The uptake of silver ions by Escherichia coli K12: toxic effects and interaction with copper ions

Summary Growth of Escherichia coli in chloridefree medium in batch culture is inhibited completely at concentrations of AgNO₃ greater than 2.5x10^-6 M. Incubation of non-growing cells in HEPES buffer (pH 7.4) at increasing levels of Ag⁺ results in the progressive saturation of two types of binding site. At one site, the Ag⁺ is not released by washing with 0.1 M nitric acid, and is probably intracellular. Silver bound to the second site is released by acid-washing, but not by buffer washing, and is assumed to be surface-bound. The amounts of Ag⁺ taken up from solution at the two sites is 1.6x10^-7 and 4.6x10^-7 mol (mg dry weight)^-1, respectively. Total accumulation of silver is 67 mg (g dry weight)^-1, similar to literature values found for silver-resistant bacteria. Binding of Ag⁺ at intracellular sites (observed at low Ag⁺]) appears to be independent of pH. Addition of AgNO₃ to growing cells in mid-exponential phase of growth in concentrations that will inhibit growth results in substantially decreased accumulation of silver. Growth yield in chemostat culture is diminished in the presence of added Ag⁺, but this effect is moderated by added Cu²⁺, which may protect copper sites from Ag⁺ or compete with Ag⁺ for other sites at which Ag⁺ exerts toxic effects. Very small amounts of Cu²⁺ are found in cell samples from the chemostat compared to the substantial amounts of Ag⁺ taken up, but uptake of Cu²⁺ is decreased at higher Ag⁺]/Cu²⁺]ratios.