Urethane-bond-degrading bacteria

#The present invention relates to a novel microorganism and a method for degrading polyurethanes through biological treatment using the microorganism.

#In recent years, plastic waste disposal has become a problem. Procedures mainly used for plastic waste disposal are incineration and reclamation. However, incineration is disadvantageous in that it accelerates global warming, while reclamation suffers from problems such as lack of landfills for reclamation. For example, polyurethanes are consumed all over the world at a rate of about 6,000,000 tons per year and in Japan at a rate of about 550,000 tons per year. Among them, foam cushioning materials made of polyurethanes are used on a massive scale as heat insulators, e.g., for refrigerators because of their excellent heat insulation properties. At present, polyurethane waste is often disposed of in landfills as noncombustible garbage, but problems are associated with its disposal, such as lack of landfills, and environmental pollution. While microbial biodegradation can be presented as a preferred disposal technique from the viewpoint of protection of the natural environment, a problem exists in that polyurethanes are not biodegradable.

#Polyurethane contains urethane bonds together with ester or ether bonds in its molecule, and degradation proceeds through cleavage of these bonds. There are some reports of ester bonds in polyol units being cleaved by fungi and/or bacteria. Darby et al. (Darby R. T. and Kaplan A. M., Fungal susceptibility of polyurethanes. Appl. Microbiol., 16, 900-905 (1968)) have performed fungal degradation tests on various polyurethanes. They have reported that ester-based polyurethanes are more sensitive to degradation than ether-based polyurethanes, and that degradation profiles vary depending on the type of isocyanate and/or polyol. Kay et al. (Kay, M. J., McCabe, R. W., Morton, L.H.G., Chemical and physical changes occurring in polyester polyurethane during biodegradation. Int. Biodeterio. Biodegrad., 31, 209-225 (1991)) have isolated 15 bacterial strains capable of degrading ester-based polyurethanes and have also reported the results of degradation profiles examined for Corynebacterium strains having a strong degradation ability.

#FIG. 1 shows the structure of synthetic urethane compounds used for screening of urethane-bond-degrading bacteria.

#FIG. 2 shows an rDNA sequence-based dendrogram including known strains.

#FIG. 3 shows the results measured for the amount of residual urethane compound I under each culture condition.

#As stated above, the present invention provides a microorganism belonging to the genus Rhodococcus and having the ability to degrade a urethane compound, particularly a low-molecular-weight urethane compound used as a source material for polyurethane synthesis, as well as providing a method for degrading a polyurethane using the microorganism belonging to the genus Rhodococcus.

#Microorganisms belonging to the genus Rhodococcus and having the ability to degrade a urethane compound may be either known or newly screened microorganisms. By way of example, screening of microorganisms may be accomplished as follows. Soil samples collected from various areas are introduced into test tubes containing a medium supplemented with a low-molecular-weight urethane compound used as a source material for polyurethane synthesis, followed by shaking culture at 30° C. After repeating subculture every one week, those samples showing cloudiness or discoloration in the culture solutions are selected and their culture supernatants are diluted and applied onto NB agar plates, followed by culturing at 30° C. for 1 to 3 days. The grown colonies are picked up and defined as candidate strains for urethane-bond-degrading bacteria. The resulting candidate strains are then cultured in a liquid medium containing, as a carbon source, a low-molecular-weight urethane compound (urethane compound I) which is obtained through reaction between toluene diisocyanate and butanol, followed by selection of strains showing the production of toluenediamine (a urethane bond hydrolysis product of urethane compound I) in their culture solutions.

#The microorganism of the present invention is not limited in any way as long as it belongs to the genus Rhodococcus and has the ability to degrade a urethane bond-containing compound. More specifically, typical examples include Rhodococcus equi TB-60-DSMZ 16175 internationally deposited on Jan. 24, 2004 with the German depository institution DSMZ [Deutsche Sammlung von Mikroorganismen und Zellkulturen GmbH (German Collection of Microorganisms and Cell Cultures), Mascheroder Weg 1 B, D-38124 Braunschweig, Germany], which is Rhodococcus equi strain TB-60 that was not accepted although an application for deposit was filed on Feb. 26, 2003 with the International Patent Organism Depositary, the National Institute of Advanced Industrial Science and Technology in Japan. Mycological properties of Rhodococcus strains can be found in BERGEY'S MANUAL OF Systematic Bacteriology (vol. 1, 1984, vol. 2, 1986, vol. 3, 1989, vol. 4, 1989).