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Tuesday, 23 November, 1999, 13:14 GMT
Bacteria with a silver lining
Silver is normally toxic to bacteria

A strain of bacteria that can manufacture tiny crystals of silver has been reported by Swedish scientists. This skill may eventually prove useful to engineers who want to fabricate extremely small optical and electronic devices.

Pseudomonas stutzeri were originally found growing on rocks recovered from silver mines.

The metal is usually highly toxic to microbes and silver compounds make effective bactericides. But P. stutzeri can survive such silver-rich environments because they gather all the metal in their systems into small granules which they store at the edge of their cells.

The silver-containing particles are deposited at the edge of cells
"It's a very clever trick," said Dr Tanja Klaus, from the Department of Materials Science at Uppsala University.

Her team cultured the bacteria in the laboratory and got them to synthesise single, silver-based crystals with well-defined shapes, including equilateral triangles and hexagons.

Most of the crystals were made from almost pure silver. They reached up to 200 nanometres (billionths of a metre) in size.

However, the Swedish team also found a second group containing silver sulphide, and a third variety whose composition is unknown but which possibly contained hydrogen, carbon, nitrogen and oxygen in addition to silver.

The way these silver crystals are made, their small size and composite nature, will be of great interest to scientists working in the field of nanotechnology.

Novel properties

Materials fabricated on the nano scale have novel properties not displayed in normal, large-scale crystalline solids or glasses of the same chemical composition. Nanophase materials, as they are often called, have unusual electrical and optical properties because of the very precise way in which their atoms are arranged.

By carefully controlling particle sizes, it is possible, for example, to make "superplastic" ceramics that stretch like chewing gum and liquids that are magnetic.

The crystals take up well-defined shapes
But fabrication of any material with dimensions on the nanometre scale tends to be costly and inefficient, which means that bacteria that can grow such particles could prove very useful indeed.

The Uppsala team suggest it may be possible to produce new types of metal films and coatings that have their properties "tuned" according to the way the bacteria are cultured.

"We are hoping that we will be able to control the size and morphology of the crystals," said Dr Klaus.

"The films we are working on are wavelength sensitive. You can collect light or energy coming from the Sun in a specific range and avoid that energy being emitted or lost.

"In this way, you can make solar collectors and use them for water heating, for example."

But much more needs to be known about the mechanism the microbes use to precipitate the crystals before any large-scale applications can even be considered, Dr Klaus said.

Details of the Uppsala research are published in the Proceedings of the National Academy of Sciences of the USA.

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