qPCR units to cell count units

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emmafiori's picture
qPCR units to cell count units

Hello all,
I am doing the EPA's enterococcus in seawater assay, and trying to compare to results to traditional culture techniques (Idexx's Enterolert). I'm getting horrible results in many ways, and one of the places I think might be a problem is "converting" or comparing qPCR units (pg/ul or copies) to MPN (most probable number) or cells/ml.
FYI, this is for beach monitoring, and so most samples have fairly low cells counts, unlike most papers out there on the topic which use sewage samples for testing, and obviously have much higher cell counts.
What I've been doing so far, is setting the qPCR settings for the standard to "copies", and then dividing the results by 4, which is how many copies of the target gene are in the particular strain of E. faecalis that EPA suggests using as a standard. But that is giving me crazy results when I compare (for example, and sample that had an Enterolert results of 86 cells/100ml had a qPCR result of 2.2x10^5 "cells/100ml" supposedly after my calculations.
Any idea what I'm doing wrong? Or am I doing the calculations right, but my qPCR reaction is still just terribly off?