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This study determined the effects of aromatic amino acids (AA) and phenyl acids (phenylacetic acid and phenylpropionic acid) on fermentation of xylan by selected rumen bacteria. The ruminal bacteria were grown in a defined medium containing oat spelts xylan as the sole energy source, plus one of the following N sources: ammonia (no AA); ammonia plus a complete mixture of 20 AA commonly found in protein (CAA); ammonia plus complete AA mixture minus aromatic AA (MAR); ammonia plus phenyl acids (no AA+PA); ammonia plus complete AA mixture without aromatic AA plus phenyl acids (MAR+PA). There was varied growth response, with Fibrobacter succinogenes S85, Ruminococcus flavefaciens FD1, and Butyrivibrio fibrisolvens JW11 not being able to grow. The supply of CAA mixture significantly (P<0.05) increased acetate production in all species, in comparison with no AA treatment, by 54, 30, 97 and 33% in Prevotella bryantii B14, Ruminococcus albus SY3, R. flavefaciens 17 and Pseudobutyrivibrio xylanivorans JK170 cultures, respectively. The addition of phenyl acids significantly (P<0.05) increased acetate yields by 34, 8 and 5% in P. bryantii B14, R. albus SY3 and P. xylanivorans JK170 cultures, respectively. Sugar utilization by all the species significantly (P<0.05) increased by 42 and 77% when CAA mixture was provided. The deletion of aromatic AA from the complete AA mixture caused a significant (P<0.05) reduction in sugar utilization in P. bryantii B14 and P. xylanivorans JK170 by 25 and 10%, respectively. Phenyl acids addition increased sugar utilization by 4.8 and 5.1% in R. albus SY3 and R. flavefaciens 17, respectively. The supply of CAA mixture significantly (P<0.05) increased microbial yield in P. bryantii B14, R. albus SY3, R. flavefaciens 17 and P. xylanivorans JK170 species. It is concluded that supplementation of complete mixture of AA is beneficial in stimulating bacterial growth in the rumen.       Key words: Aromatic amino acids, phenylacetic acid, phenylpropionic acid, rumen bacteria, xylan.

Effects of aromatic amino acids, phenylacetate and phenylpropionate on fermentation of xylan by species of rumen anaerobic bacteria