Because the sensor uses mammalian cells it only returns a positive result if the bacteria or toxins are active, and not if they have been denatured, meaning that unlike other biosensors it only signals when the pathogens pose a real threat.
The sensor, developed by researchers from Purdue University in the USA, consists of multi-well plates, each containing mouse lymphocytes suspended in collagen gel. Each well can test one sample, and there are typically 96 wells in a plate, so the technology is easily scaled to test many samples at one time.
The sensor works by detecting when a toxin or bacteria has broken the cell wall of the lymphocytes. This releases an enzyme - alkaline phosphatise - that can be marked with another chemical to produce a yellow product, which can then be detected using a spectrometer.
The higher the quantity of the toxin or bacteria, the greater the number of lymphocytes killed in this way, and the more intense the yellow colour in the well. The spectrometer can detect the exact intensity of the colour of the well, and through careful calibration it can estimate the quantity of the pathogen in the sample, which can then be used to estimate the extent of the risk.
Lymphocytes are a key part of the body's defence against such pathogens, but according to Arun Bhunia, one of the researchers, this was not the most important factor when choosing the cells to form the biosensor.
"These cells are very sensitive to toxins so we can get results very quickly….We liked these cells as they have these enzymes in abundant quantities," he reported in an interview. "It's coincidental that these cells are also part of the immune system."
So far the sensor has detected small quantities of Bacillus species and Listeria monocytogenes - a potent killer. In addition to its speed and high sensitivity to low concentrations of toxins (with results comparable to PCR and antibody based methods), the technique's key advantage is its reliance on the death of mammalian cells, so it is very selective in signalling the presence of active, but not passive, toxins.
"You can immediately determine if the toxin will cause harm to humans," said Bhunia.
Despite promising early results, the sensor will still need a lot of development before it can be used on a large scale.
"One of the limitations is that mammalian cells need a difficult growth environment. The cell life is limited, meaning it must be used within five or six days. It can be made in the lab, but it must be immediately delivered," says Bhunia. "We are trying different approaches to extend the cell life - one step could be to freeze the cells."
The researchers are also investigating whether the sensor could make use of different types of cells apart from mouse lymphocytes, which would be sensitive to other types of pathogens.
