Silver is a heavy metal with relatively high toxicity to prokaryotes, and silver salts are used as antiseptics to prevent Neisseria infections in the eyes of newborn infants and to control infections in burns (Russell and Hugo, 1994). Research on microbial interactions with silver has focused on toxicity, resistance, and accumulation (Slawson et al, 1992).

Silver resistance has been described in Gram-positive and Gram-negative bacteria (Nies, 1999) representing clinical (McHugh et al, 1975) and environmental (Deshpande and Chopade, 1994; Haefeli et al, 1984) isolates. The genes mediating plasmid-based silver resistance in Salmonella have been identified and were found to encode a periplasmic silver-binding protein and two parallel efflux pumps--a P-type ATPase and a cation/proton antiporter (Gupta et al, 1999). Silver resistance in Eschericia coli appears to be controlled by a chromosomally encoded efflux pump (Li et al, 1997) that may involve the protein encoded by gene ybdE (Franke et al, 2001). In Enterococcus hirae, a copper-exporting ATPase may also function to export silver (Solioz and Odermatt, 1995).

Some silver resistant bacteria accumulate silver instead of exporting silver. When grown in the presence of silver ions, Pseudomonas stutzerii deposited silver in regularly shaped nanometer-scale crystalline particles in the periplasmic space (Klaus et al, 1999). Research has been conducted on the ability of bacteria to recover silver ions from photographic (Shakibaie et al, 1999) electroplating (Patil and Paknikar, 2000) industry wastewaters. Fungal chitins can also bind silver ions with high affinity, and biosorption of silver by living and dead fungal biomass has been investigated (Fourest et al, 1994).

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Gupta A, Phung LT, Taylor DE, and Silver, S. Diversity of silver resistance genes in IncH incompatibility group plasmids. Microbiology. 2001 Dec;147(Pt12):3393-402.

Mullen MC, Wolf DC, Ferris FG, Beveridge TJ, Flemming CA, Bailey GW. Bacterial sorption of heavy metals. Appl Environ Microbiol. 1989 Dec;55(12):3143-9.