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Pathogenic microorganisms that are responsible for some stubborn and resistant infections in humans are mediated or caused by microbial cells that organize themselves into very complex and persistent films or community. These films are generally known as biofilms, and they are extremely resistant to some available potent antimicrobial agents. Biofilms are well-organized microbial systems consisting of different layers of microbial cells associated with surfaces, and that possess complex structural and functional characteristics. They are slimy layers of microbial cells consisting of well-organized cooperating community and assemblage of microorganisms formed on animate and inanimate surfaces. Biofilms are usually enclosed in a self-synthesized extracellular polymeric substance (EPS) that grow attached to both biotic and abiotic surfaces.

Biofilms exemplify the growth of microorganisms in the real world, and how microbes can effectively cooperate amongst themselves in their natural community or habitat to form microbial communities that is beneficial to the participating organisms. When microbes form biofilms, they become resistant to antimicrobial agents because the biofilm protects the participating organisms from harmful environmental chemicals. Biofilms also prevent microbes from being washed away from their natural habitat; and biofilms are of immense importance in industries due to their ability to clog pipes for water distribution. In the hospital environment, biofilms can form in some invasive medical devises (e.g., urine catheter) and thus prolong the hospitalization of the individual due to protracted illness.

Biofilms are notorious in developing on virtually all natural and artificial surfaces immersed in natural aqueous environment and with constant supply of nutrients required for microbial growth. Thus, an aqueous environment is vital to the formation of slimy layers (biofilms) on surfaces. According to the International Unit of Pure and Applied Chemistry (IUPAC), biofilms are aggregates of microorganisms in which cells that are frequently embedded within a self-produced matrix of extracellular polymeric substance (EPS) adhere to each other and/or to a surface. Microbes form a biofilm in response to many factors, which may include cellular recognition of specific or non-specific attachment sites on a surface, nutritional signals, or in some cases, by exposure of planktonic cells to sub-inhibitory concentrations of antimicrobial agents including antibiotics in their environment.

Standard antibiotics, antiseptics, biocides and disinfectants used in clinical settings and elsewhere often fail in their microbial onslaught when used for therapeutic purposes or for disinfection because they do not efficiently penetrate biofilms fully, irrespective of the normal bacterial resistance mechanisms of the microbes or bacteria in the biofilm community. The metabolic states of microorganisms in the biofilm stage are often recalcitrant to antimicrobial agents due to the complexity of the high level of cooperation formed amongst microbes in the biofilm community. Some bacteria are able to form biofilms by adhering to surfaces on implanted medical devices such as catheters, and this allows them to create an extracellular matrix (EPS) for other cells to adhere to. As this continues, the bacterial community becomes so large to ward-off any untoward action directed against them (e.g., the onslaught of antimicrobial agents). Biofilm formation provides microorganisms with a stable environment from which they can disperse and infect other parts of the host or attaching environment.

Additionally, the extracellular matrix and dense outer layer of cells formed in the biofilm can protect the inner cells from antimicrobial drugs and other external adverse conditions. The nutrient availability and communication amongst microbial cells in the biofilm community (through quorum sensing) is greatly enhanced. Quorum sensing allows microbial cells or bacteria to communicate amongst each other. If a biofilm contains regions that are starved of an essential nutrient for example, the microbial cells in those particular areas, which are alive but are not replicating, will survive exposure to an antimicrobial agent directed towards them. Because both active and inactive microbes are closely related and juxtaposed (i.e., placed side by side) in a biofilm, and because the bacteria that survived the antimicrobial onslaught can use already dead organisms as nutrients, the few bacterial cells remaining after the antibiotic action can restore the biofilm to its original state in a matter of hours by going ahead to build and develop new community of microorganisms. Thus, the bacterial community continues, and this allows them to ward-off any antimicrobial agent directed against them because of the formation of biofilms. Bacteria in biofilm are several folds more resistant to antibiotics and other antimicrobial agents than planktonic bacteria that are not in any biofilm community.

Bacterial adhesion to surfaces (whether biotic or abiotic) through the formation of biofilms gives them numerous advantages including (1) protection against antimicrobial agents, (2) acquisition of new genetic traits (e.g., resistance genes), and (3) access to nutrients, water and metabolic cooperation. The formation of biofilms by microorganisms (especially pathogenic strains of bacteria) has tremendous impact on the health of humans, and this has lead to increase in the sufferings of patients as a way of resistance to some available conventional antibiotics. Biofilm formation is a problematic phenomenon in the health sector owing to its contribution in a number of resistance cases including patients under catheterization and intubation. They are also a source of concern in the environment and food producing companies as well. Research has shown that microorganisms in biofilms depend critically on their ability to release signaling molecules which allows them to communicate with each other so as to better evade any external attack on them. The cell-to-cell interaction that exists in bacterial community in which biofilms is eventually formed is made possible through a biological process known as quorum sensing. Quorum sensing is a direct cell-to-cell interaction amongst microbes.

Further reading

Brooks G.F., Butel J.S and Morse S.A (2004). Medical Microbiology, 23rd edition. McGraw Hill Publishers. USA.

Gilligan P.H, Shapiro D.S and Miller M.B (2014). Cases in Medical Microbiology and Infectious Diseases. Third edition. American Society of Microbiology Press, USA.

Madigan M.T., Martinko J.M., Dunlap P.V and Clark D.P (2009). Brock Biology of Microorganisms, 12th edition. Pearson Benjamin Cummings Inc, USA.

Mahon C. R, Lehman D.C and Manuselis G (2011). Textbook of Diagnostic Microbiology. Fourth edition. Saunders Publishers, USA.

Patrick R. Murray, Ellen Jo Baron, James H. Jorgensen, Marie Louise Landry, Michael A. Pfaller (2007). Manual of Clinical Microbiology, 9th ed.: American Society for Microbiology.

Wilson B. A, Salyers A.A, Whitt D.D and Winkler M.E (2011). Bacterial Pathogenesis: A molecular Approach. Third edition. American Society of Microbiology Press, USA.

Woods GL and Washington JA (1995). The Clinician and the Microbiology Laboratory. Mandell GL, Bennett JE, Dolin R (eds): Principles and Practice of Infectious Diseases. 4th ed. Churchill Livingstone, New York.

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