Silver resistance in Escherichia coli R1

Escherichia coli strain R1, originally isolated from a patient whose burns were treated with silver sulphadiazine, contained two large plasmids of 83 kb (pJT1) and 77 kb (pJT2), and was resistant to 1 mM AgNO3. A silver-sensitive derivative, E. coli S1, cured of the 83-kb plasmid pJT1, was obtained by growth at 46°C. Studies with an Ag+ -specific ion electrode showed no significant differences in Ag+ binding by washed resting cell suspensions of strains R1 and S1, with and without glucose. However, transmission electronmicroscopy and energy dispersive X-ray analysis of whole cell mounts from actively growing cultures showed that the Ag+-resistant strain did not accumulate Ag+, whereas the sensitive strain contained dense silver particles. Both strains produced H2S, detected by blackening of lead acetate paper above inoculated broth, and reducing substances (possibly H2S) were detected only around E. coli R1 colonies when methylene blue was used as a indicator in LB agar, which may be a less sensitive assay. The mechanism of silver resistance is not known, but actively growing cells of E. coli R1 did not accumulate silver.