Biosensors: from canaries to bacteria

Impure water? Dirty work surfaces? Polluted effluent? Contaminated products or land? These days we don't use canaries in coal mines to warn of impending danger, but the same principle that protected miners then does the same job today, albeit in the form of microorganisms like bacteria. Dr Sarah Young, BIO-WISE, explains.

Pollution in many forms is a major issue for a broad range of industries. Keeping tabs on these harmful substances is becoming increasingly important as legislation gets tougher and associated clean-up costs and penalties get higher. This is where biosensors mean business.

authenticating raw materials or high value goods using biological marking systems.

With so many advantages, it is small wonder that the worldwide market in biosensors has trebled in the past four years and is now worth some £1bn.

Internal workings
A typical biosensor consists of a biological component (e.g. an enzyme, antibody or micro-organism) and an electronic device (the transducer) that converts the biological signal into a measurable output.

The biological part of the sensor reacts with the substance of interest to produce a physical or biochemical change, such as light or colour, that is detected and converted to an electrical signal by the transducer. The amplifier increases the intensity of the signal so that it can be readily measured. These components are usually housed within a single portable unit that can be placed at fixed strategic locations.

The biosensor display can be tailored to meet the needs of the application and can range from a simple output such
as switching on or off a light-emitting diode to a quantitative result displayed in graphical format.

Early warning
Biosensors have caught on in the water industry because they are simple to use, cost much less than conventional systems and can act as a back-up to existing chemical detection methods. In addition, biosensors produce results in minutes rather than days and are not confined to detecting a small range of known contaminants. They react to general toxicity and provide an all-important early warning of possible pollution. With regular use, they can lead to significant improvements in quality monitoring and provide an auditable track record of water quality.

Biosensors also cut analysis time down to a few minutes, compared to the days needed to obtain results from a conventional laboratory test. They are simple to use and are cost-effective, making them attractive to smaller companies. Biosensors can offer significant benefits for screening large quantities of samples, reducing the need to send every sample for laboratory analysis.

Already well established in the pharmaceutical and medical industries - where glucose and pregnancy testing kits are the best known examples - biosensors are now catching on in industries such as engineering, textiles, water, food, drink and chemical production.

For nearly 30 years, it has been known that a bacterium, Vibrio fischeri, found in marine environments emits visible light or 'bioluminesces'. When V. fischeri is in a state of metabolic health, the level of light emitted is high and under increasing amounts
of metabolic stress, the amount of bioluminescence drops in a predictable and reproducible manner.

This provides a powerful tool for rapidly identifying 'hot spots' of toxicity and eliminating uncontaminated areas. The cost per sample of using bioluminescence is approximately one tenth of conventional methods.

Using bioluminescence to determine the presence of toxins has been employed in the pharmaceutical sector for several years to rapidly screen for acute toxicity of lead compounds during the early stages of drug development. Increasingly, the approach
is being adopted in the environment industry, mainly in the water treatment sector, where it is used as a rapid, low cost direct toxicity assessment technique.

Xerox chose a biosensor system to monitor wastewater from a new production process at its UK manufacturing plant in Mitcheldean, Gloucester-shire. The company wanted a simple way to measure the COD of wastewater to meet consent limits. The solution was a hand-held device that measures light from an enzyme-dependent, chemi-luminescent reaction.

"Using conventional chemical methods, a result can take up to three days to obtain," says Chris Marriott, Health and Safety Officer for Xerox Environment, "Using the biosensor, a result can be obtained within an hour. The results are of sufficient accuracy to enable us to discharge our waste confident in the knowledge that the consent is not being breached."

Pollution control
Preventing pollution is not the only use biosensors can be put to as they can also indicate when to reduce dosing and additives safely, leading to substantial savings in material costs. For example, during paper manufacture, pulp is regularly dosed with biocides to control the level of bacterial contamination.

With biosensors, the bacterial content is continually monitored and biocides are added only as and when required. Not only does this reduce costs and pollution, it also improves paper quality.

The technology can be applied in any industry where the microbiological integrity of water or air has an impact on the end-product quality. Paint manufacturers use similar systems where bacterial contamination could mean the loss of up to three weeks' production.

Toxicity detection
Whereas conventional chemical monitoring systems test for known contaminants, biosensors are capable of detecting general toxicity in soil, water or air. In the case of land contamination, Remedios has developed a system to produce a computer-generated map summarising site problems.

The system was recently used by property consultant Chesterton International to assess a Scottish site with a history of various industrial uses. High levels of contaminants were present but the exact distribution of the pollution was unknown. Using the biosensor, an assessment of the site was made in less than ten working days and an area of toxicity overlooked by three previous reports was discovered.

Chesterton International Director Eric Shearer said, "I had seen the commercial value of the site years ago. Remedios, in just a few weeks, showed me the true value of the site in a presentation that even I could understand."

As well as assessing general toxicity, biosensors can home in on specific compounds, using an antibody, enzyme or even DNA fragment to detect the presence of a target substance. A further advantage of biosensors is their extraordinary sensitivity and accuracy. They can match the sensitivity of the human palate in detecting minute quantities of taste-altering contaminants, which explains their widespread use in the water and food and drink industries.

An exciting development is the use of biological tracers (usually DNA) to pinpoint the source of an effluent by marking it with a tag where it is being discharged. As environmental legislation becomes more stringent, such tracer technology will become increasingly widely used.

The same technology can be used to add identifying DNA markers during the manufacture of high-value items, such as perfumes and designer clothes, to help detect counterfeit items. Biosensors really do mean business and their range of applications set to increase.


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