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.

Development of a Screening Method and Device for the Detection of Escherichia coli from Agri-Food Production Environments and Fresh Produce.

This study was conducted to develop a screening method using Colilert-18 and a device for the detection of E. coli from agri-food production environments and fresh vegetables. The specificity and sensitivity of Colilert-18 by temperature (37°C and 44°C) were evaluated with 38 E. coli and 78 non-E. coli strains. The false-positive rate was 3.8% (3/78) and 0% (0/78) at 37°C and 44°C, respectively. The detection limit of E. coli at 37°C at <1.0 log CFU/250 ml was lower than that at 44°C. The efficiency of the developed device, which comprised an incubator equipped with a UV lamp to detect E. coli in the field, was evaluated by measuring the temperature and UV lamp brightness. The difference between the set temperature and actual temperature of the developed device was about 1.0°C. When applying the developed method and device to various samples, including utensils, gloves, irrigation water, seeds, and vegetables, there were no differences in detection rates of E. coli compared with the Korean Food Code method. For sanitary disposal of culture samples after experiments, the sterilization effect of sodium dichloroisocyanurate (NaDCC) tablets was assessed for use as a substitute for an autoclave. The addition of one tablet of NaDCC per 50 ml was sufficient to kill E. coli cultured in Colilert-18. These results show that the developed protocol and device can efficiently detect E. coli from agri-food production environments and vegetables.