sequences allows for the design of PCR primer sets that specifically amplify the genes encoding the enzyme from DNA samples extracted from the natural environment (Henckel et al. 1999, Meyer et al. 1999, Mesarch et al. 2000). Although the PCR method is both specific and sensitive, such standard reactions are not quantitative. To obtain quantitative data from PCR-based analyses, statistical methods based on most probable number (MPN) estimations have been used (Wand et al. 1997). In MPN-PCR, DNA extracts are diluted before PCR amplification and limits are set on the number of genes in the sample by reference to known control dilutions. Another way to quantify PCR-amplified products for comparison is to include an internal control in the PCR reaction (Leser et al. 1995, Mesarch et al. 2000). Here, a known amount of target DNA is added to a PCR reaction containing DNA from the mixed microbial population. The known target DNA is complementary to the same primers and thus competes with the target sequences in the extracted DNA sample. By preparing a dilution series of the known and unknown DNA species, it is possible to quantify the amount of product produced from die complementary gene in the extracted DNA. The known DNA target can be generated by cloning the gene of interest or purifying the PCR-amplified product after which a deletion is introduced to give a differently sized PCR product. There exists many variations of the standard PCR technique. The sensitivity and specificity of the PCR may be improved by adopting a nested approach. The initial amplification is carried out with a pair of primers that are not organism-specific, whereafter a second round of amplification is conducted on the product using primers specific for an organism with target sites internal to the first primer pair (el Fantroussi et 1997, Levesque et al. 1997, Rheims and Stackebrandt 1999).