Organic solvents in microbial production processes

Michiaki Matsumoto, Sonja Isken, Jan A. M. de Bont Division of industriai Microbioiogy Department of Food Technoiogy and Nutritionai Sciences Wageningen University, Wageningen, The Netheriands 

Solvents are not dominating compounds in the biosphere of our planet. Under natural conditions, their presence in appreciable amounts is restricted to specific areas. Only a very limited number of solvents is of biological origin and some may reach higher concentrations in nature. The best known example is ethanol. However, also butanol and acetone can be 

With the advent of the chemical industry, this picture has changed dramatically. In polluted locations, microorganisms may be confronted with a large number of solvents at high concentrations. With a few exceptions only, it has turned out that microbes can be found that are able to degrade these compounds if their concentration is low. This degradative potential is not unexpected in view of trace amounts that may be present locally in the natural biosphere. But the exposure of cells to unnatural high concentrations of these solvents usually leads to irreversible inactivation and finally to their death. 

Free or immobilized enzymes have been exploited already in a number of systems. Here, biocatalysis may take place in reversed micelles or in an aqueous phase in eontaet with an organic solvent. In a powdered state some enzymes are able to funetion in pure organic solvents. Furthermore, modified enzymes such as polymer bound enzymes or surfactant-coated enzymes have been developed so that they ean solubilize in organie solvents to overcome diffusion limitation. The advantages of enzymatie reaetions using organic solvents can be briefly summarized as follows  

4) bioproducts and biocatalysts can easily be reeovered from flie systems eontaining organic solvents. 

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The applications of solvent-tolerant strains in microbial production processes are at present limited, but two strategic options are currently available to use such bacteria. Relevant genes can be introduced into solvent-tolerant organisms in order to produce the required product. This approach has been followed successfully by J. Wery in our laboratory who employed an 1-octanol-aqueous system. Methylcatechol was produced from toluene by solvent tolerant P. putida S12. Alternatively, the efflux pump can be expressed in a suitable solvent-sensitive host which would then be more tolerant for a particular solvent. 

Many important fine chemicals, including catechols, phenols, aldehydes and ketones, low molecular epoxides and diepoxides, medixun-chain alcohols, and terpenoids fall within the range of 1 < log Pq/w < 4. The discovery of solvent-tolerant bacteria leads to the new possibility of biocatalytic reaction systems containing organic solvents. By using solvent-tolerant bacteria, a variety of fine chemicals can be formed in microbial production processes. 

Other Applications. Fermentation-based processes have also been developed for the mass production of certain materials. Most notably, these processes have focused on the bulk production of alternative fuels, particularly ethanol and other solvents, and biogas, which is primarily methane. Processes employing Clostridia bacteria, for example, are useful in the production of acetone and 1-butanol, two extremely important organic solvents. In addition, as the applied science and technology behind fermentation becomes more developed, other areas of science will use the ability to tailor microbial processes for the purposes of analytical and test procedures and production methods. 

The solution came, in part, by the application of biotransformations. These can be defined as biological processes that modify organic compounds via simple chemical reactions (oxidations, reductions) by means of enzymes contained in microbial, plant, or even animal cells. The aim is usually a one-step reaction to a recoverable product in a sequence of steps in which the majority of conversions are chemical steps (i.e., synthesis). In fermentation processes the whole sequence of reactions is carried out by microorganisms, be it a carbohydrate breakdown to alcohol (or other solvents), the production of antibiotics, or even enzymes. 

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