Many bacterial strains were isolated from samples of activated sludges at sewage treatment plants in Tokyo. These organisms were tested for their ability to produce flocs in lactose broth by shake culture method.
The floc-forming organisms isolated by us (S-32, S-43-2, S-63) were non-motile, Gram negative or variable coccoid cells (0.8 to 1.2 microns in diameter) and showed following characteristics: rounded: convex, opaque colonies (1 to 2mm in diameter) with narrow zones of slight hemolysis on blood agar; no acid or gas production from lactose and glucose; gelatin was not liquefied; nitrates were not reduced. Similar floc-producing organisms were also isolated from the film covering the stones taken from the trickling filter at Maebashi sewage treatment plant in Gunma Prefecture.
Spore-forming, gram-positive rods with marked ability to liquefy gelatin were also isolated from the activated sludge. They showed almost no floc-producing ability, but mixed shake culture with the floc-forming organism (S-63) gave excellent floc-production wite clear supernatant.