Silver resistance In Acinetobacter baumannii BL54 occurs through binding to a Ag-Binding Protein

The mechanism of plasmid mediated silver (Ag) resistance was investigated in Acinetobacter baumannii BL54. The intracellular accumulation of Ag in both original strain BL54 and Escherichia coli K12 transconjugant containing plasmid pUPI276 began immediately and reached a maximum within 60 minutes. This initial accumulation was followed by net loss of Ag which reached a maximum within 180 min. Pre-treatment of cells with 0.5 mM 2,4 dinitrophenol (DNP); 20 M N, N-dicyclohexylcarbodiimide (DCCD); 3% toluene; 25 g/ml cefotaxime and polymyxin-B resulted in considerable decrease in the accumulation process. Ags plasmid less cured derivative (BL54.1) also accumulated silver but only one-fourth the amount compared to the resistant strain BL54. The intracellular accumulated silver is detoxified by binding to a cysteine rich metal binding protein. The purified Ag-binding protein exhibited maximum absorption at 280/215 nm. From the above data it could be concluded that the intracellular detoxification of silver in A. baumannii BL54 is achieved through binding to a cysteine rich metalloprotein.