Biofilm structure and surrounding matrix
The ability of microbes to attach to a surface and make that first step in the development of a biofilm is affected by what particles are already coating that surface. The presence of organic or inorganic particles (known as a conditioning film) on a surface enhances attachment of the first microbes arriving and beginning to colonise. Anything that is present in the fluid surrounding a surface can settle out and become part of a conditioning layer. Attributes of the surface, like its charge, can influence how likely it is to accumulate a conditioning layer. Similarly, local environmental variables, such as availability of nutrients, temperature and surface roughness, all contribute to the likelihood of bacterial adhesion. Physical forces of attraction affect bacterial adhesion to and include: van der Waals’ forces; steric and electrostatic interactions; and hydrophobic, hydrophilic and osmotic interactions.
Some microbes only remain attached to a surface for a short time while others become immobilised and irreversibly attached. Microbes are covered in hairs and other appendages and these help them overcome any physical forces of repulsion and attach irreversibly to the surface. The first colonisers grow in surface-attached microcolonies. Initial colony growth is slow but then rapidly increases – this is described as the exponential growth phase. As the microcolonies develop, other species, the so-called secondary colonisers, join the microcolonies and increase the mass of the biofilm and its species complexity.
The ability of microbes to colonise is regulated by expression of a number of genes they have in their DNA. For example, expression of genes for the production of cell surface proteins and excretion products increases. Surface proteins (porins) such as Opr C and Opr E enable each microbe to transport extracellular products into its cell and excrete other materials out of its cell, e.g., extracellular polymeric substances (EPS) mainly polysaccharides (complex sugars) proteins, nucleic acids and lipids; they provide the mechanical stability of biofilms, mediate their adhesion to surfaces and form a cohesive, three-dimensional polymer network that interconnects and immobilises biofilm cells. In addition, the biofilm matrix acts as an external digestive system by keeping extracellular enzymes close to the cells, enabling them to metabolise dissolved, colloidal and solid biopolymers. They also help microbes within the biofilm survive under the harsh and changing environmental conditions they find themselves in. Differences in gene expression between planktonic microbes (living free in liquid) and sessile microbes (living attached to surfaces in biofilms) have been identified; as many as 57 proteins are associated with microbes growing as biofilms and were not found in planktonic microbes.
Further reading on biofilm structure and surrounding matrix
The Biofilm Matrix, Nature Reviews Microbiology, 8(9):623-33, September 2010. DOI:10.1038/nrmicro2415.
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