As is clear from the definition above, at the heart of every biosensor is a biological entity, the purpose of which is to react with the target analyte(s) and generate a readily quantifiable signal. Whereas traditional biosensors are based on the unique specificity of enzymes to their substrates, antibodies to antigens or that of nucleic acids to their complementary sequences, we have been promoting the use of a different concept, that of whole cell biosensors. An intact live cell, containing a selected gene promoter fused to a reporter gene, serves both as the sensing and the reporting element; the specificity of the system is controlled by the choice of gene promoter, and the type of signal generated (and hence the hardware needed for measuring it) is dictated by the choice of reporter systems.

The small size requirements, rapid responses and sensing versatility of bacterial-based whole-cell biosensors allow their convenient and efficient adaptation for laboratory as well as field applications, for environmental, pharmaceutical, security and industrial uses. The relative ease by which molecular sensing and reporting elements can be fused together to generate dose-dependent quantifiable physical responses (luminescent, fluorescent, colorimetric, electrochemical) to pre-determined conditions allows the construction of diverse classes of sensors. Over the last two decades, we have employed this principle to design and construct microbial bioreporter strains for the sensitive detection of either:

(a) Specific chemicals (e.g. TNT or DNT)
(b) Groups of chemicals sharing either chemical characteristics (e.g. heavy metals) or biological activity (e.g. oxidants)
(c) Global biological effects such as general toxicity or genotoxicity.
 

In many of these cases, additional molecular manipulations beyond the initial sensor-reporter fusion may be highly beneficial for enhancing the performance of the engineered sensor systems.