Silver

Silver is a naturally occurring element and, as silver nitrate, has some bactericidal activity against selected species (3.0 log10) at 0.032 mg/l. Fungicidal or mycobactericidal activity is largely unknown. Its antimicrobial activity is explained by membrane interaction and induction of the VBNC state. Cellular defence mechanisms include various tolerance genes such as silE, silA, silP and silS, plasmids, efflux pumps and cellular silver uptake and accumulation. Silver reduces the formation of a developing biofilm in some species. Biofilm removal by silver nanoparticles is moderate or strong. High MIC values, indicating tolerance to silver nitrate, have been reported mainly for sil-positive isolates of E. coli (≤512,000 mg/l), E. cloacae (≤512,000 mg/l), P. aeruginosa (≤128,000 mg/l), Klebsiella spp. (≤5500 mg/l), P. stutzeri (4250 mg/l), S. Typhimurium (1700 mg/l) and A. baumannii, P. mirabilis, S. agalactiae and S. aureus (all 512 mg/l), but their relevance is uncertain. Low-level exposure results in no MIC change in 11 species, a weak MIC change in two species and a strong MIC change in seven species (two of which are stable), resulting in MIC values as high as 1024 mg/l (E. coli) or 1000 mg/l (E. cloacae). Cross-tolerance to copper is possible, mainly via efflux pumps. Cross-resistance to various antibiotics has been described in selected isolates of Gram-negative species. No silver-resistant isolates have been described with an insufficient efficacy in suspension tests or tests under practical conditions, or with a contamination of disinfectants. The overall probability of resistance to silver of practical relevance is low in the absence of sil genes and biofilm, but may be moderate in the presence of sil genes and biofilm.