Biofilms are accumulations of many different bacterial species adhering to some form of extracellular matrix. they often form as a response to which is determined by the cell density.

They often respond to Quorum sensing which is determined by the cell density. Bacteria cells secrete molecules that can be detected by other bacteria, Quorum sensing allows bacteria to sense the concentration of these signalling molecules to monitor the local density of cells. It used by bacteria to coordinate certain behaviors , such as the production of biofilms.