Science: Bacteria with a taste for the dirty work - A magnetic microbe offers hope of a breakthrough in detoxifying industrial waste, says Martyn Kelly
Monday 17 January 1994
There is almost no place on earth where bacteria are not found. Several bacteria, for example, are naturally tolerant of heavy metals; others can be bred for resistance in the laboratory. Place these in an industrial effluent and toxic chemicals in it will be accumulated by the bacteria to such an extent that the effluent becomes clean enough to be discharged. That, at least, is the theory. The problem lies in moving from laboratory fermentors to factory-
sized installations. Having allowed your bacteria to accumulate metals, how do you then remove them en masse from the solution?
Three biotechnologists - Professor Derek Ellwood, of the Westlake Industrial Park in Cumbria; Professor Jim Watson, of Southampton University; and Mike Hill, of European Cancer Prevention - have hit upon a novel technique: make the bacteria magnetic. Then if the treated effluent is exposed to a magnetic field, the bacteria and toxins can be collected on to a solid surface.
'The bacteria can accumulate some metals to up to 30 per cent of their body weight,' Professor Ellwood says. 'This is a lot more concentrated than many natural metal ores and, depending upon the metal, can be very valuable.'
But how do you make a bacterium magnetic? Bacteria are usually grown in the laboratory on a growth medium containing simple sugars. Dr Alistair Dean, a microbiologist at Oxford University, gave some bacteria a sugar that was bound to a phosphate molecule. In order to use the sugar, the bacteria had to secrete an enzyme to split it from the phosphate. The sugar was then absorbed by the bacterium, while the phosphate remained outside the cell.
Phosphate is relatively insoluble. If the solution contains dissolved metals, these will form precipitates with phosphate on the nearest available surface - in this case the bacteria. If the metal is magnetic, then it is a simple job to harvest the bacteria by passing a current through some steel wool. Professor Ellwood and his colleagues first tested this with solutions containing radioactive isotopes common in effluents produced by British Nuclear Fuels. Typically, concentrations of uranium, caesium, strontium and other radioactive metals were reduced by a factor of 20.
However, not all metals readily form insoluble phosphates, and the next step was to find a similar way of removing these from solution. The organism they struck upon was a bacterium called Desulphovibrio. This organism is common in places where there is no oxygen for respiration. Its respiration is based, rather, on sulphate. The sulphate ion (one sulphur atom plus four oxygen atoms) is taken into the cell, the oxygen removed and sulphur released, usually as hydrogen sulphide. Sulphide is highly insoluble, so if there are any metals in the solution, these are precipitated on to the cell surface.
The results, Professor Ellwood admits, were a complete surprise. When tested on a large volume of industrial effluent containing two parts per million mercury, a pilot system for a company in the Netherlands succeeded in reducing the concentration to five parts per billion: a reduction of more than 99 per cent.
The chemical mechanism is not clear, but it seems that the bacteria become covered with a 'feather boa' of iron sulphide strands. This massive surface area - more than 200 square metres per gram - makes it extremely efficient at mopping up stray metals. The irony of this case is that the company decided it was cheaper to pay the fine for discharging the untreated effluent than invest in a new treatment plant. So much for leaving environmental control to market forces.
By contrast, some of these metals, such as rhodium, are extremely valuable. Jim Watson is presently assessing the feasibility of using natural populations of Desulphovibrio to clean up the sediments of New York harbour.
'The value of the recovered metals in this situation,' Professor Ellwood says, 'could potentially pay for the entire process.' Laboratory tests suggest that any industry that uses precious metals - the photographic processing industry, for example - might be able to profit from using magnetic bacteria systems to recycle waste metals.
It is a tempting prospect: companies not so much penalised for polluting as able to profit from good environmental practice. With extensive trials under way at Southampton General Hospital to exploit the same system in treating victims of nuclear accidents, the potential of these magnetic bacteria may be limited only by the ingenuity of biotechnologists.
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