Question 1:
I have difficulty correctly identifying fluorescing wells using IDEXX SimPlate for heterotropic plate count (HPC) method. What do you recommend for troubleshooting?

Answer 1:
The method can be tricky. With IDEXX SimPlate, you're looking for a chromogenic or actually a fluorogenic endpoint, and there is no comparator available for it. We recommend that you run a QC sample along with it. Another thing that will help you out is IDEXX sells a viewing box or cabinet (WCM10 – catalog #). We call it a dark box. Basically, you put the fluorescence lamp on the top of the box, and then you look in through a viewing port and you hold the sample inside the box, it makes it a little bit easier to read SimPlate results. Another thing that is good to do is to read it from the reverse side of the plate. For some folks, that is easier. And you can check the wells as you go.

Question 2:
Do you have suggestions on how to have a better count on fecal coliform MPN?

Answer 2:
If running a 15 tube test, we suggest running the additional series of tubes for more accurate results. When you're only doing the 15 tubes, you're limiting the range that you are covering, and also by doing that additional set of tubes, you add an additional level of accuracy to that test that you don't normally have.

Question 3:
For enzyme substrate methods, so you have any advice for incubation at elevated temperature for fecal coliform?

Answer 3:
We have seen that you need to be careful when you’re doing this because it’s very sensitive to temperature. We recommend using a water bath and definitely maintain the temperature tolerance prescribed in the method of plus or minus 0.2. If you get outside of that, it’s very possible that you can cook the organisms and they will not grow. One way to help you to avoid from cooking the bugs is, if you're using a water bath, use a circulating bath. Keep the trays off the bottom or off the walls. Make sure that the trays are incubated facing up and remember that you're looking for a color change only, you're not looking for fluorescence. These things will not fluoresce under those conditions. The reason we tell you to incubate them facing up is because the wells can trap water and that water can get hot and it can cook the organisms.

Question 4:
Why are CFU and MPN results assessed separately?

Answer 4:
Membrane filtration methods are not equivalent to MPN methods. If we were to evaluate them together, the standard deviation would be relatively large, which would possibly make any evaluation not appropriate. Evaluating like with like is best, so membrane filtration is evaluated with membrane filtration, and MPN is evaluated either with multi-well or multi-tube.