ARS | Publication request: pH Dynamics and Bacterial Community Composition in the Rumen of Lactating Dairy Cows

Submitted to: American Dairy Science Association Abstracts
Publication Type: Abstract
Publication Acceptance Date: March 9, 2009
Publication Date: N/A

Technical Abstract: The effect of pH dynamics on ruminal bacterial community composition (BCC) was studied in 8 ruminally cannulated Holstein cows fitted with indwelling electrodes that recorded pH at 10-min intervals over a 2.4-d period. Cows were fed a silage-based TMR supplemented with monensin. Ruminal samples were collected each day just before feeding, and at 3 and 6 h after feeding. Solid and liquid phases were separated at collection, and extracted DNA was subjected to PCR amplification followed by automated ribosomal intergenic spacer analysis (ARISA). Although cows displayed widely different pH profiles (mean pH= 6.11 to 6.51, diurnal pH range = 0.45 to 1.39), correspondence analysis of the ARISA profiles revealed that 6 of the 8 cows had similar BCC. The 2 cows having substantially different BCC from the other 6 had intermediate mean pH values (6.30 and 6.33) and intermediate diurnal pH ranges (averaging 0.89 and 0.81 pH units). These two cows displayed the lowest milk fat percentages of the 8, along with markedly higher ruminal populations of one bacterial Operational Taxonomic Unit (OTU) and reduced populations of two other OTUs. Cloning and sequencing of a DNA segment from the elevated OTU revealed phylogenetic similarity to Megasphaera elsdenii, a species known to produce trans-10, cis-12 conjugated linoleic acid (CLA), a known regulator of mammary lipogenesis. The higher populations of both M. elsdenii and OTU246 in these 2 cows were confirmed using quantitative real-time PCR with species-specific primers, and the population sizes of these two taxa were highly correlated (r2=0.99, p<0.001). By contrast, the relative population sizes of Streptococcus bovis and genus Ruminococcus, two taxa expected to respond to ruminal pH, did not differ among cows (mean = <0.01% and 10.6%, respectively, of rRNA gene copy number, determined by real time-PCR). The results indicate that cows with widely differing pH profiles can have rather similar ruminal bacterial community compositions, and that low milk fat can occur at intermediate ruminal pH. The results support recent reports that milk fat depression is associated with shifts in bacterial community composition in the rumen, and are specifically related to the relative population size of Megasphaera elsdenii.