Microbe of the Month: How a flashing fish could save your life: Luminescent bacteria from the deep are being used to safeguard food and water supplies, says Bernard Dixon

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THE DEEP sea is by no means as Bible-black as we might think. True, little or no sunlight penetrates the oceans' great depths - which is why green plants are unable to grow there. However, thanks to microbes such as Photobacterium phosphoreum, in many parts of the world the sea bottom is a place where light plays a role just as important to some species as it does on land.

Exploiting P. phosphoreum's remarkable luminescence, fish are able to emit brilliant flashes of light, which they use to attract prey, divert or escape from their enemies, and communicate for sexual and other purposes.

The mutually beneficial relationship known as symbiosis is ubiquitous in the living world. Many of the participants are microbes - for example, the bacteria that fix nitrogen in nodules on the roots of legumes, and those that break down cellulose in the cow's rumen. Yet few symbiotic relationships are as striking as that of luminescent bacteria and the special organs on the skin of deep-sea fish.

One fish that plays host in this way is Anomalops katoptron. An inhabitant of the Indonesian coast, it has a large, kidney-shaped aperture under each eye, covered with a membrane that it opens from time to time to emit a flash from the luminescent microbes within. Many other fish have similar structures, often with sophisticated shutters, lenses and reflectors to control the emission. The benefits to the fish are balanced by the protected environment and supply of nutrients it provides for the bacteria.

Now, to make matters more curious, humans are beginning to harness luminescent microbes. Research at the universities of Nottingham and Brighton shows that bioluminescence may be used as the basis for a whole range of ingenious new sensing devices. These can measure tiny quantities of pollutants in waters and foods, and detect bacteria whose presence indicates danger to health.

These possibilities began to emerge a few years ago when microbiologists pondered the fact that any substance which impairs the vital processes inside a luminescent bacterium, such as P. phosphoreum, inhibits its light output. Normally, each cell produces about 1,000 quanta of light per second, which can be measured with a simple photometer. The emission, however, is exquisitely sensitive to toxic agents, which almost instantly cut down the amount of light produced.

This suggests that the microbe could be used as the centrepiece for a sensor to monitor the levels of various substances in the environment. Using this elementary principle, the California-based Microbics Corporation has marketed its 'Microtax' system, which harnesses P. phosphoreum to detect chemical pollutants.

Several international research groups have now evolved a strategy for making devices of this sort even more sensitive and targeted to respond to specific toxicants. It involves genetic engineering to transfer the so-called lux genes, which are responsible for luminescence, into bacteria that are normally 'dark' but that respond to their environment in potentially exploitable ways. Thus systems for measuring a wide range of substances - from cadmium in water to antibiotics in milk - have been developed. Given the wealth of microbes in nature that are sensitive to different chemicals, opportunities for using such tailor-made bacteria as environmental sensors seem limitless.

One innovation by Professor Gordon Stewart in Nottingham and Professor Stephen Denyer in Brighton uses a bioluminescent version of the common bowel bacterium Escherichia coli to monitor disinfectants in cooling waters. The risk of legionnaire's disease has led to greater stringency in disinfecting cooling towers, which can harbour the bacterium responsible, and in monitoring the effectiveness of such treatment.

Another strategy is to splice lux genes not into bacteria but into bacteriophages, viruses that specifically infect certain strains of bacteria. Viruses have no metabolic machinery of their own and are thus 'dark'. However, they can ferry lux genes into host bacteria which, as a result, are lit up. Appropriately targeted, the modified phages could, for example, reveal food-poisoning bacteria, and could revolutionise microbial screening in the food industry.

Based on the traditional method of water testing, a further possibility is to use phages to enumerate bacteria that do not themselves cause disease but whose presence indicates faecal contamination. Professor Stewart and his co-workers have already been able rapidly to monitor microbes of this sort on a factory meat-processing line.

Techniques founded on bacterial luminescence will probably not entirely supplant existing methods of identifying and enumerating unwanted bacteria. But in surveillance of bacteria, and of chemical pollution, they have the enormous advantages of simplicity and on-site operation. What could be more environmentally friendly than the use of naturally occurring bacteria to safeguard our food and water supplies and to monitor the condition of the biosphere?