## 2 Results and Discussion
### Acertainment of Affected Bacterial Pathways
Generated nitriles can be transformed into the corresponding carboxylic acid and ammonia [71]. In this context, benzonitrile is the metabolite provided from the processing of phenylalanine. Decreased levels of this VOC indicate the enzymatic inhibition of the first step of the referred path, which comprises amino acids conversion to nitrile by action of cytochrome p450 [71]. The esterification  
Figure 8: Pathway network analysis presenting the relations between most relevant metabolites (squares) and likely associated bacterial processes (circles) for (**a**) _E. coli_, (**b**) _E. faecalis_, (**c**) _K. pneumoniae_ and (**d**) _P. mirabilis_. P1 = metabolism of S-containing amino acids, P2 = propanoate metabolism, P3 = Ehrlich pathway—reduction of keto acids, P4 = fatty acids pathway—fatty acid biosynthesis, P5 = fatty acids pathway—decarbonylation of fatty aldehydes, P6 = Ehrlich pathway—esterification of fused alcohols, P7 = Ehrlich pathway—oxidation of fused aldehydes, P8 = fatty acids pathway—decarboxylation of keto fatty acids, P9 = aldoxime-nitrile pathway—phenylalanine metabolism, P10 = fatty acids pathway—reduction of fatty acyl-ACP, P11 = lipid peroxidation, P12 = pyruvate fermentation, P13 = fatty acids pathway—decarboxylation of fatty acids, P14 = fatty acids pathway—reduction of methyl ketones, P15 = Ehrlich pathway—decarboxylation of keto acids, P16 = butanoate metabolism, P17 = amino acids metabolism—tryptophan metabolism. of fused alcohols also presented itself as boosted in _E. coli_. The overall increasing in the production of esters, which are final product of Ehrlich pathway, can be connected with an amplification of the catabolism of amino acids assimilated by this path--in this particular strain, it is possible that such mechanism is being used by cell as an alternative process to promote energy-efficient NADH regeneration, due to the detriment of respiration as a main energetic source. In _E. faecalis_, as for _E. coli_, the majority of altered metabolites belongs to the biochemical path of lipids. However, in this case, fermentation processes also configure as considerably hampered. Regarding _K. pneumoniae_, the decarboxylation of keto fatty acids appears reduced, factor evidenced by the suppressed emission of several methyl ketones. Early steps of fatty acids path were not so impacted as it was observed for other bacterial species. Depletion of fermentation processes is also expressive. Apart from the examination of qualitative VOC profiles, the tracking of impaired biochemical routes also suggests that _P. mirabilis_ was the bacterium more susceptible to the presence of bio-AgNPs. For this species, nanoparticles caused decreased response of numerous S-containing metabolites and great impairment of fatty acids routes (mostly affecting biosynthesis and \(\beta\)-carboxylation steps). Moreover, _P. mirabilis_ displayed increasing trends for three metabolites possibly descendant from lipid peroxidation. Depletion of pyrazines emerge as an indicator of restricted catabolism of tryptophan. In agreement with the general remarks provided by this analysis, an in silico approach aiming to predict the interaction between AgNPs and proteins revealed that inhibition is expected for enzymes involved in the metabolism of several amino acids, glycolysis, TCA cycle and ions transportation, evidencing the interruption of essential cellular activities and the participation of oxidative processes [72]. A greater bacterial resistance against bio-AgNPs can be inferred from VOC patterns of _E. faecalis_, once these were the less affected in terms of conservation of the original volatilome. Indeed, previous studies on molecular profiles of Gram-positive and Gram-negative bacteria, showed that this first group presented to be superior is terms of resistance against bio-AgNPs, especially when a longer incubation time with the agent is considered. Protein patterns acquired by MALDI-TOF-MS technique for _S.