(Fwd) SCIENCE-WEEK February 18, 2000

From: Stirling Westrup (sti@cam.org)
Date: Sun Feb 20 2000 - 20:05:51 MST

From: "Science-Week" <prismx@scienceweek.com>
Organization: Science-Week
Date sent: Tue, 15 Feb 2000 11:23:36 -0600
Subject: SCIENCE-WEEK February 18, 2000


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February 18, 2000 -- Vol. 4 Number 7


Heavy metal ions are usually toxic to biological systems,
although certain heavy metal ions, in trace quantities, serve as
enzymatic catalyst cofactors. Silver is highly toxic to most
microbial cells and can be used as a biocide or antimicrobial
agent. It has been reported, however, that several bacterial
strains are silver-resistant and may even accumulate silver at
the cell wall to as much as 25 percent of the dry weight biomass.
... ... T. Klaus et al (4 authors at Uppsala University, SE)
report the biological synthesis of silver-based crystals in a
bacterial species, the authors making the following points:
     1) The authors report the biosynthesis of silver-based
single crystals with well-defined compositions and shapes, such
as equilateral triangles and hexagons, in the bacterium
Pseudomonas stutzeri AG259, a bacterial strain originally
isolated from a silver mine. The crystals were up to 200
nanometers in size and were often located at the cell poles.
Electron microscopy, x-ray analysis, and electron diffraction
studies established that the crystals comprise at least 3
different types in both whole cells and thin sections.
     2) In contrast to many other metal-resistant bacteria, for
which efflux of toxic ions is the main detoxification mechanism,
the majority of the accumulated silver in P. stutzeri AG259 is
deposited as particles in vacuole-like granules between the outer
membrane and the plasma membrane (periplasmic space). X-ray
analysis shows silver and sulfur in the proportion 2:1, thus
suggesting Ag(sub2)S (silver sulfide). The electron diffraction
pattern suggests acanthite, a stable crystalline form of silver
sulfide. The authors suggest the formation of acanthite crystals
may be caused by a reaction of silver particles with H(sub2)S gas
produced by P. stutzeri.
     3) The authors conclude: "Metal-insulator composite
materials have interesting optical and electrical properties that
favor their application in microelectronics, for example, or as
functional optical thin-film coatings. The possibility of
synthesizing metal particles directly in an organic matrix points
toward new uses of metal-containing bacteria as precursors in
thin film and surface coating technology, for which a composite
or *cermet structure can yield controlled optical, electrical,
and other properties."
T. Klaus et al: Silver-based crystalline nanoparticles,
microbially fabricated.
(Proc. Natl. Acad. Sci. US 23 Nov 99 96: 13611)
QY: Tanja Klaus [Tanja.Klaus@angstrom.uu.se]
Text Notes:
... ... *cermet structure: The term "cermet" refers to any of a
group of composite materials made by mixing, pressing, and
sintering CERamic with METal material.

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