Cellulose, a component of the plant cell wall, is the most abundant renewable organic resource on Earth. Cellulose is insoluble and highly resistant to degradation because of its crystalline structure. Microorganisms have evolved diverse strategies to digest cellulose.
Some secrete soluble extracellular cellulases that function synergistically (noncomplexed cellulase systems) and some produce cellulosomes composed of multiple cellulases and cellulose-binding modules (complexed cellulase systems) to digest cellulose efficiently, producing glucose or short oligomers extracellularly. Cellulases involved in these two strategies include endoglucanases and exoglucanases. The endoglucanases attack cellulose in the amorphous regions and release free ends. The exoglucanases bind to these ends and processively cut cellulose chains and are essential in the breakdown of cellulose in the crystalline regions.
Cytophaga hutchinsonii is a soil gliding bacterium that can digest cellulose. It appears to use a different cellulose utilization mechanism from the noncomplexed and complexed cellulase systems described above. The cellulases of C. hutchinsonii are cell-associated but this organism does not have cellulosomes. More interestingly, only endoglucanases but no exoglucanases were predicted according to the genome analysis. C. hutchinsonii appears to transport cellooligosaccharides, the initial cellulose digestion products, across its outer membrane and digests these internally in the periplasm. Unlike the other cellulolytic microorganisms, cellulose degradation products do not accumulate extracellularly while cells of C. hutchinsonii digest cellulose.
We found that the novel type IX protein secretion system is required for C. hutchinsonii cellulose utilization. Five endoglucanases (Cel5A, Cel9A, Cel9B, Cel9D, and Cel9E) are predicted to be delivered by the type IX secretion system to the cell surface. These enzymes are thought to be required for cellulose digestion. We recently deleted most of the endoglucanase encoding genes and found that two periplasmic endoglucanases Cel5B and Cel9C are also essential for C. hutchinsonii cellulose utilization.
Based on all previous studies, we proposed a model for C. hutchinsonii cellulose utilization. In this model, cells attach to cellulose by using cellulose binding-proteins. The five cell surface endoglucanases perform the initial extracellular digestion of cellulose, producing cellooligosaccharides. The cellooligosaccharides are transported across the outer membrane and further digested in the periplasm by Cel5B and Cel9C, producing cellobiose and short oligomers. Periplasmic β-glucosidases complete the digestion of these products to glucose.
There are still many mysteries in C. hutchinsonii cellulose utilization. Future research will identify novel proteins involved in this process. These proteins may be useful in enhancing the conversion of plant biomass into biofuels or other valuable products in the industry.
These findings were described in the article entitled The unusual cellulose utilization system of the aerobic soil bacterium Cytophaga hutchinsonii, published in the Applied Microbiology and Biotechnology. This work was led by Yongtao Zhu from the University of Wisconsin-Milwaukee.