Biofilms

A biofilm is a population of bacteria, algae, yeast, or fungi that is growing attached to a surface. The surface can be living or nonliving. Examples of living surfaces where biofilms may grow include the teeth, gums, and the cells that line the intestinal and vaginal tracts. Examples of nonliving surfaces include

rocks in watercourses, and implanted medical devices such as catheters.

Rudimentary knowledge of the presence of biofilms has been known for centuries. For example, the bacterium Acetobacter aceti attached to wood chips has been used to manufacture vinegar since the nineteenth century. Despite this history, biofilms were viewed as more of a curiosity until the 1980s. Indeed, much of what is known about microorganisms and about specific areas such as bacterial antibiotic resistance has resulted from the use of bacteria growing as floating (planktonic) populations in liquid growth sources.

Beginning in the 1980s, evidence accumulated that has led to the recognition that the floating form of bacterial growth is artificial, and that the biofilm form of growth is the natural and preferred mode of growth for microbes. Now, it is accepted that virtually every surface that comes into contact with microorganisms is capable of sustaining biofilm formation.

Much of what is known about biofilms has come from the study of bacteria. Typically, the biofilm studied in the laboratory consists of one bacterial type. Observation of only one growing bacteria makes study of the formation and behavior of the biofilm easier to accomplish. In a natural setting, however, a biofilm is often comprised of a variety of bacteria. Dental plaque is a good example. Hundreds of species of bacteria can be present in the biofilm that forms on the surface of the teeth and gums.

The formation of a biofilm begins when floating bacteria encounter a surface. Attachment can occur nonspecifically or specifically. Specific attachment involves the recognition of a surface molecule by another molecule on the surface of the microorganism. Bacterial attachment can be aided by appendages such as flagella, cilia, or the holdfast of Caulobacter crescentus.

Attachment is followed by a more long-lasting association with the surface. For bacteria, this association involves structural and genetic changes. Genes are expressed following surface attachment. A particularly distinctive result of this preferential genetic activity is the production of a large amount of a sugary material known as the glycocalyx or the exopolysaccharide. The sugar layer buries the bacterial population, creating the biofilm.

As times passes, a biofilm can become thicker. An older, more mature, biofilm differs from a younger biofilm. Studies using instruments that can probe into a biofilm without physically disturbing its structure have demonstrated that the bacteria deeper within a biofilm stop producing the expopolysaccharide and slow their growth rate to become almost dormant. In contrast, the bacteria at the edge of the biofilm grow faster and produce large amounts of exopolysaccharide. These activities occur at the same time and indeed are coordinated. The bacteria can chemically communicate with one another. This phenomenon, which is called quorum sensing, allows a biofilm to grow and encourages bacteria to leave a biofilm and form new biofilms elsewhere.

Another difference in biofilms that develop over time concerns their three-dimensional structure. A young biofilm is fairly uniform in structure, with the bacteria arranged evenly throughout the biofilm. In contrast, a well-established biofilm consists of bacteria clustered together in microcolonies, with surrounding regions of exopolysaccharide and open channels of water that allow food to easily reach the bacteria and waste material to easily pass out of the biofilm.

Bacterial biofilms are important in the establishment and treatment of infections. Within the biofilm, bacteria are very resistant to chemicals like antibiotics that would otherwise kill the bacteria. Antibiotic resistant biofilms occur on inert surfaces such as artificial heart valves and urinary catheters, and on living surfaces, such as gallstones and in the lungs of those afflicted with cystic fibrosis. In cystic fibrosis, the biofilm formed by bacteria, mainly Pseudomonas aeruginosa, protects the bacteria from the host's immune system. The immune response may persist for years, which irritates and damages the lung tissue.