studies on the products emitted by _B. amyloliquefaciens_ and _B. subtilis_ (Table 1). [PERSON] et al. [49] also detected pentadecan-2-one in _Bacillus_ sp., while their results confirmed the presence of this compound only in the volatile emission of _B. thuringiensis_ (Table 1). The second group of compounds found in the volatile emissions of _Bacillus_ sp. were alcohols. [PERSON] et al. [23] found phenethyl alcohol in the composition emitted by _B. amyloliquefaciens_ and _B. subtilis_. The same authors also identified 3-methylbutan-1-ol in _B. subtilis_. In contrast, [PERSON] et al. [47] detected heptano-2-ol in _B. subtilis_, and [PERSON] et al. [41] detected the presence of 3-methylbutan-1-ol in _B. thuringiensis_. All the above information on the content of different alcohols in the different _Bacillus_ species was also confirmed by the results of our study (Table 1). In addition, [PERSON] et al.