Metal accumulation in Pseudomonas aeruginosa occur in the form of nanoparticles on the cell surface

In this study the mechanism of chromium (Cr) and copper (Cu) resistance in Pseudomonas aeruginosa was investigated. For this reason, 50 isolates of this microorganism were separated from 345 burn patients hospitalized in burn unit of Kerman hospital, Iran, during May 2001 to April 2002. Susceptibility/resistance of the isolates to KCrO4, CuSO4, 5 H2O, AgNO3 and HgCl2 was determined by the agar dilution method. Among them, 6% were highly resistant to KCrO4 (MIC 50 mM), 56% were resistant to CuSO4, 5 H2O (MIC 10 mM), while, all the isolates were sensitive to HgCl2 and AgNO3 with MIC range 0.5 -1 mM, respectively. Metal resistant isolates exhibited different rate of Cr and Cu accumulation. Isolates 14, 39 and 50 accumulated 11,14 and 15 mM/g biomass chromate, similarly, isolate 24 accumulated 8 mM/g biomass copper. The accumulation of Cr and Cu was mainly surface bound (biosorption), since considerable quantity of these heavy metals was lost from the cell biomass after treating the cells with 50 mM EDTA. Furthermore, P. aeruginosa isolates did not produce H2S. X-ray diffraction analysis of the cell surface exposed to the above heavy metal ions revealed that Cr and Cu were mainly deposited on the cell surface in the form of chromium and copper sulfide (CrS and CuS). These complexes were in the form of electron dense nanoparticles ranging in size from 10 to 40 nm in diameter. However, cells treated with EDTA did not show such complexes.