Biofilms

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.