Biofilms are mixed communities of different kinds of bacteria and other microbes that are attached to a surface and to each other, forming a multilayer conglomerate of cells and intracellular material. Usually there is a "pioneer" colonizer, a bacterium that initially attaches to a surface, such as a tooth or the lung tissue (figure 6.8). Other microbes then attach either to those bacteria or to the polymeric sugar and protein substance that inevitably is secreted by microbial colonizers of surfaces. Organisms in a biofilm may start to communicate and behave as a functioning group instead of in the single cell planktonic form. Group communication allows for the division of labor allowing some cells to perform functions while other are doing something completely different. In many cases, once the cells are attached, they are stimulated to release chemicals that accumulate as the cell population grows. By this means, they can monitor the size of their own population. This is a process called quorum sensing. They can also communicate with neighboring and different species generating a large interactive community.