Cellulose is probably the most abundant biopolymer on earth (5
). The β-1,4-linked glucose chains of cellulose form highly ordered crystalline fibrils that are insoluble and relatively recalcitrant to degradation. Cellulolytic bacteria produce a suite of enzymes that act synergistically to convert crystalline cellulose to glucose (38
). The cellulolytic bacteria that have been studied most use either of two strategies to digest cellulose. Some, such as aerobic actinomycetes belonging to the genera Streptomyces
, secrete soluble extracellular cellulolytic enzymes that usually contain carbohydrate binding modules (CBMs) that anchor them to the substrate (71
). These enzymes attack the insoluble substrate and release glucose, cellobiose, and short oligomers that are taken up and metabolized by the cells. In contrast, anaerobic bacteria of the genera Clostridium
produce multiprotein complexes, called cellulosomes, that often remain attached to cells (7
). Cellulosomes contain cellulose-binding proteins, enzymes involved in the hydrolysis of cellulose and of other polysaccharides, and scaffolding proteins that hold the multiprotein complex together. Cellulosomes often anchor the bacteria to their substrate and result in localized release of sugars that are taken up by the cells.
(Fig. ), the type species of the genus Cytophaga
, is an abundant aerobic cellulolytic soil bacterium (37
). C. hutchinsonii
is a gram-negative bacterium that belongs to the phylum Bacteroidetes
(also known as the Cytophaga-Flavobacterium-Bacteroides
group). C. hutchinsonii
utilizes very few substrates as sole carbon and energy sources. Besides cellulose, its only known substrates are cellobiose and glucose (37
). Wild strains generally use these soluble sugars poorly and exhibit a preference for crystalline cellulose. C. hutchinsonii
requires direct contact with cellulose for efficient digestion, and most of the cellulolytic enzymes appear to be cell associated (16
). Another unusual feature of cellulose degradation by C. hutchinsonii
is that reducing sugars such as glucose and cellobiose do not accumulate in the medium when it digests cellulose (64
). This is probably not simply the result of efficient uptake and metabolism of these sugars, since incubation of cells under anaerobic conditions, which should interfere with these processes, does not result in accumulation of reducing sugars.
Scanning electron micrograph of C. hutchinsonii cells digesting cellulose filter paper. Bar, 10 μm.
was selected for genome sequencing for several reasons. First, biochemical and physiological studies suggested that it might use a novel strategy for cellulose utilization. Second, techniques to genetically manipulate C. hutchinsonii
are available (44
). Most cellulolytic bacteria have resisted genetic analysis, so the development of these techniques promises new insights into bacterial cellulose utilization. Third, C. hutchinsonii
exhibits a form of rapid gliding motility whose mechanism remains unexplained despite decades of study (43
). Gliding may help to facilitate cellulose digestion, since gliding cells align themselves with and move along cellulose fibers as they digest them (64
). Finally, few genome sequences have been reported for members of the phylum Bacteroidetes
, and none are closely related to C. hutchinsonii
. This paper highlights novel features of the C. hutchinsonii
genome, with particular emphasis on genes and proteins likely to be involved in cellulose utilization and gliding motility.