Hydroxyapatite powder cake filtration reduces false positives associated with halophilic bacteria when evaluating Escherichia coli in seawater using Colilert-18.

Escherichia coli is an important fecal indicator bacterium that is used to evaluate the microbiological quality of water. The Colilert-18 (Quanti-Tray/2000) is a widely used, rapid, and simple quantitative method for detecting E. coli in drinking water, bathing water, and wastewater. However, Colilert-18 method is less reliable for seawater; false positives are often caused by halophilic bacteria such as Vibrio. While false positives can be avoided by diluting the sample by 10 times or more, the resulting decrease in detection limit makes it difficult to quantify E. coli in seawater. In this study, we combined cake filtration, using hydroxyapatite powder, with the Colilert-18 method to remove salinity without diluting the water sample. When quantifying E. coli in river water, the E. coli concentration obtained from the cake filtration/Colilert-18 method showed a high quantitative value of 90% or more, on average, compared to the concentration obtained with the original Colilert-18 method. The E. coli concentrations in seawater determined using the developed method were similar to those determined using the modified m-TEC method, with no false positives. Highly reliable quantitative values can be obtained using the proposed method because it is possible to measure 100 times as much sample compared to the dilution method. Thus, the developed method is expected to be a powerful tool that can eliminate the problem of false positives.