Chitin degradation by the facultatively aerobic cellulolytic bacterium <i>Cellulomonas uda</i>
Microbes, traditionally characterized by their capacity to decompose cellulose anaerobically, were examined for the ability to degrade the chemically-related polymer, chitin. Our results suggest that the ability to degrade and utilize chitin as a source of nitrogen might be widespread among cellulolytic bacteria. We suggest that, in natural environments, the ability to degrade chitin may confer an enormous ecological advantage on cellulolytic microorganisms as chitin may serve as a source of nitrogen, which is often limiting in environments where cellulose accumulates.
The soil isolate, Cellulomonas uda ATCC 21399, was selected for further studies inasmuch as cultures of this strain rapidly degraded cellulose and chitin, aerobically and anaerobically, and active cellulases and chitinases were present in culture supernatant fluids. Our studies indicate that the cellulase and chitinase systems of C. uda are distinct based on differences in their protein composition and the regulation of their production by growth substrates. C. uda efficiently degraded chitin with a relatively simple chitinase system composed of a major enzyme component, an endochitinase designated ChiA, and one or more exochitinases. ChiA was purified from culture supernatant fluids and characterized. It was found to be a glycoprotein with a Mr of 70kDa, pI of 8.0 and an endochitinase mode of action, ChiA degraded chitinous substrates with different degrees of crystallinity and strongly bound to both chitinous and cellulosic substrates. Optimum temperature for activity against colloidal chitin was 60°C, and optimum pH was 7.0. Preliminary information on the nucleotide sequence of chiA, the gene coding for ChiA of C. uda, suggested that ChiA might be closely related to chitinases from Streptomyces species, such as ChiC from Streptomyces lividans.
The influence of nitrogen availability on growth of C. uda was studied, C. uda cells grew as a biofilm in response to conditions of nitrogen limitation. Addition of sources of nitrogen, including products of chitin degradation, resulted in biofilm detachment. The biofilms attached to biotic surfaces such as cellulose and chitin and appeared to be involved in their degradation. Chitinase activity was detected in supernatant fluids from cultures where cells were growing as a biofilm, suggesting that nitrogen availability might also regulate the production of chitinases. Our results suggest that bacterial biofilms play an important role in the degradation of polymers in nature.