In the last decade, molecular biology has made significant progress, and innovative molecular methods have become available to analyze microbial community structures. Among them, we applied the FISH (Fluorescent in situ Hybridization) method to analyze activated sludge in wastewater treatment plants (WWTPs). As a result, domain- or division-level community structures in activated sludge were determined successfully without cultivation. We also applied the PCR (polymerase chain reaction) -DGGE (Denaturing Gradient Gel Electrophoresis) method for laboratory nitrifying sludge in order to investigate more detailed microbial community structure. By this method, genus- or species-level community structures were characterized well. This method was also found to be powerful for monitoring the change of microbial community structures. For example, the behavior of Nitrosomonas group was successfully detected in the reactor with nitrification by the PCR-DGGE method.

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