Each soil sample was tested in duplicates for amplifiable DNA
and for PCR inhibition by analyzing 2 ml of the 101 dilution of the
same Powersoil DNA template as used for the double assays. The
qualitative 16S TaqMan PCR assay was performed as described
previously (Woegerbauer et al., 2014). The cut-off threshold for a
positive 16S rRNA TaqMan assay was set at Cp 27. Samples with a
negative 16S TaqMan assay (Cp > 27) were excluded from statistical
analysis if the nptII and/or nptIII PCR results were negative to avoid
false negative results. Absolute quantification was performed using
an external standard which was prepared by a semi-logarithmic
dilution of a plasmid with an E. coli 16S gene insert (Ingenetix).
Except for using 2 ml of a 102 dilution of the original PowerSoil
total DNA eluate and testing the samples in quadruples the same
procedure as described abovewas followed for the quantitative 16S
rRNA TaqMan assay.