Single-celled organisms generally exhibit two distinct modes of behavior. The first is the familiar free floating, or planktonic, form in which single cells float or swim independently in some liquid medium. The second is an attached state in which cells are closely packed and firmly attached to each other on a solid surface. The change in behavior is triggered by many factors, but prominent among these is the quorum sensing mechanism. When a cell switches modes, it undergoes a phenotypic shift in behavior in which large suites of genes are up- and down- regulated.
Formation of a biofilm begins with the attachment of free-floating microorganisms to a surface. These first colonists adhere to the surface initially through weak, reversible van der Waals forces. If the colonists are not immediately separated from the surface, they can anchor themselves more permanently using cell adhesion molecules such as pili. The first colonists facilitate the arrival of other cells by providing more diverse adhesion sites and beginning to build the matrix that forms the basis of a biofilm. Some species are not able to attach to a surface on their own but are often able to anchor themselves to the matrix or directly to earlier colonists. It is during this colonization that the cells are able to communicate via quorum sensing. Once colonization has begun, the biofilm grows through a combination of cell division and recruitment. The final stage of biofilm formation is known as development, and is the stage in which the biofilm is established and may only change in shape and size.
Biofilms are usually found on solid substrates submerged in, or exposed to, some aqueous solution. Given sufficient resources for growth, a biofilm will quickly grow to be macroscopic and can ultimately contain many different types of microorganism - e.g. bacteria, fungi, protozoa and algae - each group performing specialized metabolic functions. However, some organisms will form monospecies films under certain conditions.
Bacteria living in a biofilm usually have significantly different properties from free-floating bacteria of the same species, as the dense and protected environment of the film allows them to cooperate and interact in various ways. One benefit of this environment is increased resistance to detergents and antibiotics, as the dense extracellular matrix and the outer layer of cells protect the interior of the community and compounds are secreted which actively disrupt host defense mechanisms.
Biofilms are very widespread and can tolerate extreme conditions - e.g. from the hot, acidic pools of Yellowstone National Park to the glaciers of Antarctica - and this further renders biofilm growth problematic in many healthcare, domestic and industrial environments.
Thus, biofilm is a significant contributor to mortality or life-threatening morbidity in conditions such as chronic lung disease, endocarditis or chronic wounds, and causes major costs to the healthcare industry in infections related to medical device use. Bacterial biofilms contribute significantly to the growth of dental plaque and to microbial keratitis associated with contact lens use. In the home, biofilms contaminate food preparation surfaces, reduce efficiency in air conditioning systems and block waste water systems. In industries such as oil- or water- or food-processing, biofilm development is a cause of corrosion, blockage, efficiency loss and bacterial contamination, which leads to atttendant high costs in maintenance, downtime and attempted disinfection.