The increasing threat of silver-resistance in clinical isolates from wounds and burns

Purpose: The widespread use of silver-containing compounds has led to emergence of silver-resistant bacteria. Few studies are available on the detectability of plasmid-mediated silver-resistance in developing countries. Therefore, we aimed to detect silver-resistance in isolates from wounds and burns, and to genetically characterize plasmid-mediated silver-resistance genes (sil genes). Methods: One hundred and fifty clinical isolates were obtained from burns and wounds. They were identified using the suitable Analytical Profile Index and MicroScan identification systems. Their antimicrobial susceptibility was tested by the disk diffusion and broth microdilution methods. Their silver nitrate (AgNO3) minimum inhibitory concentration (MIC) was determined using the broth macrodilution method. The presence of different sit genes on plasmids extracted from silver-resistant isolates and the replicon types of the extracted plasmids were investigated using polymerase chain reaction (PCR). The ability of these plasmids to impart silver-resistance was tested by transformation. Results: All except two isolates were multidrug-resistant. Nineteen silver-resistant bacterial isolates (12.6%) were detected; with AgNO3 MIC >= 512 mu g/mL. They were identified as Klebsiella pneumoniae (n=7), Staphylococcus aureus (n=4), Escherichia coli (n=2), Enterobacter cloacae (n=2), Pseudomonas aeruginosa (n=2) and Acinetobacter baumannii (n=2). PCR revealed the presence of different sit genes on the extracted plasmids. Plasmid transformation resulted in the transfer of silver-resistance to the resulting transformants. The extracted plasmids had different replicon types. Conclusion: Plasmid-mediated silver-resistance was detected for the first time, in clinical P aeruginosa, A. baumannii and S. aureus isolates; in addition to its detection in K. pneumoniae, E. coli and Enterobacter cloacae. Therefore, strict monitoring on the use of silver compounds in medical settings is required; with implementation of an approved standardized method for silver-resistance detection.