average of 3.6 copies of 16S rDNA per cell in the total population and two copies of the amoA gene per
ammonia-oxidizing bacterial cell, the ammonia oxidizers examined represented 0.0033% 0.0022% of the total
culturable and nonculturable organisms in environmental samples, including AOB (16, 23, 31, 35, 45), although its use with
N.J.). The program used for the ampliﬁcation was as follow: 5 min at 94°C,
followed by 50 cycles consisting of 30 s at 94°C, 30 s at 62°C, and 30 s at 72°C and
values were 4.3 ( 2.0 × 1011 for bacteria, 3.7 ( 3.2 × 1010 for Nitrospira, 1.2 ( 0.9 × 1010 for all AOB, and 7.5 ( 6.0 × 109 for N. oligotropha-like AOB. 
The percent of the nitrifying population was 1.7% N. oligotropha-like AOB based on the N. oligotropha amoA assay, 2.9% total AOB based on the AOB 16S rDNA assay, and 8.6% nitriteoxidizing bacteria based on the Nitrospira 16S rDNA assay. 
Ammonia-oxidizing bacteria in the wastewater treatment plant were estimated to oxidize 7.7 ( 6.8 fmol/hr/cell based on the AOB 16S rDNA assay and 12.4 ( 7.3 fmol/hr/cell based on the N. oligotropha amoA assay.
Standards consisted of the plasmid pCR2.1 carrying the M-20 amoA gene (GenBank accession number AF420299) (9) adjusted to 10 to 1.0 × 107 copies per PCR. PCR amplification consisted of 3 min at 50 °C, 10 min at 95 °C, 55 cycles at 95 °C for 30 s, 56 °C for 60 s.
