English: Two side-by-side tubes show growth of Zoogloea resiniphila, a bacterium found in activated sludge wastewater treatment plants, with (left) or without (right) the ability to form floc, a specialized type of biofilm that forms suspended in water rather than on a surface. The tube on the left contains wild-type cells. The tube on the right is turbid because of planktonic growth by a mutant strain of the same species. Cells in that tube are unable to organize into flocs because of knock-out mutation of a regulatory system that controls expression of proteins needed for floc formation.