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Silver Colloids Impregnating or Coating Bacteria


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Introduction
In this communication we demonstrate that deposits of
metallic silver may be generated selectively inside or on bacteria
in a controlled manner and discuss several aspects of the
behavior of the combined metal-bacteria system. The methods
developed here are potentially of wide general use, for example,
in the study of the biochemistry and biophysics of bacteria or
in material science. Specifically, the methods can be applied
to the identification of bacteria, on one hand, as well as to the
control of the production of colloids in materials processing,
on the other hand, in line with the directions outlined recently
by Mann.



Results and Discussion
Electron Microscopy. Figure 1 shows an electron micrograph
of Escherichia coli with a silver colloid deposited mainly
on the bacterium wall (“wall colloid”). The TEM measurements
are done to determine the location of the silver deposits when
compared to various controls that show the bacterial image as
much less dense. Thus the micrographs shown here are made
without any of the usual preparations to enhance the image.
They utilize only the silver from the experiment to add density
to the bacterial image.


Surface-Enhanced Raman Scattering. Figure 6 shows the
Raman spectrum obtained from E. coli coated with a silver
colloid. The system consists of a Coherent Innova 90/5 laser,
a Jarrell Ash MonoSpec 27 single stage monochromator (Model
82499) with a ruled grating of 600 gr/mm, one or two Kaiser
Holographic SuperNotch filters, HSNF-514.5, and a PI 512
element intensified diode array.


Summary
In summary, we showed here that metal silver colloids can
form within and on bacteria. On one hand, the bacteria modify
the properties of the colloid, which is important for material
science. Specifically, stable nanocolloidal dispersions are
obtained under conditions that yield only powdery deposits in
the absence of the bacteria.
Raman spectra are known to produce unique fingerprints for individual chemicals. Therefore, there is some possibility of using these spectra to identify or classify suspects of bacterial agents through their chemical differences. The biggest obstacle to this is the weak Raman signal that arises from trace chemicals. In the future this can be overcome by using Surface Enhanced Raman Spectra (SERS), which can increase a signal of several orders of magnitude. It has been shown that the reduction of silver ions in and on bacterial cells produces silver colloids leading to intense SERS of chemical components of the cell. Transmission electron microscopy, absorption spectroscopy and elemental analysis were used to explore the distribution of silver in and on cells produced by two different protocols. The first protocol gives rise to an internal colloid of nanometer-sized silver particles evenly distributed within the cells. The second protocol produces a rough (aggregate) layer of silver on the surface of the cells. Both protocols largely preserve the shape of the cells although there is a reduction in size, which will be detailed in a later report. Treatment with potassium cyanide removes silver and reverts the cells to a size and shape similar to the original. This is a basic research that is expected to improve the understanding of the SERS spectrum (and hence related IR spectra) arising from bacteria. This knowledge will facilitate the development of instrumentation for the rapid classification of suspects of biological agents.
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