WEF Discussion Forums
Laboratory Management and Technical Issues
E. coli by IDEXX
We are looking at an E. coli limit instead of fecal for our new permit so we have been running Colilert w/Quanta-Tray for a couple weeks now. What we found is that the E. coli values are much higher than the fecal values. This is not what we expected so we started cranking up the chlorine to find out how much extra chemical costs would be incurred by E. coli testing. Also we looked for literature on this subject and found out this is not an unusual occurrence to see E. coli higher than fecal, especially in using chlorine as a disinfectant. UV does not show this phenomenon as much. Any discussion or help on this subject?
Since E. coli is a subset of the fecal coliform, one would expect the values to be lower than the fecal coliform values, but this is not always reflected in the results of the two tests. Assuming that you are doing fecal coliform by membrane filtration (such as with mFC media), it is important to realize the additional stress being applied to the bacteria using that method. The bacteria are already stressed by being dosed with chlorine. They are then collected and held at a temperature of <6 C, so you add in temperature stress. They are then filtered, adding mechanical stress. Next, they are tossed in a water bath at 44.5 C, so you get additional temperature stress. With the Colilert method, the mechanical stress is eliminated, and the temperature stress is less severe due to the lower incubation temperature for the IDEXX method. Also, because the quantitrays are placed in an incubator instead of a water bath, the temperature increase is more gradual. This allows the stressed organisms to better survive the test procedure. Your E. coli values aren't really higher. Your fecal coliform values are underreported because the bacteria aren't able to survive the test procedure.
Gregg, the first thing I would do is to analyze the same sample with FC by MPN and filter technique to determine if you current results are biased low by not recovering the stressed organisms. When you are sure of the FC method results then you can compare to the e-coli method which should be slighly lower or the same as the FC. I would try a different e-coli method if the current mothod is higher than the FC.
The EPA method 1603 for E. coli using modified mTEC agar (membrane filtration) calls for a 2 hour incubation at 35 deg C prior to incubating at 44.5 deg C in an effort to reduce the heat stress and to resusciate any injured or stressed bacteria. The fecal method does not call for this so one could be losing some organisms as bsims pointed out.