## 2 Results and Discussion
### Differences in VOC Profiles Composition  
The Venn diagram exhibited in the Figure 4a refers to the incidence of compounds common among the investigated bacteria in untreated cultures. These values can indicate the degree of similarity between the original metabolism of the referred strains. In total, 189 different compounds were detected as being emitted by unstressed cultures; from these, 27 were coincident among all bacteria (approximately 14% of the total profile). The greatest profile similarity was observed for _E. coli_ and _P. mirabilis_, which shared 20 VOCs. A total of 7, 10, 12 and 30 volatiles were found exclusively in the profiles of _E. faecalis_, _E. coli_, _K. pneumoniae_ and _P. mirabilis_, respectively. In this sense, _E. faecalis_ produces the less distinct pattern of volatiles, while _P. mirabilis_' VOC profile is the most diverse among the studied strains. Few compounds were incident consistently in all replicates of a specific bacterium in the control cultures, namely 2-propanol and pentadecane for _E. coli_; acetoin, indazole and 3-phenylindole for _E. faecalis_; 3-methyl-1-butanol propanoate and 2-tetradanone for _K. pneumoniae_; and methyl thiolacetate, 2-methylpropanoic acid, 5-methyl thiobbutyrate, 2-methylpyrrole-3-carbonitrile, octanoic acid, 2,4-dimethyldecane and methyl 4-tert-butyl-thiobenzoate for _P. mirabilis_. Figure 3: HCA associated with heatmap, displaying changes in VOCs’ intensities in studied volatile bacterial profiles and showing the assigned clusters (Pearson method). EC = _E. coli_, EF = _E. faecalis_, KP = _K. pneumoniae_, PM = _P. mirabilis_. The network analysis (Figure 4b-e) provides a graphical representation of the relationships existent between bacterial strains (rectangles) and their emitted VOCs (circles). These graphs can demonstrate the dynamics of alterations that occur in the set of bacterial metabolites coming from different species when increasing concentrations of bio-AgNPs are added to the medium. The addition of 12.5 ug mL\({}^{-1}\) primarily induces a reduction in the variety of detected VOCs (139). Additionally, there is an increase in the occurrence of compounds generated by more than one bacterium: although the original set of VOCs had been reduced by 26%, the number of compounds shared among the three species increased (32), and exclusive volatiles are no longer observed in case of _P. mirabilis_. In contrast to this, for _E. coli_ and _E. faecalis_ the number of characteristic VOCs increased, indicating the activation of metabolic processes which were previously absent. The supplementation with 25 and 50 ug mL\({}^{-1}\) of bio-AgNPs lead to progressive depletion in the number of produced bacterial metabolites. Overall, the VOC patterns of _K. pneumoniae_ and _P. mirabilis_ tend to converge, while _E. coli_ develops the most distinct VOC profile. Considering also the experiments involving bio-AgNPs treatment, a total of 220 compounds were detected. Figure 5a presents distribution of found VOCs according to their functional groups. Specifically, 102, 76, 94 and 144 different volatiles were found in the unstressed cultures of _E. coli_, _E. faecalis_, _K. pneumoniae_ and _P. mirabilis_, respectively. Native VOC profiles from _E. coli_, _E. faecalis_, _K. pneumoniae_ and _P. mirabilis_ were mostly composed by hydrocarbons (14, 26, 25 and 19%), alcohols (14, 19, 14 and 10%), ketones (10, 10, 14 and 9%), volatile nitrogen compounds (VSCs) (7, 3, 14 and 18%), volatile sulfur compounds (VSCs) (21, 10, 11 and 14%), acids (14, 13, 3 and 5%), aldehydes (7, 6, 6 and 5%) and esters (10, 10, 8 and 7%), respectively. In terms of incidence, hydrocarbons, followed by alcohols and VSCs (or VSCs, in case of _E. faecalis_), were the predominant chemical classes among the obtained profiles. The differing numbers of VOCs detected in untreated cultures can be interpreted as being associated with the individual metabolic variability pertinent to each of the bacterial species, at the given environmental and nutritional conditions.