## 2 Results and Discussion
### Investigation of Discriminating Features
Indeed, a study showed that membrane disruption provoked by AgNPs triggered the biosynthesis of lipids in _E. coli_, possibly as a mechanism to recover membrane integrity [32]. Decanal and 2-undecencal (elevated in _E. coli_ and _E. faecalis_, respectively) can be obtained through the \(\beta\) and \(\alpha\) scission of oleic acid after its peroxidation in the position C8 [30]. Propanal (augmented in _K. pneumoniae_) can be generated during fermentation of monosaccharides, during dehydration of propane-1,2-diol [45; 46]. Alternatively, mentioned saturated aldehydes could be obtained by the reduction of intermediaries from fatty acid elongation step [18]. The fermentation of glucose under conditions deprived of oxygen can produce acetoin as metabolite, which in its turn can be reduced to 2,3-butanediol. 2,3-Butanediol is dehydrated to 2-butanone, which may be reduced to 2-butanol--a VOC augmented in _E. coli_, possibly evidencing an upregulation of anaerobic sugar fermentation [47]. The increasing of compounds such as 2-methyl-1-propanol and 2-methyl-1-butanol (in _E. coli_, and the latter also in _P. mirabilis_) suggest enhancement of Ehrlich pathway. Moreover, \(p\)-cresol, found elevated in _E. faecalis_, can be obtained by fermentation of tyrosine and it is typically produced by _Clostridium difficile_, another Gram-positive bacterium [48]. \(p\)-Cresol has bacteriostatic properties--in a specific microbiota, its production can confer competitive advantages over other colonizing pathogens. In that way, release of \(p\)-cresol is related to bacteria mechanisms for enhancement of their pathogenesis and survival [49]. C2 resulted in the appearing of 2-methylpyrazine, 2-butanone and methyl formate for _E. coli_, nonane for _E. coli_, _K. pneumoniae_ and _E. faecalis_; 2,6,7-trimethyldodecane for _K. pneumoniae_ and _E. faecalis_; \(p\)-xylene for _K. pneumoniae_ and _P. mirabilis_; ethanol for _P. mirabilis_; and butanediolic acid in _E. faecalis_. Pyrazine derivatives are possibly descendant from the biotransformation of tryptophan [18]. They are outlined as biologically active bacterial volatiles with antifungal and antimicrobial properties. Besides that, there is evidence on their antioxidant activity, a property beneficial for resistance to free radicals [50; 51]. As described previously, 2-butanone is a product of pyruvate-diacetyl/acetoin pathway [47], evidencing a stimulation of alternative pathways for obtaining of energy. However, its reduction product (2-butanol) became absent at the present concentration level (C2), indicating that the conversion of 2-butanol to 2-butanone has been interrupted; this can be possibly related to a lack of reduced co-factors in the environment. Additionally, 2-butanone was found inducing bacterial motility in _Pseudomonas aeruginosa_[52]; therefore, this can be a VOC with biologically active response that suits to the stress experienced by bacteria in this circumstance. Faculative anaerobic bacteria such as those here studied can perform glucose fermentation. The process begins with the obtaining of phosphoenolpyruvate (PEP), which is then transformed into pyruvate and can be subsequently reduced to ethanol. PEP can also be transformed to oxaloacetate, which is enzymatically converted into succinate (butanediolic acid). Research on _Actinobacillus_ sp. and _E. coli_ showed that ratio of production of ethanol and butanediolic acid are dependent on the CO\({}_{2}\) concentrations and used source of electrons [53]. As for 3-methyl-1-butanol, supplementation with ethanol at low concentrations in _Staphylococcus_ cultures proved to enhance the formation of biofilm and engender the expression of biofilm-promoting genes, potentially increasing resistance and pathogen potential of bacteria [44; 54]. Residual levels of silver ions are commonly present dissolved or adsorbed to the nanoparticles coating.