Plasmid-Determined Resistance to Silver in Enterobacter cloacae Isolated from Sewage

Microbial silver toxicity is found in situations of industrial pollution, especially those associated with the use of photographic film. In hospitals, silver salts are the preferred antimicrobial agents for burns. It is well known that silver-resistant bacteria with plasmids (extrachromosomal DNA molecules) encoded for silver resistance can be isolated from polluted industrial sites and hospitals. The silver resistance of five strains of Enterobactercloacae isolated from contaminated sewage was investigated. In one of these strains, a high frequency of inactivation of silver resistance was achieved by treatment with ethidium bromide and acriflavine (inactivation frequencies of 23.3% and 17.7%, respectively). The maximum concentration of AgNO 3 at which plasmid-less Ag-sensitive strains could be grown was 20 mg/l. The maximum concentration of AgNO 3 at which Ag-resistant strains were able to grow was 10 000 mg/l, i.e., 500 times that for silver-sensitive strains. Agarose gel electrophoresis profiles showed that the DNA with higher mobility, i.e., plasmids, present in the silver-resistant cells was inactivated by the treatment with ethidium bromide and acriflavine. Viable organisms of Ag-resistant and Ag-sensitive strains at about 10 8 colony-forming units (CFU) per ml were inoculated into broth containing AgNO 3 . The concentrations of AgNO 3 in the supernatants were measured by atomic absorption spectrophotometry. A decrease in the concentration of AgNO 3 was observed. The concentrations decreased to below 50% of the initial value after 24 h, and to almost 0% (trace) after 1 week.