Biotechnology and Bioinformatics (1+1)

6.5.1. Introduction

A biosensor is an analytical device incorporating a bio recognition element intimately associated with or integrated within a transducer that converts the biological response into an electrical signal.

A great variety of detection techniques can be included in this definition.

• The biological response could be anything from enzyme activity or antibody /receptor binding to cell responses.
• The transduction to an electrical signal could also be diverse.

These techniques have been adapted to detect analytes of interest based on the interaction with or functionality modification of a biological target, which could be nucleic acids, enzymes , antibodies, receptors, cell organelles or whole cells. The specificity of the detection is determined by the biological component of the method.

For example, a method based on binding to a specific antibody would be very specific, however whole cell-based biosensors usually lack that degree of specificity and that characteristic could be used as an advantage in a broad-spectrum detection/monitoring technique. The sensitivity, on the other hand, as well as the portability of the device, depends on the signal transducer.

Biosensor assays may have mainly two designs, a direct or an indirect format.

• The direct format is based on the detection of analyte binding to a target or being cleaved by an enzyme, for example.
• In the indirect format an additional reaction has to occur in order to detect the analyte, for example the analyte may inhibit the interaction of the biological target with a “reporting element”.

Indirect assays are often used in food analysis because they usually display lower interferences with complex matrixes. Actually, sample preparation is commonly a critical step in method development when working with food samples due to their complexity, and seafood is not an exception. Therefore, attention should be paid not only to the efficiency and sensitivity of the biosensor assay but also to the sample preparation procedure.

Biosensors refer to intact, living cells that have been genetically engineered to produce a measurable signal in response to a specific chemical or physical agent in their environment. Biosensors contain two essential genetic elements, a promoter and a reporter gene. The promoter is turned on when the target agent is present in the cell’s environment. The promoter in a normal cell is linked to other genes that are then likewise transcribed and then translated into proteins that help the cell in either combating or adapting to the agent to which it has been exposed. In the case of a biosensor, these genes, or portions thereof, have been removed and replaced with a reporter gene. Consequently, turning on the promoter now causes the reporter gene to be turned on. Activation of the reporter gene leads to production of reporter proteins that ultimately generate some type of a detectable signal.

Therefore, the presence of a signal indicates that the biosensor has sensed toxic levels of a particular target agent in its environment. Biosensors can be designed to signal levels of pollutants toxic to prokaryotic or eukaryotic systems in general by selecting appropriate host organisms. Bacterial and zebrafish biosensors have been designed and are being used currently as prokaryotic and eukaryotic biosensors, respectively.