Researchers at the U.S. Department of Energy's Argonne National Laboratory, in collaboration with colleagues at Wichita State University, the University of Notre Dame, and the University of Southern California have found a new way to study individual living bacterial cells and analyze their chemistry. In research published in the October 22, 2004 issue of Science, the scientists used high-energy X-ray fluorescence measurements to obtain spatially resolved chemical analyses and produce chemical “maps” of individual free-floating (planktonic) and surface-attached (biofilm) cells of the bacterium Pseudomonas fluorescens. The results showed differences between the planktonic and attached cells in morphology, elemental composition, and sensitivity to hexavalent chromium (Cr⁶⁺), a heavy-metal contaminant and a known carcinogen. The cells in the attached biofilm were more tolerant of the contaminant, while it damaged or killed the planktonic cells. The mechanism by which the biofilm cells acquire this tolerance appears to involve a calcium- and phosphorous-rich “coating” of extracellular organic material produced by the bacterial film.  X-ray spectroscopic techniques showed that the hazardous Cr⁶⁺ was chemically reduced to nontoxic Cr³⁺ by this layer.  Other differences also were observed in the distribution of transition metal abundance within surface adhered cells relative to planktonic cells. By advancing the development of high-energy X-ray microprobes and methods for using these microprobes to investigate single bacterial cells, this work pioneers a potentially revolutionary new technique for investigating microbiological systems in natural subsurface environments. Previously, there have been no techniques available that have the spatial resolution needed to analyze individual bacterial cells noninvasively and nondestructively. The analyses reported here were made possible by recent developments at the Advanced Photon Source (APS) at Argonne that have enabled the production of X-ray beams small enough to probe single bacterial cells, which are typically one-hundredth the diameter of a human hair. Funding for this project came from the Natural and Accelerated Bioremediation Research program of the U.S. Department of Energy's Office of Biological and Environmental Research.

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