Biosensor detects foodborne pathogen

Sunday, 19 December, 2004

A team of food scientists at Purdue University has developed a sensor that can detect the potentially deadly bacteria Listeria monocytogenes in less than 24 hours at concentrations as low as 1000 cells per millilitre of fluid. The sensor is also selective enough to recognise only the species monocytogenes.

Listeriosis, the illness caused by consuming Listeria-contaminated foods like deli meats or cheese, is claimed to lead to higher rates of hospitalisation and mortality than any other foodborne illness.

According to the Centers for Disease Control and Prevention, approximately 2500 people develop listeriosis every year, and approximately one in every five cases is fatal. The elderly, pregnant women, newborn infants and individuals with compromised immune systems are most at risk of contracting the disease.

The bacteria classified as Listeria include six different species, but only L.monocytogenes can infect humans. This makes it especially important to develop highly selective sensors that can detect only L.monocytogenes.

The sensor is also selective enough to recognise cells of L. monocytogenes when other types of foodborne contaminants, such as salmonella or E.coli, are present.

Known as an 'optical biosensor', the device uses light to detect the presence of a target organism or molecule.

The sensor is made of a small piece of optical fibre coated with an antibody, which specifically recognises L.monocytogenes and captures it, binding it to the fibre. When the fibre is placed in a liquid food solution, any L.monocytogenes in the sample will stick to the fibre.

The presence of L.monocytogenes is verified by the addition of a second antibody, which not only recognises L.monocytogenes but also carries a molecule that produces a fluorescent glow when exposed to laser light. This antibody attaches to the L.monocytogenes bound to the fibre and acts as a flag, signalling the pathogen's presence when laser light is passed through the liquid.

Bhunia expects the sensor to be ready for industrial use in another year.

Many tests currently in use require a high concentration of pathogen cells. The tests also rely on 'enrichment', which occurs when a sample believed to be contaminated grows for a period of time in a nutrient broth to allow any pathogen cells present to multiply.

The enrichment process increases the concentration of cells present, making it possible for today's sensors to detect their presence, but it can take as long as seven days to complete a test using conventional methods.

Other tests rely on DNA markers, but these can also take days to process. That's a problem because by the time test results come back, products may already be in food suppliers' warehouses or on store shelves.

The ability to detect L.monocytogenes at low levels is essential because most of the foods susceptible to Listeria contamination are ready-to-eat products, which are cooked or otherwise processed for human consumption before they make it to a grocer's shelves. Detection at low levels also is important because Listeria can grow at refrigeration temperatures. So if a product has a level of Listeria low enough to evade detection when it's tested at the processor, that Listeria still can grow in the home refrigerator to a level that makes it infective to people at risk.

Cooking would kill many of the L.monocytogenes cells that can grow at refrigeration temperature, but many read-to-eat products, such as deli meats, smoked fish, cheeses and hot dogs, aren't always cooked by consumers before consumption.

Bhunia said his next goal is to optimise the test conditions of the biosensor so a sample can be processed in one working day and be monitored remotely via computer.

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