We developed standard curves based on plasmids containing a 16S rRNA gene of a member of one of the three genera Dehalobacter, Desulfitobacterium, and Dehalococcoides. A large difference in amplification efficiency between the standard curves was observed ranging from 1.5 to 2.0. The total eubacterial 16S rRNA gene copy number determined in a sample DNA by using eubacterial primers and the three standard curves led to differences in the estimated copy numbers of a factor up to 73. However, the amplification efficiencies for one specific standard curve were the same independent of the PCR primer pair used. This allowed the determination of the abundance of a population expressed as fractional number, hence, the percentage of genus-specific copy numbers within the total eubacterial 16S rRNA gene copy numbers. Determination of the fractional numbers in DNA mixtures of known composition showed the accuracy of this approach. The average difference in threshold value between two 10-fold dilutions of DNA of pure cultures, mixtures thereof and of environmental samples was -3.45 +/- 0.34, corresponding to an average almost optimal amplification efficiency of 1.95. This indicated that the low amplification efficiency of certain standard curves seemed to be mainly a problem of the plasmid DNA used and not of the 16S rRNA gene of the target genera. (C) 2004 Elsevier B.V. All rights reserved.