Establishing the lower bacterial concentration threshold for different optical counting techniques

This work compares several physical and optical techniques used in fundamental research and industrial applications to detect bacteria in water. Optical techniques such as, UV-absorbance spectroscopy, laser particle counting, turbidimetry and Z-Sizer light scattering, and a direct observational physical technique, the plate count method, were compared when measuring the concentration of E.coli in tenfold dilution from a stock solution. Estimates of the detection threshold limit of E.coli for the different optical counting techniques and the relationship between colony-forming units (CFU) and tenfold dilutions was established. Optical methods have generated interest due to the rapid response of just minutes, non-destructive approach and minimal sample preparation but their use is still limited to concentrations of up to 4 Log E.coli/mL. In contrast, the plate count method is still a reliable technique for water quality analysis despite its long response time of 24 h.