Denaturating gradient gel electrophoresis (DGGE) and fluorescent in situ hybridization (FISH) as a tool for monitoring temporal changes in bacterial community structure in activated sludge

Wastewater treatment based on activated sludge is known to be one of the most effective and popular wastewater purification methods. Activated sludge, as a mixture of microorganisms, is an excellent research material both for microbiology and technology, resulting in finding a method to effectively utilize different chemicals. An estimation of microbial community changeability in activated sludge allows us to observe the correlation between particular bacterial groups' appearance and the effectiveness of substantial removal of chemicals. This research is focused on microbial community temporal changes in membrane bioreactors' treating wastes containing a high level of ammonia nitrogen. Samples for this study were collected from two membrane bioreactors A and B with the activated sludge age of 12 and 32 days respectively. The activated sludge microbial community was adapted to the removal of ammonia nitrogen at the level till 0,56 g N-NH₄/d·g VSS. The methods - denaturating gradient gel electrophoresis (DGGE) based on 16S rRNA gene PCR products and fluorescent in situ hybridization (FISH) with 16S rRNA gene probes are the techniques that lead to an analysis independent of the possibility of pure bacterial culture cultivation. The usage of these methods revealed significant differences in the microbial community structure in both bioreactors, caused mainly by the age variety. According to the results obtained in this study bioreactor A (age of 12 days) is characterized by much higher community diversity than bioreactor B (age of 32 days). Interestingly the appearance of the particular species of nitrifying bacteria is steady during the experiment in both bioreactors. Changes occurs only in case of Nitrosomonas oligotropha lineage bacteria, which appear in bioreactor B in the final stage of the experiment, while in bioreactor A they are constantly present. The differences in community structure in bioreactor B is based mainly on the variety of the number of bacteria species other than nitrifiers present in the environment during time, while the newly appearing nitrifying species could be the cause of bioreactor A community changes. This study demonstrates that the bacterial community of bioreactors operated with different sludge ages differs significantly in the number of bacteria. Nevertheless these changes don't have influence on the bioreactors' performance.