  We introduce a new image cytometer design for detection of very small
particulate and demonstrate its capability in water analysis. The device is a
compact microscope composed of off the shelf components, such as a light
emitting diode (LED) source, a complementary metal oxide semiconductor (CMOS)
image sensor, and a specific combination of optical lenses that allow, through
an appropriate software, Fourier transform processing of the sample volume.
Waterborne microorganisms, such as Escherichia coli (E. coli), Legionella
pneumophila (L. pneumophila) and Phytoplankton, are detected by interrogating
the volume sample either in a fluorescent or label-free mode, i.e. with or
without fluorescein isothiocyanate (FITC) molecules attached to the
micro-organisms, respectively. We achieve a sensitivity of 50 cells/ml, which
can be further increased to 0.2 cells/ml by preconcentrating an initial sample
volume of 500 ml with an adhoc fluidic system. We also prove the capability of
the proposed image cytometer of differentiating microbiological populations by
size with a resolution of 3 um and of operating in real contaminated water.
