Complex Flocculation of Biofilm-forming Bacteria in the Presence of Flocculating Bacteria Isolated from Activated Sludge

We demonstrated that complex flocculation of biofilm-forming Pseudomonas aeruginosa PAO1 and Bacillus cereus SM-5 occurred in the presence of floc-forming Pseudomonas oryzihabitans AO-363 isolated from activated sludge. Laser diffraction analysis showed a drastic decrease in the detected frequency of planktonic PAO1 cells (1.3 μm) after they were mixed with AO-363 flocs. In addition, the most frequent diameter of SM-5 flocs shifted from 152 to 175 μm after mixing with the AO-363 culture. Complex flocculation was also supported by analysis of the bacterial composition of the floc fraction obtained by low-speed centrifugation. Such complex microbial flocs could contribute to the development of biofilms on polystyrene surfaces. When AO-363 was co-cultured with PAO1 or SM-5, the amount of biofilm formation increased by 3-4 times compared to that in the absence of the floc-forming AO-363 cells. These results suggest that microbial flocs containing extracellular polymeric substances (EPS) could increase the development of biofilms that contain bacterial colonies and the EPS components.