AN RIBOSOMAL-RNA APPROACH FOR ASSESSING THE ROLE OF OBLIGATE AMINO ACID-FERMENTING BACTERIA IN RUMINAL AMINO-ACID DEAMINATION

Ruminal amino acid degradation is a nutritionally wasteful process tha t produces excess ruminal ammonia. Monensin inhibited the growth of mo nensin-sensitive, obligate amino acid-fermenting bacteria and decrease d the ruminal ammonia concentrations of cattle, 16S rRNA probes indica ted that monensin inhibited the growth of Peptostreptococcus anaerobiu s and Clostridium sticklandii in the rumen, Clostridium aminophilum wa s monensin sensitive in vitro, but C. aminophilum persisted in the rum en after monensin was added to the diet. An in vitro culture system wa s developed to assess the competition of C. aminophilum, P. anaerobius , and C. sticklandii with predominant ruminal bacteria (PRE), PRE were isolated from a 10(8) dilution of ruminal fluid and maintained as a m ixed population with a mixture of carbohydrates, PRE did not hybridize with the probes to C. aminophilum, P. anaerobius, or C. sticklandii. PRB deaminated Trypticase in continuous culture, but the addition of C . aminophilum, P. anaerobius, and C. sticklandii caused a more-than-tw ofold increase in the steady-state concentration of ammonia, C. aminop hilum, P. anaerobius, and C. sticklandii accounted for less than 5% of the total 16S rRNA and microbial protein. Monensin eliminated P. anae robius and C. sticklandii from continuous cultures, but it could not i nhibit C. aminophilum. The monensin resistance of C. aminophilum was a growth rate-dependent, inoculum size-independent phenomenon that coul d not be maintained in batch culture, On the basis of these results, w e concluded that the feed additive monensin cannot entirely counteract the wasteful amino acid deamination of obligate amino acid-fermenting ruminal bacteria.