Population changes of fibrolytic rumen bacteria in the presence of phenolic acids and plant extracts

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Phenolic acids inhibit rumen microbial digestion of forage, but information on the interactions of the mixed population with natural phenolics from plants is required to elucidate fully the anti-quality aspect of phenolic compounds in forages. The objective of this study was to evaluate the digestion of leaf blades of Coastal bermudagrass ( Cynodon dactylon L. Pers.) and Italian ryegrass cv. RvP ( Lolium multiflorum L.) and the morphological types of colonizing bacteria during incubation with rumen fluid in the presence of phenolic acids [ trans- p-counmaric acid (t-PCA), cis- p-coumaric acid, trans-ferulic acid, or diferulic acid] or phenolic extracts from maize ( Zea mays L.) stems or barley ( Hordeum vulgare L.) straw. The presence of phenolics resulted in less degradation of plant cell walls, as determined by scanning electron microscopy and often fewer bacteria associated with the plant walls, as determined by transmission electron microscopy, when compared with control treatments. In the presence of t-PCA, the ability of microbial suspensions that had not been previously exposed to t-PCA to colonize and degrade leaf blade cell walls was completely inhibited. This inhibitory effect was reduced, but not totally eliminated, if microbial suspensions were exposed to t-PCA for 48 h in culture prior to inoculation of leaf blades. Trans-PCA and phenolics from maize stem caused a reduction in the proportion of fiber-associated bacteria resembling Bacteroides succinogenes, which appeared to be the most active fiber digester in this system. Phenolics extracted from cell walls of barley straw showed slight to no inhibition and were similar to control treatments.