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which at present is expensive and bulky. The ability to extract a wide range
of information by appropriate staining procedures is very attractive. While
very useful in cell sorting, rare cell detection remains challenging. Sample
processing is again very important in preventing system blockages and also
in prestaining the pathogen of interest. SPC offers one potential solution.
Spectroscopic techniques avoiding the use of any labels have an imme-
diate advantage in terms of simplified sample preparation (no need for any
staining or rinsing steps) and reduced reagent cost. In some cases relatively
high LODs have been reported, although spectra from single cells have
been recorded, showing the potential for single cell “fingerprinting”. These
noninvasive methods can also be used for subsequent testing, if required.
There is also the potential for these methods to provide highly detailed
information allowing identification to the strain level as well as the ability
to distinguish between viable, nonviable and temporarily damaged bacteria.
However, there are several major challenges with this technology.
Most importantly, a spectral library is needed to characterize the results
of numerous pathogen screens, enabling identification of an unknown sam-
ple. More evidence is also required to determine the impact of environment
factors on the spectroscopic results. Some studies claim a high degree of
sensitivity to, for example, the age of oocysts, with this presented as evidence
of the high-quality information delivered by this approach. Others report
robustness of detection between samples with different environmental his-
tories, claiming this proves the technique can cope with a wide variety of
input waters. Finally, instrument miniaturization would also be beneficial to
spectroscopic techniques, as would the design of new instruments capable
of faster sample scanning. With some setups acquiring a spectrum can be
very time-consuming, and work is ongoing to reduce acquisition times.
Overall it seems as though there are several promising optical technolo-
gies, although for all of them instrument design and miniaturization are the
key to more widespread acceptance in the water quality monitoring arena.
REFERENCES
1. Wang Z, Guo W, Li L, Luk'yanchuk B, Khan A, Liu Z, et al. Optical virtual imaging at
50 nm lateral resolution with a white-light nanoscope. Nature Communications 2 :218.
2 . Robertson LJ, Gjerde BK. Cryptosporidium oocysts: challenging adversaries? Trends in
Parasitology 2007; 23 :344-7 .
3 . Iturriaga R, Zhang S, Sonek GJ, Stibbs H. Detection of respiratory enzyme activity in
Giardia cysts and Cryptosporidium oocysts using redox dyes and immunofluoresce tech-
niques. Journal of Microbiological Methods 2001; 46 :19-28 .
4 . van de Linde S, Heilemann M, Sauer M. Live-Cell Super-Resolution Imaging with
Synthetic Fluorophores. Annual Reviews of Physical Chemistry 2012; 63 :519-40 .
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