Abstract
Although Fibrobacter succinogenes S85 is one of the most proficient cellulose degrading bacteria among all mesophilic organisms in the rumen of herbivores, the molecular mechanism behind cellulose degradation by this bacterium is not fully elucidated. Previous studies have indicated that cell surface proteins might play a role in adhesion to and subsequent degradation of cellulose in this bacterium. It has also been suggested that cellulose degradation machinery on the surface may be selectively expressed in response to the presence of cellulose. Based on the genome sequence, several models of cellulose degradation have been suggested. The aim of this study is to evaluate the role of the cell envelope proteins in adhesion to cellulose and to gain a better understanding of the subsequent cellulose degradation mechanism in this bacterium. Comparative analysis of the surface (exposed outer membrane) chemistry of the cells grown in glucose, acid-swollen cellulose and microcrystalline cellulose using physico-chemical characterisation techniques such as electrophoretic mobility analysis, microbial adhesion to hydrocarbons assay and Fourier transform infra-red spectroscopy, suggest that adhesion to cellulose is a consequence of an increase in protein display and a concomitant reduction in the cell surface polysaccharides in the presence of cellulose. In order to gain further understanding of the molecular mechanism of cellulose degradation in this bacterium, the cell envelope-associated proteins were enriched using affinity purification and identified by tandem mass spectrometry. In total, 185 cell envelope-associated proteins were confidently identified. Of these, 25 proteins are predicted to be involved in cellulose adhesion and degradation, and 43 proteins are involved in solute transport and energy generation. Our results supports the model that cellulose degradation in F. succinogenes occurs at the outer membrane with active transport of cellodextrins across for further metabolism of cellodextrins to glucose in the periplasmic space and inner cytoplasmic membrane.
Highlights
Cellulose, an abundantly occurring organic polymer in the plant kingdom [1], has immense potential for the production of alternate fuels such as bioethanol [2]
F. succinogenes S85 grew on glucose, AS cellulose and MC cellulose with growth rates of 0.20, 0.098 and 0.084 h-1 respectively
Previous studies [7, 8] have suggested that cellulose degradation in F. succinogenes is a cell envelope-associated process, which includes adhesion to cellulose as a pre-requisite step
Summary
An abundantly occurring organic polymer in the plant kingdom [1], has immense potential for the production of alternate fuels such as bioethanol [2]. Low cost production of fuel from cellulose necessitates the development of inexpensive pre-treatment techniques [2]. Enzymatic degradation of cellulose using microorganisms could be a promising low cost alternative to existing cellulose degradation strategies. There are many microorganisms capable of enzymatic degradation of cellulose, as reviewed by Lynd et al [3]. The microbial consortia in the rumen of herbivores are well-specialised for cellulose degradation [4, 5]. Fibrobacter succinogenes S85 is a dominant cellulose degrading bacterium of the rumen community and actively degrades crystalline cellulose. The mechanism by which F. succinogenes degrades cellulose remains unknown
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