Bacteria in the genus <
i>
Ruminococcus<
/i>
are ubiquitous members of the mammalian gastrointestinal tract. In particular, they are important in ruminants where they digest a wide range of plant cell wall polysaccharides. For example, <
i>
Ruminococcus albus 7<
/i>
is a primary cellulose degrader that produces acetate usable by its bovine host. Moreover, it is one of the few organisms that ferments cellulose to form ethanol at mesophilic temperatures <
i>
in vitro<
/i>
. The mechanism of cellulose degradation by <
i>
R. albus 7<
/i>
is not well-defined and is thought to involve pilin-like proteins, unique carbohydrate-binding domains, a glycocalyx, and cellulosomes. We used a combination of comparative genomics, fermentation analyses, and transcriptomics to further clarify the cellulolytic and fermentative potential of <
i>
R. albus 7<
/i>
. A comparison of the <
i>
R. albus 7<
/i>
genome sequence against the genome sequences of related bacteria that either encode or do not encode cellulosomes revealed that <
i>
R. albus 7<
/i>
does not encode for most canonical cellulosomal components. Fermentation analysis of <
i>
R. albus 7<
/i>
revealed the ability to produce ethanol and acetate on a wide range of fibrous substrates <
i>
in vitro<
/i>
. Global transcriptomic analysis of <
i>
R. albus 7<
/i>
grown at identical dilution rates on cellulose and cellobiose in a chemostat showed that this bacterium, when growing on cellulose, utilizes a carbohydrate-degrading strategy that involves increased transcription of the rare carbohydrate-binding module (CBM) family 37 domain and the tryptophan biosynthetic operon. Our data suggest that <
i>
R. albus 7<
/i>
does not use canonical cellulosomal components to degrade cellulose, but rather up-regulates the expression of CBM37-containing enzymes and tryptophan biosynthesis. This study contributes to a revised model of carbohydrate degradation by this key member of the rumen ecosystem.