## 2 Results and Discussion
### Investigation of Discriminating Features
Spearman correlation coefficients (rho) were calculated for each of the detected VOCs, aiming to point out the compounds that, in a significant manner, had their responses greatly modulated by the applied concentrations of agent. General guidelines for the interpretation of presented coefficients is described by the literature on statistical methods [64]. A very strong positive correlation was found between the compound area and the concentration level of bio-AgNPs for 2-undecenal in _E. coli_ (rho = 0.97, \(p\) = 7.88 \(\times\) 10\({}^{-8}\)), _E. faecalis_ (rho = 0.89, \(p\) = 1.03 \(\times\) 10\({}^{-4}\)) and _K. pneumoniae_ (rho = 0.93, \(p\) = 8.79 \(\times\) 10\({}^{-6}\)); 2-decenal in _E. faecalis_ (rho = 0.86, \(p\) = 3.11 \(\times\) 10\({}^{-4}\)); nonanoic acid in _K. pneumoniae_ (rho = 0.83, \(p\) = 7.74 \(\times\) 10\({}^{-4}\)) and 2,4-decadienal in _P. mirabilis_ (rho = 0.85, \(p\) = 5.15 \(\times\) 10\({}^{-4}\)). A strong positive correlation was identified for 2,6-dimethylnonane in _E. coli_ (rho = 0.71, \(p\) = 8.36 \(\times\) 10\({}^{-3}\)) and 2-ethyl-5-methyl-pyrazine in _E. faecalis_ (rho = 0.80, \(p\) = 1.84 \(\times\) 10\({}^{-3}\)). These are VOCs which were proportionally elevated in response to increasing amounts of nanoparticles. On the other hand, very strong negative correlation was detectedfor 2-nonanone in _E. coli_ (rho = \(-\)0.84, \(p\) = 6.11 \(\times\) 10\({}^{-4}\)) and _P. mirabilis_ (rho = \(-\)0.82, \(p\) = 1.28 \(\times\) 10\({}^{-3}\)). A strong negative correlation was presented by propyl proppanoate in _E. faecalis_ (rho = \(-\)0.80, \(p\) = 1.62 \(\times\) 10\({}^{-3}\)), _K. pneumoniae_ (rho = \(-\)0.76, \(p\) = 4.10 \(\times\) 10\({}^{-3}\)); 2-octanol (rho = \(-\)0.74, \(p\) = 5.77 \(\times\) 10\({}^{-3}\)) in _E. faecalis_; in _K. pneumoniae_, allyl thioacetate (rho = \(-\)0.71, \(p\) = 8.42 \(\times\) 10\({}^{-3}\)), 2-nonanone (rho = \(-\)0.75, 4.30 \(\times\) 10\({}^{-3}\)) and S-methyl propanethoate (rho = \(-\)0.79, \(p\) = 1.92 \(\times\) 10\({}^{-3}\)); in _P. mirabilis_, methyl isopropyl disulfide (rho = \(-\)0.73, \(p\) = 6.87 \(\times\) 10\({}^{-3}\)), S-methyl-3-methylbutanetthioate (rho = \(-\)0.73, \(p\) = 6.57 \(\times\) 10\({}^{-3}\)), methyl ethyl disulfide (rho = \(-\)0.74, \(p\) = 5 \(\times\) 10\({}^{-3}\)) and 2-butanol (rho = \(-\)0.78, \(p\) = 2.71 \(\times\) 10\({}^{-3}\)). The antimicrobial mechanism of action of AgNPs implies in the interaction of nanoparticles with bacterial biomolecules, eventually promoting their degradation--directly or indirectly--due to the release of ROS in the cell. As mentioned, aldehydes and fatty acids that elevated their levels as progressively greater concentrations of agent were applied can be pondered as by-products of lipid peroxidation. The crescent responses of 2,6-dimethylnonane may be associated with similar processes targeting branched lipids. The occurrence of branched fatty acids among bacterial lipids is common and has been associated to functions related to regulation of membrane fluidity. The ratio of _iso.anteiso_ branched fatty acids also seems to be involved in bacteria adaptation to pH and temperature [65]. The decarboxylation of fatty acids can give rise to corresponding alkanes [18]. Apart from this, alkanes can be obtained in the course of peroxidation of a fatty acid followed by its cleavage and posterior reduction [66]. In this sense, elevation of branched species can be linked to the degradation of constituting lipids. Similar hypothesis can be drawn regarding the previously mentioned appearing/augmentation of 2,6,7-trimethyldodecane and nonane, observed at C2. Interestingly, most of the compounds negatively regulated were VSCs, demonstrating that the catabolism of S-containing amino acids appears largely impaired for _K. pneumoniae_ and _P. mirabilis_. The biogenesis of VSCs, including S-methyl thioesters, is often related to methionine and cysteine derivatives, and part of these volatiles are assigned as bioactive molecules [18].