Any surface, whether it is animate or inanimate, is of considerable importance as a microbial habitat owing to the adsorption of nutrients. A nutrient-rich microenvironment is thus produced in a nutrient-poor macroenvironment whenever a surface–liquid interface exists.
BIOFILMS
Any surface, whether it
is animate or inanimate, is of considerable importance as a microbial habitat
owing to the adsorption of nutrients. A nutrient-rich micro environment is thus
produced in a nutrient-poor macro environment whenever a surface–liquid
interface exists. Consequently, microbial numbers and activity are usually much
greater on a surface than in suspension. Hence, in many natural, medical and
industrial settings bacteria attach to surfaces and form multi-layered
communities called biofilms. These
commonly contain more than one species of bacteria, which exist cooperatively
together as a functional, dynamic consortium. Moreover, biofilms commonly
possess unique properties that are distinct to unattached cells. Biofilm
formation usually begins with pioneer cells attaching to a surface, either through
the use of specific adhesins such as fimbriae, or non specifically by EPS. Once established, these
cells grow and divide to produce microcolonies, which with time, eventually coalesce
to produce a biofilm. A key characteristic of biofilms is the enveloping of the
attached cells in a matrix of EPS and other macromolecules. This helps to
cement cells to the surface and to each other, and protects the bacteria from
hazardous materials such as antibiotics and biocides, from desiccation and from
engulfment by macrophages and phagocytes in much the same way as the capsules
and slime layers mentioned above. In addition, strands of EPS hold the
bacterial cells at a distance from one another, enabling small water channels
to form in the biofilm. These channels act as a primitive circulatory system
carrying trapped nutrients and oxygen to the enclosed cells and take waste
products away.
Biofilms have a number
of significant implications in medicine and industry. In the human body the resident
cells within the biofilm are not exposed to attack by the immune system and in
some instances can exacerbate the inflammatory response. An example of this is
shown by the growth of Ps. aeruginosa
as an alginate-enclosed biofilm in the lungs of cystic fibrosis patients. Bacterial
biofilms are also profoundly less susceptible to antimicrobial agents than
their free-living, planktonic counterparts. As a consequence, bacterial
biofilms that form on contaminated medical implants and prosthetic devices, manufacturing
surfaces or fluid conduit systems are virtually impossible to eliminate with
antibiotics or biocides. In these situations antimicrobial resistance occurs as
a population or community response.
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