Fibrolytic activities and cellulolytic bacterial community structure in the solid and liquid phases of rumen contents.

Four sheep were fed an alfalfa hay diet. Rumen content samples were collected three hours after feeding in order to total microorganism population (TP), solid attached population (SAP) and solid attached firmly population (SAFP). Fibrolytic specific activities (xylanase, CMCase and beta-glycosidases) were estimated by the amount of reducing sugars or p-nitrophenol released from the appropriate substrate. The distribution of the three main cellulolytic bacterial species (Fibrobacter succinogenes, Ruminococcus albus and Ruminococcus flavefaciens) was quantified by dot-blot hybridisation using specific 16S-rRNA-targeting probes. Specific activities of polysaccharidase enzymes were higher in SAP than in TP, and in SAFP than in SAP. The sum of RNA of the three cellulolytic bacterial species represented on average 9% of the total bacterial RNA, and increased after filtration. In all samples, the relative population size of F. succinogenes was higher than that of R. albus and of R. flavefaciens. These results demonstrate that the most active enzymes are secreted by the particle-associated microorganisms. The differences in composition of the microflora between the solid and liquid phase suggest that bacteria are not equally distributed throughout the rumen content: the cellulolytic species are present in a higher proportion in the solid phase of rumen contents.