Microbial excreted polymers

Microbial mats are communities in surface water ecosystems where bacteria and bacterial processes dominate. In microbial mats dissolved nutrients and metabolites are transformed by one-dimensional (vertical) molecular diffusion. The distinction between microbial mats and biofilms is not sharp. By definition (Fenchel et al, 1998), microbial mats are typically stratified vertically with respect to different functional types of bacteria. Microbial mats are thicker (often several millimeters) than biofilms. In microbial mats various types of filamentous prokaryotes are the most conspicuous part and they are responsible for the mechanical coherence of the mat. The mechanical stability of microbial mats is reinforced by the bacterial excretion of mucous polymers, producing a gelatinous matrix. 

F. Donot, A. Fontana, J.C. Baccou, and S. Schorr-Galindo, Microbial exopolysaccharides Main examples of synthesis, excretion, genetics and extraction. Carbohydr. Polym. 87,951-962 (2012). 

Microbial EPS production is usually not confined to just one type of EPS as product but rather a mixture of diflbrent polymers, each being synthesized by a certain gene cluster. Generally, the availability of the precursors encoded by these genes has a high impact on the yield and structure of the EPS excreted by the cell. ... 

For that reason microorganisms excrete enzymes into the environment, which can attach to the polymer surface and cleave the polymer chains, as long as the degradation products become short enough to be water soluble (this biological system has been developed by the evolution to use natural polymers or other poorly bio-available substrates for microbes). Then these intermediates can diffuse into the surrounding environment of the plastics, be incorporated into the microbial cells and metabolised there to form biochemical end products such as water and carbon dioxide (and many others). 

Microbial polymers (e.g. poly(3)-hydroxybutyrate-hydroxyvalerate) are excreted or stored by micro organisms cultivated on starch hydrolysates or lipidic mediums. Isolation and purification costs could be high for those products that are obtained Ifom complex mixtures. Monsanto stopped the commercialisation of its product Biopol in 1999. Since then, production has been low but some new producers are entering the market (e.g. Coopeazucar in Brazil which has built new facilities for a pilot plant production of these polyhydroxyalkanoates). 

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