Team:Toronto/Project/Background

BIOFILMS - AN INTRODUCTION

Unlike how they are usually conceived by those involved with laboratory experiments, bacteria such as E. coli do not always exist as free-floating, independent cells. In natural environments, they often live in adhesive, structured communities known as biofilms. These semi-rigid structures offer cells protection from harsh environmental conditions which include changes in osmolality and temperature, as well as medical incursions like antibiotics. In nature, biofilms are often heterogeneous with different species of bacteria forming large, layered complexes; however, individual species can form biofilms by themselves. In biofilms, individual cells change their morphology and protein expression and secrete adhesive matrix polysaccharides (such as cellulose, PGA, etc) in response to environmental stress. (Beloin et al., 2008) Some of the cellular and extracellular components upregulated during an E. coli biofilm response are discussed below.

Curli

Curli are amyloid fibrils composed of two separate subunits, CsgA and CsgB, the latter of which is associated with the outer membrane. The transcription regulator CsgD controls subunit production by upregulating transcription of the csgBA operon. Curli fibrils assist in the processes of surface attachment, as well as playing a key role in intercellular interactions. Thus, curli are involved in both the initial adhesion of a biofilm onto a surface and further fortification of its structure (Beloin et al., 2008; Zhou et al., 2013).

A model for the production and export of curli subunits.
Barnhart M.M., Chapman M.R. Curli biogenesis and function. Annu Rev Microbiol. 60, 131-47.

Fimbriae

Type I fimbriae are long, surface-exposed polymeric filaments. They are composed of fimA, which forms the major component of the stalk, and fimH, which is found at the end of the filament and binds ligands in a mannose-dependent manner (Beloin et al., 2008). The transcription of the gene cluster responsible for Type I fimbrial synthesis is dependent on the competition between fimB and fimE to turn the transcription of the fimbriae production operon fimAICDFGH on or off, respectively, by inverting the orientation of fimS, which contains the promoter for the operon (Ecocyc, 2013). The adhesive ability of fimbrae make them important for initial binding of E. coli to surfaces.

Hahn E., Wild P., Hermanns U., Sebbel P., Glockshuber R., Häner M., Taschner N., Burkhard P., Aebi U., Müller S.A. Exploring the 3D molecular architecture of Escherichia coli type 1 pili. J Mol Biol. 323(5),845-57.

Colanic acid

Colonic Acid is a polymer of fucose, glucose, glucuronic acid, and galactose which forms a protective capsule around the cells in a biofilm. The stress sensor RcsC along with RcsD and RcsB form a three-component system that upregulates genes involved in colonic acid synthesis. While colonic acid reduces initial surface attachment, it is believed that it assists in later biofilm maturation. (Beloin et al., 2008)

c-di-GMP

Cyclic-di-GMP (c-di-GMP) is a small second messenger molecule that is found in bacteria, triggering different internal processes. It influences everything from biofilm formation to extracellular signalling. c-di-GMP is synthesised by diguanylate cyclases and broken down during the phosphorous stress response. In general, c-di-GMP promotes biofilm formation and decreases motility. For example, diguanylate cyclase YdeH increases the pool of cdiGMP which, in the case of this specific diguanylate cyclase, increases PGA, an extracellular polysaccharide used in biofilm maturation (Povolotsky et al, 2012).

References