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Bacterial Biofilms

Fig. 1: The biofilm life cycle, taken from Reference [12]. 1: individual cells populate the surface. 2: EPS is produced and attachment becomes irreversible. 3 & 4: biofilm architecture develops and matures. 5: single cells are released from the biofilm.

A majority of bacteria in natural and clinical settings are contained in biofilms, which are surface-mounted, integrated communities of cells. Biofilms are highly structured and physically dynamic, with their structure and mechanical properties defined by extracellular polymeric substances (EPS), which serve as a scaffolding or glue holding the biofilm together.

The life cycle of a biofilm is characterized by attachment of planktonic bacteria to a surface or by migration or division of sessile cells to cover an empty region of the surface, production of extracellular polymeric substances (EPS) to adhere cells irreversibly to the substrate, and then by additional EPS production , cellular motility and reproduction, and phenotypic differentiation to produce a mature, thick and spatially structured biofilm.[4]

In EPS production and in many other ways, bacteria in biofilms are phenotypically distinct from their genomically-identical planktonic counterparts. Bacteria in biofilms can be up to 1000 times more resistant to antibiotics, and less conspicuous to the immune system, because antigens are hidden and key ligands are suppressed. [5]

The majority of these phenotypic differences, and of the structural and dynamic properties of biofilms, arise from complex, dynamic patterns of intercellular interaction and signaling that are not present for planktonic cells. Other characteristics seem to arise from interaction with a surface alone. Therefore, it is impossible to understand biofilms solely from studies of planktonic cells; systematic studies of the cooperative properties of self-cohering and of surface-bound prokaryotes, in and out of biofilms and in the process of biofilm formation, are essential. [6-11]

Quorum sensing is the regulation of bacterial gene expression in response to fluctuations in local signal concentration. This is done via the production and release of molecular autoinducers that increase in concentration as a function of cell density in liquid culture; when quorum-sensing cells detect a minimum threshold stimulatory concentration of an autoinducer, gene expression changes. [7, 9] Quorum sensing is manifested in many different species and environments to produce a variety of phenomena, such as the production and release of virulence factors and antibiotics by many animal and plant pathogens, and competence in Bacillus and Streptococcus. [6, 10, 11] These observations have led to widespread, current interest in quorum sensing and related concepts in sociomicrobiology. [8]

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  5. M. R. Parsek & P. K. Singh, Annual Reviews in Microbiology 57, 677-701 (2003).
  6. F. Harrison, Microbiology 153, 917-923 (2007).
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  8. J. V. McArthur, Microbial Ecology: an evolutionary approach Academic Press, Boston (2006).
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  12. Biofilm hypertextbook, Montana State University Center for Biofilm Engineering