Individual cellulose fibers associate into bundles to form a 3D matrix for the cells, as you can see in this thin slice through a Gluconacetobacter hansenii biofilm. Not all biofilms contain cellulose, but all have an extracellular matrix, typically made of proteins secreted by the component cells. These proteins have properties that help cells aggregate, like the unipolar polysaccharide you just saw. They may also have other properties useful to the community; for instance, Bacillus subtilis secretes a hydrophobic protein that forms a resistant coat around the biofilm. Matrices also contain lipids and extracellular DNA, and dead cells contribute material to the superstructure. Keep in mind that the lab-grown biofilm you see here consists of a single species, but in nature a biofilm often contains multiple species.
