Biomedical Engineering Reference
In-Depth Information
attached to magnetic beads by antigen-antibody coupling and sorted with a magnet.
This example illustrates the current efforts to move and control populations of live
cells, isolate them, and quantify their amount and their characteristics.
Live cells can also be used as sensors: in cell-based biosensors, the cell itself
is the detector. For instance, some cells are extraordinarily sensitive to particular
chemicals and can be used as sensors for traces of these molecules. In others cases,
if a drug has to be tested on a particular organ, the response of cultured cells of
this organ are monitored by electrical or optical means and, although the details
of the response are not always fully understood, they directly carry the biological
information [10].
Cells arrays take advantage of the high parallelism of the microarrays technol-
ogy. They consist of arrays of live cells or cells clusters that are each transfected
with a different gene. The response of these different cells to external stimuli can
then be monitored in parallel [11] for instance, by monitoring the expression of a
GFP-labeled protein (see Section 8.2.1.2) or by measuring their electrical response
using microelectrodes or patch-clamp techniques.
8.1.2.2 Bacteria
Bacteria are smaller than eukaryotic cells (typically a few
micrometers vs. a few
tens of micrometers).
The bacteria family is very diverse; they display a wide range of morphologies
and their mode of locomotion varies from one species to the next.
Escherichia coli
for instance is a flagellated bacterium that swims by a succession of “runs” and
“tumbles.”
Bacteria cell walls consist of peptidoglycans; two families are often distinguished
according to the properties of this cell envelope: Gram (+) bacteria have a thicker
cell wall compared to Gram(-) bacteria.
Although very different from eukaryotic cells, some of the practical situations
dealt with these cells are very similar to concerns mentioned earlier. For instance,
the fight against bioterrorism has emphasized the importance of checking for the
presence of a few virulent bacteria within relatively large samples.
Bacteria are also used by biologists to produce useful molecular tools. Since the
genome of some of them have been entirely sequenced, it has now become a routine
work to modify it using molecular biology tools for instance by inserting exogenous
genes that can express particular proteins. These bacteria (and
E. coli
in particular)
are thus literally transformed in protein factories.
8.2 Microparticles as Biotechnological Tools
Synthetic microparticles find a natural use either as a macroscopic “handle” for the
manipulation of molecules or cells, or to add or to enhance a signal in the various
detection schemes. This last category includes immunofluorescence or immuno-
electron microscopy in which a fluorescent dye or a metal colloid is coupled to an
antibody that specifically targets the molecule of interest.
