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Biofilms

Biofilms

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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 bio-films 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 exopolysac-charide. 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 bio-film formed by bacteria, mainly Pseudomonas aeruginosa, protects the bacteria from the hosts immune system. The immune response may persist for years, which irritates and damages the lung tissue.

Research has established that the interior of a bacterial biofilm involves chemical communication between individual bacteria that aids in coordinating the development of the biofilm and the sloughing off of peripheral bacteria, which settle on more remote surfaces to form a new biofilm. Knowledge of these chemical signals may help retard or even prevent the formation of biofilms in cystic fibrosis and other diseases and infections.

Resources

BOOKS

Ghannoum, Mahmoud and George A. OToole. Microbial Biofilms. Washington, DC: ASM Press, 2004.

Pace, John L., Mark Rupp, and Roger G. Finch. Biofilms, Infection, and Antimicrobial Therapy. Boca Raton: CRC, 2005.

Wilson, Michael, and Deirdre Devine. Medical Implications of Biofilms. Cambridge: Cambridge University Press, 2003.

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Biofilms

Biofilms


Marine microbiology began with the investigations of marine microfouling by L. E. Zobell and his colleagues in the 1930s and 1940s. Their interests focused primarily on the early stages of settlement and growth of microorganisms , primarily bacteria on solid substrates immersed in the sea. The interest in the study of marine microfouling was sporadic from that time until the early 1960s when interest in marine bacteriology began to increase. Since 1970 the research on the broad problems of bioadhesion and specifically the early stages of microfouling has expanded tremendously.

The initial step in marine fouling is the establishment of a complex film. This film, which is composed mainly of bacteria and diatoms plus secreted extracellular materials and debris, is most commonly referred to as the "primary film" but may also be called the "bacterial fouling layer," or "slime layer." The latter name is aptly descriptive since the film ultimately becomes thick enough to feel slippery or slimy to touch. In addition to the bacteria and diatoms that comprise most of the biota, the film may also include yeasts, fungi , and protozoans.

The settlement sequence in the formation of primary films is dependent upon a number of variables which may include the location of the surface, season of the year, depth, and proximity to previously fouled surfaces and other physiochemical factors.

Many studies have demonstrated the existence of some form of ecological succession in the formation of fouling communities, commencing with film forming microorganisms and reaching a climax community of macrofouling organisms such as barnacles, tunicates, mussels, and seaweeds.

Establishment of primary films in marine fouling has two functions: (a) to provide a surface favoring the settlement and adhesion of animal larvae and algal cells, and (b) to provide a nutrient source that could sustain or enhance the development of the fouling community.

Formation of a primary film is initiated by a phenomenon known as "molecular fouling" or "surface conditioning." The formation of this molecular film was first demonstrated by Zobell in 1943 and since has been confirmed by many other investigators. The molecular film forms by the sorption to solid surfaces of organic matter dissolved or suspended in seawater. The sorption of this dissolved material creates surface changes in the surface of the substrate which are favorable for establishing biological settlement. These dissolved organic materials originate from a variety of sources such as end-products of bacterial decay, excretory products, dissolution from seaweeds, etc., and consist principally of sugars, amino acids, urea, and fatty acids.

This molecular film has been observed to form within minutes after any clean, solid surface is immersed in natural seawater. The role of this film in biofouling has been shown to modify the "critical surface tension" or wetability of the immersed surface which than facilitates the strong bonding of the microorganisms through the agency of mucopolysaccharides exuded by film-forming bacteria.

Bacteria have been found securely attached to substrates immersed in seawater after just a few hours. Initial colonization is by rod-shaped bacteria followed by stalked forms within 2472 hours. As many as 4050 species have been isolated from the surface of glass slides immersed in seawater for a few days.

Following the establishment of the initial film of bacteria and their secreted extracellular polymer on a solid substrate, additional bacteria and other microorganisms may attach. Most significant in this population are benthic diatoms but there are also varieties of filamentous microorganisms and protozoans. These organisms, together with debris and other organic particular matter that adhere to the surface create an intensely active biochemical environment and form the primary stage in the succession of a typical macrofouling community.

Considering the enormous economic consequences of marine fouling it is not at all surprising that there continues to be intense interest in the results of recent research, particularly in the conditions and processes of molecular film formation.

[Donald A. Villeneuve ]


RESOURCES

BOOKS

Corpe, W. A. "Primary Bacterial Films and Marine Microfouling." In Proceedings of the 4th International Congress of Marine Corrosion and Fouling, edited by V. Romansky. 1977.

PERIODICALS

Zobell, L. E., and E. C. Allen. "The Significance of Marine Bacteria in the Fouling of Submerged Surfaces." Journal of Bacteriology 29 (1935): 239251.

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"Biofilms." Environmental Encyclopedia. . Encyclopedia.com. 18 Aug. 2017 <http://www.encyclopedia.com>.

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