Optical Detection Technologies for Waterborne Pathogens - Waterborne Pathogens: Detection Methods and Applications - page 127

considerable variation between spectra from the same species, depending
upon the previously experienced environmental conditions.
An advantage of this technique over the traditional culture-based
approaches is that it has the ability to detect bacterial injury. For exam-
ple, sublethally injured bacteria may not be detected by other means, e.g.
culture-based test, but since they have the ability to repair themselves and
resume growth under favorable conditions, a culture method may give a
false negative result whereas IR spectroscopy would detect them. These
authors employed Fourier-Transform-IR (FT-IR) spectroscopy in the
mid-IR range of the electromagnetic spectrum (4000-400 cm −1 ) and con-
centrated 50 mL of sample using aluminum oxide membrane filtration. In
contrast to other membranes, this aluminum oxide membrane contributes
no spectral features between 4000 and 1000 cm −1 thus allowing direct entry
to the FT-IR system. Better reproducibility with direct membrane capture,
as opposed to prior centrifugation, is also obtained. 40
FT-IR has also been used to study the impact of stress on Salmonella
as well as to distinguish between viable and nonviable E. coli among other
studies focusing on foodborne pathogens. 38 The approach could easily be
translated to waterborne pathogens. Bacterial IR detection has also been
performed in bottled drinking water samples. 41
ATR-FT-IR has been applied to study the surface properties of Cryp-
tosporidium oocysts. This article highlighted the large spectral differences
between oocysts from different sources (see Fig. 6 in the original article). 42
The technique has also been applied to study surface adhesion of oocysts. 43
5.2.2. Raman spectroscopy
The Raman effect, which is the inelastic scattering of light, was discov-
ered by Chandrashekhara Venkata Raman in 1928. When monochromatic
light shines on a sample, the majority of it passes through or is absorbed,
depending upon the nature of the sample and the wavelength of the light.
However, a small proportion (around 1%) of this light is scattered, either
elastically, with the same frequency as the incident light (Rayleigh scat-
tering), or inelastically, at a different frequency (Raman scattering) (see
Fig. 5.10 ). The sample gains (or loses) some energy from the light and thus
the frequency shift corresponds to vibrational energy shifts in molecules
within the sample. Raman scattering works for vibrations where a polariz-
ability change occurs. The resulting Raman spectrum is a “fingerprint” of
vibrational modes with a molecule, providing a uniquely identifiable “sig-
nature” (see Fig. 5.11 ). Raman scattering can also be used for imaging, and
an excellent review of this area is provided in Ref. 44 .