Biofilm

A biofilm is a community of Micro-organisme S (Bactérie S, Champignon S, Algue S, or Protozoaire S), adhering between them and on a surface, and marked by the secretion of an adhesive and protective matrix. Its structure is heterogeneous, often in the form of an extracellular matrix, and made up of substances Polymère S. the definition of a biofilm can slightly vary according to the sources: certain authors choose for example to exclude from the " term; biofilm" bacterial communities not producing their own extracellular matrix.

Until mainly being attached there to study the cells for themselves and independently of their medium, microbiology integrates the recent developments of the concept of interactions with the medium and turns now to the Biotope S, and in particular the biofilms.

General information

The lifestyle in biofilm is one of the two modes of behavior of the organizations Unicellulaire S - the other alternative being the free floating of type known as " planktonique " , in a liquid medium, fluid or even solid. John William Costerton proposed in 1978 the term of biofilm by suggesting that it would be the natural lifestyle of the majority of the micro-organisms. This proposal, which was based initially on the comparison of the number of bacteria in planktonique form on the one hand, and within biofilms in a river on the other hand, is from now on generally allowed by the microbiologists.

The biofilms except exceptions are observed in the mediums aqueous or exposed to moisture. They can develop on any type of natural or artificial surface, that it is mineral (rock, interfaces air-liquid…) or organics (skin, digestive tract of the animals, roots and sheets of the plants), industrial (drains, hulls of the ships) or medical (prostheses, catheters),… It is possible with a biofilm to adhere on materials " anti-adhésifs" like the Polytetrafluoroethylene (or Teflon). See the article on the micro-organisms extrémophile S for astonishing diversity their possible habitats in the ranges of heat, cold, pressure, and other extremes. Under optimum conditions for growth a biofilm can quickly become macroscopic, until reaching the meter thickness if the environment allows it.

The particular environment of the biofilm allows the cells, or the force with, to cooperate and act the ones with the others in a different way that in free environment. The bacteria living in a biofilm have properties appreciably different from those of the bacteria " flottantes" same species.

Composition, properties, functions

  • Composition

Almost all the micro-organisms have mechanisms of adherence on the surfaces and/or the ones with the others. They can be integrated into a biofilm in formation, just as they can be detached from the biofilm under the action of the mechanical forces or chemical of the environment. Adhesion is the fact of Protéines of various types: Fimbriae, Pili, Adhésine S… It is thus mechanically possible for a biofilm to shelter several different species. In fact only some biofilms is composed of only one type of organization - phenomenon related to the surrounding conditions more often than with the nature even of the organizations. The natural biofilms are only seldom clonaux but on the contrary shelter often many types of micro-organisms - Bactérie S, Protozoaire S and/or Algue S, each group carrying out of the specialized metabolic functions.

The matrix of the biofilm strictly speaking, in addition to containing the elements whose the organizations are made that it shelters (Protéine S, Lipide S, DNA, ARN,…) also consists of Polysaccharide S, (Peptidoglycane S, Cellulose) and of an important proportion of water.

  • Properties, functions

One of the major aspects of the biofilms is the induction of changes in the Phénotype S corresponding to the change of mode of behavior (of “ planktonique ” and individual, with fixed and Community). Whole series of genes see changing the duration and the rate/rhythm of their mechanisms of activation, thus corresponding also to changes of functions . The particular environment of the biofilm allows the cells, or the force with, to cooperate and act the ones with the others in a different way that in free environment. The bacteria living in a biofilm have properties notably different from those of the bacteria " flottantes" same species.

It is interesting to note that certain stocks of bacteria of laboratory lost their capacity to form biofilms, either by the cultures and successive selections of planktonique bacteria, or by the loss of their known natural Plasmide S to support the formation of biofilm.

The most spectacular property of the biofilms is doubtless astonishing the capacity of resistance which they provide to their participants against various aggressions, compared with the situation of the same organizations in state known as " planctonique". This matrix is in addition itself rather resistant so that under certain conditions the biofilms can fossilize .

The micro-organisms are in more than one way protected and connected to each other by the matrix which the biofilm makes:

Protection passivates

By its simple presence this matrix passively protects the cell S in a role from simple physical barrier against the entry of the antimicrobic, detergent and antibiotic agents: the dense extracellular matrix and the external layer of cells protect the interior of the community.

metabolic Protection .

Another factor of increased resistance: for reasons which remain to be determined the bacteria surrounded by biofilm are less active métaboliquement , therefore less receptive with the antimicrobic agents and environmental disruptions. Some biofilms proved to contain aqueous channels which in addition to the distribution of Nutriment S allow that of Molécule S of indication, establishing the communication between cells by biochemical signals . The formation of and by the biofilm is controlled by signals of cell-with-cell, and mechanisms known as of Quorum sensing, or perception of the quota , based on the principle of Critical mass . The systems of perception of the quota in the gram-negative bacteria detect the density of the cells by using signals of cell-with-cell depending on the population, generally a molecule of acyl Homosérine lactone. When this reached certain critical concentration, it activates a regulator transcriptionnel which induces specific target genes. The nature and thus the function of the molecules announcing the exchanges of cell-with-cell change starting from a given concentration of the bacteria.

genetic Protection

In certain cases, resistance to antibiotics can be exponentially multiplied. Indeed, at the time of their establishment in a biofilm the genetic expression of the bacteria is modified. This environment of exchanges of genetic material allowing the transfer of information is thus favourable with the acquisition of new characters.

Formation and development of the biofilm

(Information drawn from Center for Biofilm Engineering, Montana State University)

Considering the biofilm with the direction " embryologique" i.e. like an entity/functional unit of organized multicellular structure, one can also speak about development cycle or of life cycle because the model in five stages proposed hereafter can be repeated indefinitely:

  1. the first stage is the adhesion (reversible) of mobile micro-organisms on a surface, mainly by noncovalent or weak chemical bonds. These connections between the cell (especially its proteins: the curlis) and surfaces it attachment are of type Van der Waals, electrostatics, or acid-bases of Lewis.
  2. Comes then the permanent adhesion by the formation from Molécule S proteinic S called Ligand S, and from structures such as the pili. These first fixed points increase the capacity of anchoring other micro-organisms while increasing and by varying surfaces of anchoring. To note that certain species are not able to anchor themselves and is integrated into other species already installed in colonies while sticking to their biofilm. There are here the first steps of the structure of the biofilm: its diversity of natures and structures lets consider a diversity of functions .
  3. the micro-organisms divide , thus beginning microcolonies. Starting from a sufficiently dense concentration individuals, the microcolonies begin the secretion of the biofilm itself.
  4. the biofilm grows and matures , thickening until becoming macroscopic, even giant in optimal conditions.
  5. the fifth stage is the phase of dispersion , known as planktonique phase : induced by the ageing of the biofilm, certain stresses or deficiencies, the micro-organisms can actively separate from the biofilm, sometimes consuming the matrix which represents an energy source. These micro-organisms turn over at the state known as " planctonique" from freedom of movement and can go to colonize new surfaces, thus supplementing the cycle. In the lifestyle of the biofilm and according to this model in five stages, the phase " planctonique" can then be seen like a phase of dispersion.

Use

The biofilms can also be used for constructive goals (see Bioremédiation). For example, much of installations of water treatment of sewer include a stage of treatment in which the free waters pass on and through biofilms developed on the filters, which extract and digest the harmful organic compounds. In such biofilms, the bacteria are mainly responsible for the displacement of the organic matter (biological Demande oxygenates or DBO of it); while the protozoa are mainly responsible for the displacement of the solids in suspension (MY, or Movement of the Solids in Suspension), including pathogenic microbes and other micro-organisms.

Sources and notes

  • Klingler C., Filloux A. & Lazdunski A. (2005) bacterial biofilms, fortresses. Research , 389.
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