CHARACTERIZATION OF THE BINDING OF GALLIUM, PLATINUM, AND URANIUM TO PSEUDOMONAS-FLUORESCENS BY SMALL-ANGLE X-RAY-SCATTERING AND TRANSMISSION ELECTRON-MICROSCOPY

Small-angle X-ray scattering (SAXS) was used to determine the binding of Ga, U, and Pt to Pseudomonas fluorescens in aqueous buffer. Atomic absorption spectrophotometry was used to quantify the heavy metals dur ing bulk analysis, whereas transmission electron microscopy of whole m ounts and thin sections was used to determine the locations of the cel l-bound metal precipitates, as well as their sizes and physical struct ures. Energy-dispersive X-ray spectroscopy confirmed the compositions and identities of the precipitates and helped show that they were asso ciated primarily with the envelope layers of the bacteria. Unlike Ga a nd Pt, which were located only at the cell surface, U was also found i ntracellularly in approximately 10% of the cells. This cytoplasmic loc ation ultimately killed and lysed the cells. Surface-bound Ga and U we re spread over the entire cell envelope (outer membrane-peptidoglycan- plasma membrane complex), whereas Pt was associated only with the lipo polysaccharide-rich, external face of the outer membrane. SAXS confirm ed these data and showed that the bacteria were metal-enshrouded parti cles that were 1.0 to 1.5 mum in diameter. SAXS also provided a statis tically significant representation of the bound metal precipitates, wh ich ranged in size from 10 nm to 1 mum. The correlation between the mi croscopic data and the scattering data was extremely good. Since SAXS is performed in an aqueous milieu, it yields a more representative pic ture of the physical state of the metal bound to cell surfaces.