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2.3.3.1.2 Detecting biological contaminants

According to the European Food Safety Authority there were a total of 5,311 foodborne outbreaks, involving 47,251 people and resulting in 5,330 hospitalizations and 24 deaths in 23 Member States during 2005, the majority caused by Salmonella and Campylobacter[i].  Many of these illnesses are caused by bacterial enterotoxins, which are not easily removed from food, as they are often stable at temperatures used in normal cooking.  To combat this, it is of critical importance to be able to detect food spoilage through bacterial, fungal or viral contamination at each stage in the food processing industry. 

This is a major market, with an estimated 558 million tests performed each year, worth 1.45 billion euros.  More than 90% are performed by service laboratories; however the use of rapid test kits is increasing.  70% of all tests are for Salmonella and Listeria⁷³.

At present such methods are largely based on classical immuno- (e.g. ELISA) or DNA (e.g. PCR) assays, which require some sample preparation and have a turnaround of a day or two.  Although this is much quicker than other techniques (such as isolation and cultivation of microbes), there is still scope for greater sensitivity and faster detection times.  The key drivers are lower detection limits, real-time detection, higher throughput, and discrimination between different species.

Most systems detect microbial components, rather than intact cells.  Protein detection systems are favoured, as this increases the probability that the intact microbe is present and also screens for the presence of important bacterial enterotoxins and fungal mycotoxins (which can be present in the absence of viable microbes, and are responsible for significant illnesses).  In general, such systems must be able to detect the presence of 10-100 infectious particles per ml.  There are various biosensor platforms in development which are based on nanostructured materials, while there are large amounts of research on the development of electronic platforms (principally amperometric, but also voltametric and impedance) there are also efforts in the area of optical and mass change detection.  In each case the nanostructured material is decorated with biomolecules capable of interacting specifically with the target analyte.  This interaction is transduced by the nanomaterial into a quantifiable signal:

• electronic biosensors, based on protein conjugated nanowires[ii]^,[iii], and carbon nanotubes[iv].  These directly quantify the presence of specific analytes (e.g. proteins, nucleic acids, metabolites) which directly or indirectly indicate the presence of the microbe.  As the output is an electrical signal, such platforms have the potential to be linked to other devices allowing data to be transmitted, shared and analysed further.  By virtue of the nanoscale dimensions these demonstrate much faster electron transfer rates than microelectrodes, which manifests as higher sensitivity.  CNTs have been combined variously with nanoparticles (e.g. gold or platinum nanoparticles or quantum dots) and polymer matrices to form composite materials with improved robustness and high porosity (facilitating entry of target biomolecules)⁷⁹.  Such composite electrodes exhibit even greater sensitivity. 
• optical biosensors, with readout by a number of different techniques including surface plasmon resonance (SPR), fluorescence, colourimetric changes and based on a number of biomolecule-conjugated platforms including CNTs[v]; silica[vi], gold[vii]^,[viii], and latex[ix] nanoparticles.
• mass-change biosensors, based on cantilever arrays, and piezoelectric devices[x]^,[xi].  Binding of analyte to the conjugated biomolecule results in changes in the resonant frequency of the nanomaterial, which is directly proportional to the amount of target bound, and can be read by, for example deflection of a laser beam.  

Most of these technologies are still at the level of basic research; however Biophage Pharma Inc, in collaboration with NRC-Biotechnology Research Institute, has developed electronic biosensors capable of discriminating between different bacteria (in a process termed Electric Cell-Substrate Impedance Sensing, or ECIS).  This is now at the pre-commercialisation stage and is expected to have applications for the detection of bacteria in water, food, and biological fluids[xii].