Extracellular Polymers in Biofilms

Abstract

Many of those who study biofilms view them as a collection of living organisms at an interface but this definition should be expanded to include the products of those organisms. A major product is the matrix in which biofilm cells are found. It is somewhat surprising that there is such an emphasis on the biotic component of the film because this phase occupies only a small fraction of the volume (Characklis & Cooksey, 1983). It is often the biofilm matrix that causes many of the economic problems associated with biofilm formation since it acts as a layer of immobilized water. It is in fact highly hydrated and contains 98–99% water (Christensen and Characklis, 1990). This matrix, which is really a collection of polymers rather than a single material, is made by many organisms in biofilms. The polymers have been referred to collectively as capsules, sheaths, slime and glycocalyces. Costerton et al (1981) proposed the term glycocalyx for use in procaryotic biology. They defined a glycocalyx as “those polysaccharide-containing structures of bacterial origin, lying outside the integral elements of the outer membrane of Gram-negative cells and the peptidoglycan of Gram-positive cells”. They further subdivided glycocalyces into (1) glycoprotein subunits at the cell surface and (2) capsules. “Capsules” were further subdivided into (a) those that are rigid and exclude particles such as Indian ink ( a classical negative “stain” in bacteriology); (b) those, which in contrast to (a), are flexible and include Indian ink; (c) integral capsules that are closely associated with the cell surface and (d) those capsules that are peripheral to the cell and can be lost to the aqueous phase. In a brief but comprehensive review (Geesey, 1982), Geesey used a less structured term for the high molecular weight material extracellular to cells.