Utilization of inorganic sulfate by rumen microorganisms. I. Incorporation of inorganic sulfate into amino acids.

The ability of rumen microbiota to incorporate inorganic sulfate into cystine and methionine was suggested by Loosli et al. (1949) and demonstrated by Block et al. (1951) through use of radioactive sodium sulfate. The significance of this incorporation as applied to ruminant nutrition has been discussed by Davis et al. (1954). Formation of microbial protein in the rumen of cattle is accompanied by protein degradation and absorption. The complexity of this situation is further intensified by passage of both the substrate and the fermentation products through the alimentary tract (Pearson and Smith, 1943). Proteins are deaminated during the course of rumen fermentation with the liberation of ammonia (McDonald, 1952). Free amino acids occur in only trace amounts durinig this process probably due to their rapid deamination (Lewis, 1955). The liberated ammonia is absorbed, partially returned to the rumen in the form of salivary urea, and partially excreted in the urine (McDonald, 1948). The complex nature of protein metabolism by rumen microbiota indicates the need for some tracer technique to define the individual reactions and pathways. The use of radioactive sulfate enables one to follow the synthesis of sulfur containing amino acids, to determine the contribution of different strains of microorganisms to this synthesis, and to evaluate the symbiotic effects between strains. This study was initiated to determine the ability of the total rumen microbiota to utilize inorganic sulfate when acting in vitro upon substrates representative of normal cattle feeds.