## 3 Discussion
Although silver is usually considered a benign material, it can also produce secondary effects such as: the formation of ulcers in burns treated with silver, the staining or destruction of skin cells when directly applied to the treatment of vulgar warts, and sometimes increased serum levels of silver, even argiria and argirosis in self-medicated patients with colloidal silver solutions. There is also a concern about the toxicity of silver nanoparticles to other organisms (especially aquatic organisms) [42]. One of the problems raised by the large use of silver-based antiseptics is the emergence of silver resistance. The endogenous resistance has been described mostly in Gram-negative bacteria and involves multiple mechanisms, such as: derepression of the chromosomal Cus system or of the silver-resistance genes (sil genes) [43]; loss of outer-membrane porins (OmpF or OmpF/C), leading to reduced outer-membrane permeability; active efflux of silver out of the cell (transporter CusCFBA) [44]. CHASRI is a copper-homeostasis and silver-resistance island that is involved in silver resistance after mutation in CusS and/or silS genes of members of _Enterobacteriaceae_ family [45]. Components of the cus operon (CusF and CusB) are overexpressed in _E. coli_ silver-resistant strains [45]. In 1975, an outbreak of _Salmonella enterica serovar Typhimurium_ that was resistant to silver was recorded in a burn unit. The epidemic strain proved to have an exogenous silver resistance due to the acquisition of a plasmid harboring the sil system [43; 44]. Later, it was shown that plasmid gene-mediated resistance to silver can be conferred by sil and the related gene operon present in the copper-resistant operon (pco) [37]. The plasmid pMG101 is 180 kb and provides resistance to silver, mercury, tellurite, and antibiotics (ampicillin, chloramphenicol, tetracycline, streptomycin and sulfonamides). The silver-resistance gene cluster contains nine genes, out of which seven are named, and two are lesser known with open reading frames (ORFs): silP, ORF105, silA, silB, ORF96, silC, silS, silR and silE. Silver-resistance gene box encodes two efflux pumps (silP-ATPase and silCBA-chemiosmotic), and two periplasmic proteins that bind Ag\({}^{+}\) (silE), regulatory genes (silS-membrane kinase sensor and silR) [45]. These proteins are responsible for silver resistance through a combination of mechanisms involving silver sequestration in the periplasm (via SilE and SilF binding) and active efflux (via the resistance-modulation-division (RND)-type efflux transporter SilCBA and the putative P-type ATPase transporter SilP) [45]. As silver ions are widely used in treating leg ulcers, burns, plantar ulcers, the investigation of silver susceptibility and resistance profiles of pathogenic strains isolated from this type of wound is a topic of great interest. Therefore, in this study, 125 bacterial strains isolated from 103 patients with venous ulcers were investigated to elucidate the susceptibility of the skin microbiome to silver-nitrate solutions and study the genetic determinants for this resistance. These bacterial strains were previously characterized for their virulence and antibiotic-resistance features at the phenotypic and genotypic level [46; 47]. The isolated strains, both in the planktonic and biofilm growth phase, showed sensitivity to the standard concentrations of 1/6000 silver-nitrate solution that is currently used as an antiseptic substance in treating leg ulcers. Regarding the genetic support of silver resistance, [PERSON] et al. studied silE, silP, and silS genes in strains isolated from leg ulcers, observing their presence in strains of Enterobacter aerogenes, E. cloacaae, Klebsiella pneumoniae, K. oxytoca, E. coli, Citrobacter, Proteus, Providencia, Salmonella and P. aeruginosa[35; 47]. [PERSON] et al. analyzed 859 strains belonging to Staphylococcus, Escherichia, Pseudomonas, Klebsiella, Enterococcus, and Enterobacteriaceae genera.