Sensing techniques form an integrated part of our modern life. We like to be accurately and
constantly informed about the quality, security and composition of products that we consume or
encounter in our daily life. Medical tests need to provide instantaneous answers on health parameters,
blood values or presence of potential pathogenic organisms. Industrial processes rely on constant
physical and chemical sensing of process parameters, system inflow or outflow. Sensors come in
thousand and more forms and shapes, principles and output. Future demand calls for further
miniaturization, continuous sensing, rapidity, increased sensitivity or flexibility.
One of the emerging domains in sensing technology is the use of living (microbial) cells or
organisms. Whereas this principle is arguable very old (for example, mine canaries were used in
Roman times to sense carbon monoxide), it is only since the last twenty years that living cell-based
sensing assays have gained impetus and developed into a scientific and technological area by itself.
The question we would like to discuss here is why one would use living cells and organisms for
sensing? What are the specific purposes for basing sensing methods on living cells and what are the
advantages that cellular-based sensing can have over other sensing techniques? In this overview we
will concentrate specifically on bacteria- (microbe-) based sensor (MBS) methods. We will shortly
rehearse the major design principles in MBS and give some examples of potentially useful applications
that have been achieved up to now. Furthermore, we will focus our attention on the concepts
bioavailability and bioaccessibility, which are useful to explain the central conceptual differences
between sensing based on living cells and other sensing methods.