Biology Reference
In-Depth Information
with particular focus on the testing of water samples and on whole-bacteria
detection. Where the information is available, the performance characteris-
tics of the biosensors e.g. LOD, assay time, and specificity, are given. More
consideration is given to
E. coli
, with this first section illustrating typical
results obtained for the different types of biosensors.
7.5.1.
Escherichia coli
O157:H7
Comprehensive reviews of biosensors for this pathogen were published in
2008 for food
52
and as a topic chapter in 2011 for water.
45
Many different
optical, electrochemical, and mass-sensitive biosensors have been developed
and optimized, with most work being done using the mass-sensitive meth-
ods. A comparison of the LOD achieved by optical and electrochemical
transduction methods is shown to be relatively similar, with both methods
achieving an LOD of 10
2
cells mL
−1
. However, with recent developments
in mass-sensitive techniques, the limit of single-cell detection is reported to
have been achieved.
In 2003, a commercially available (Analyte 2000, Research International)
fiber optic waveguide biosensor was applied using antibodies to capture the
bacteria and fluorescent labels for detection. Positive samples were then
subjected to further analysis by traditional culture or molecular methods.
An alternative antibody-based indirect sensor with fluorescent labeling
was reported by Ho et al. in 2004. In this approach, capture antibodies were
immobilized on the interior surface of a microcapillary through which the
test sample was subsequently flowed. Next, liposome secondary antibody
conjugate was passed through the capillary, followed by a rinse to remove
any unbound conjugate. The final detection step involved the lysis of these
liposomes to release the encapsulated fluorescent molecules, giving an LOD
of 360 cells mL
−1
in 45 min. Liposomes can encapsulate 10
5
-10
6
fluorescent
molecules, thus offering a means of signal amplification that resulted in the
low LOD reported.
Another optical biosensor approach taken by Song and Kwon was a
photodiode array, using a secondary antibody conjugated to alkaline phos-
phatase, giving an LOD of 10
4
cells.
53
Signal detection depends upon the
action of the enzyme producing a blue precipitate, the absorbance of which
is detected. This sensor is very sensitive to pH.
Other optical biosensor technologies employed for
E. coli
O157:H7
have utilized optical waveguides and/or optical fibers. In 2003, Horvath
and coworkers employed a reverse symmetry waveguide sensor for
E. coli
detection with an LOD of 10
6
cells mL
−1
.
54
In 2005, Rijal et al. reported
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