Bacterial auto-aggregation and co-aggregation in activated sludge

Microbial auto-aggregation and co-aggregation, leading to biofilm or floc formation, widely occur in natural systems. This study was conducted to investigate the extent of bacterial auto-aggregation and co-aggregation in activated sludge and their contributions in activated sludge floc formation. In this study, an activated sludge sample was separated into a supernatant part (planktonic community) and a sediment part (floc community) by centrifugation at 650xg for 2 minutes. 197 strains were isolated from both communities. These isolates belong to 18 phylotypes based on molecular identifications. Similar phylogenetic distributions of these isolates were observed in planktonic and floc communities, suggesting a similar community structure exists in these communities. 1 strain was picked from each phylotype to conduct further characterization on auto-aggregation and co-aggregation. Three types of bacterial auto-aggregation patterns were observed: (1) aggregating bacteria (F): bacteria have constant high aggregation abilities. (2) bacteria with varying aggregation capability at different growth stage (VF). (3) Non-aggregating bacteria (NF): bacteria do not aggregate. The VF type bacteria are the most dominant one. The results of co-aggregation characterization indicated that co-aggregation relationship between bacteria in activated sludge is complex, varying at different growth time. Strain S13, which is identified as Acinetobacter johnsonni, has a negative effect on the aggregation of other strains.