Biofilms are found in most natural environments and are probably the most common form of microbial existence. They are relatively stable, three-dimensional communities of microbial cells encased in a complex mixture of extracellular polymers. Biofilms normally form at interfaces after adherence of free-living (planktonic) cells, which grow and divide to form clusters. The cells generate chemical signals that aid communication between members of the same species. As the numbers of cells and signals within clusters increase and a critical level (or quorum) is exceeded, gene expression within the cells changes and biofilm development is initiated.