Fig. 1: The biofilm life cycle, 1: individual cells populate the surface. 2: EPS is produced and attachment becomes irreversible. 3 & 4: biofilm architecture develops and matures. 5: single cells are released from the biofilm.
A biofilm is a complex aggregation of microorganisms growing on a solid substrate. Biofilms are characterized by structural heterogeneity, genetic diversity, complex community interactions, and an extracellular matrix of polymeric substances.
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 holds the biofilm together. Only some species are able to attach to a surface on their own. Others are often able to anchor themselves to the matrix or directly to earlier colonists. Once colonization has begun, the biofilm grows through a combination of cell division and recruitment.
Biofilms are usually found on solid substrates submerged in or exposed to some aqueous solution. Biofilms consist of many species of bacteria and archaea living within a matrix of excreted polymeric compounds. This matrix protects the cells within it and facilitates communication among them through chemical and physical signals. Some biofilms have been found to contain water channels that help distribute nutrients and signalling molecules. This matrix is strong enough that in some cases, biofilms can become fossilized.
Bacteria living in a biofilm can have significantly different properties from free-floating bacteria, 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.
Biofilms are common in nature, as bacteria commonly have mechanisms by which they can adhere to surfaces and to each other. Dental plaque is a biofilm. In industrial environments, biofilms can develop on the interiors of pipes and lead to clogs and corrosion. In medicine, biofilms spreading along implanted tubes or wires can lead to pernicious infections in patients. Biofilms on floors and counters can make sanitation difficult in food preparation areas.
Biofilms can also be harnessed for constructive purposes. For example, many sewage treatment plants include a treatment stage in which waste water passes over biofilms grown on filters, which extract and digest harmful organic compounds.
Dental plaque is a yellowish biofilm that build up on the teeth. If not removed regularly, it can lead to dental caries.
A majority of bacteria in natural and clinical settings are contained in biofilms, which are surface-mounted, integrated communities of cells. Biofilms are highly structured and physically dynamic, with their structure and mechanical properties defined by extracellular polymeric substances (EPS), which serve as a scaffolding or glue holding the biofilm together.
The life cycle of a biofilm is characterized by attachment of planktonic bacteria to a surface or by migration or division of sessile cells to cover an empty region of the surface, production of extracellular polymeric substances (EPS) to adhere cells irreversibly to the substrate, and then by additional EPS production , cellular motility and reproduction, and phenotypic differentiation to produce a mature, thick and spatially structured biofilm.
In EPS production and in many other ways, bacteria in biofilms are phenotypically distinct from their genomically-identical planktonic counterparts. Bacteria in biofilms can be up to 1000 times more resistant to antibiotics, and less conspicuous to the immune system, because antigens are hidden and key ligands are suppressed.
The majority of these phenotypic differences, and of the structural and dynamic properties of biofilms, arise from complex, dynamic patterns of intercellular interaction and signaling that are not present for planktonic cells. Other characteristics seem to arise from interaction with a surface alone. Therefore, it is impossible to understand biofilms solely from studies of planktonic cells; systematic studies of the cooperative properties of self-cohering and of surface-bound prokaryotes, in and out of biofilms and in the process of biofilm formation, are essential.