Real-Time PCR and Competitive PCR were applied to quantify 16S ribosomal and ammoniamonooxygenase (amoA) genes in the genome of ammonia-oxidizing bacteria. Two sets of PCR primer targeting the 16Sribosomal gene and one set targeting amoA gene were selected to implement both PCR-based quantification methods. Alinear response was observed over more than 3 orders of magnitude at real time PCR using a dilution series of Nitrosomonas europaea DNA, ranging from 10⁵ to 10² copy number of 16S ribosomal gene and from 10% to 0.01% tototal DNA, respectively. The DNAs extracted from nitrifying activated sludge sample contained 3.0×10⁴ and 1.3×10⁵ copies/ng in 16S ribosomal gene and 1.8×10⁵ copies/ng in amoA gene, that corresponded to 16, 43 and 55% to the genes at a N.europaea DNA equivalent. The number of amoA gene in the DNA extracted from the activated sludgeincreased with increase in the ammonia-oxidizing activities determined by batch experiments, while the abundance of16S ribosomal gene showed almost constant.

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