Rumen bacterial community transition during adaptation to high-grain diet

Transitional changes of the ruminal bacterial community structure in cows during the switch from roughage to high-grain diet were monitored by PCR amplification and sequencing of 16S rDNA clone libraries. In total, one hundred fifty 16S rDNA sequences of almost full-length (1.4 kb) were analysed from three libraries prepared from the rumen fluid on day 0, 3, and 28 of switch to high-grain diet. In the first library (day 0, hay diet) of 51clones, 90.2% of sequences were belonging to the low G+C Gram-positive bacteria (LGCGPB) phylum, with the minor inclusion of the Cytophaga-Flavobacter-Bacteroides (CFB;3.9%), Proteobacteria (3.9%) and high G+C Gram-positive bacteria (HGCGPB; 2.0%) phyla-related sequences. Six LGCGPB sequences were clustered with the well-known cellulolytics of the rumen, Ruminococcus flavefaciens and R. albus. In the second library (day 3 of high-grain diet) of 58 clones, the LGCGPB-related sequences still dominated (72.4%), albeit being represented by other species than in the first library In particular, this library was enriched by representatives of Selenomonas-Succiniclasticum-Megasphaera group IX (17.2%), lactobacilli (6.9%) and Butyrivibrio fibrisolvens lineage 3-related (8.6%) sequences. Other phyla were represented by CFB (22.4%) and HGCGPB (3.4%). In the third library (day 28 of high-grain diet) of 41 clones, 95% of sequences fell into the LGCGPB phylum. About half of them (46%) were clustered within the Selenomonas-Succiniclasficum-Megasphaera group in Clostridium cluster TX. No HGCGPB-related sequences were detected and CFB was represented by only a single clone. No Streptococcus bovis-related sequences were detected in any of the three clone libraries. (C) 2000 Academic Press.