The extreme oxygen sensitivity and fastidious growth requirements of many rumen mi­croorganisms mean that they may often be difficult to culture, or even unculturable. At the very least, the task of completing the description of rumen microbial diversity through cultivation looks daunting. An attractive alternative, therefore, is to build metagenomic li­braries of rumen DNA that can be screened for activities of interest, or randomly sequenced to gain information on the genes present. This approach has been successfully applied to the recovery of genes encoding esterases, amylases and cellulases, xyloglucanases from the rumen (80, 81). Most of the glycoside hydrolases so far recovered appear to have simple domain structures, resembling those from the Prevotella group rather than those typical of known cellulolytic bacteria. This is most likely to reflect the relative ease of recovery of planctonic, rather than surface-attached bacteria, and perhaps also the relative ease of lysis
of Gram-negative cells. It may also reflect greater numbers of secondary utilizers of plant polysaccharides compared with primary degraders within the community. Nevertheless, the value of the approach is demonstrated by the fact that the collection includes representatives of new enzyme families. Information on sequence diversity can also be obtained without the need for library construction, either through amplification of specific genes by degenerate PCR, or directly by 454 sequencing.