ECOLOGY, METABOLISM, AND GENETICS OF RUMINAL SELENOMONADS

Selenomonas ruminantium is one of the more prominent and functionally diverse bacteria present in the rumen and can survive under a wide ran ge of nutritional fluctuations. Selenomonas is not a degrader of compl ex polysaccharides associated with dietary plant cell wall components, but is important in the utilization of soluble carbohydrates released from initial hydrolysis of these polymers by other ruminal bacteria. Selenomonads have multiple carbon flow routes for carbohydrate catabol ism and ATP generation, and subspecies differ in their ability to use lactate. Some soluble carbohydrates (glucose, sucrose) appear to be tr ansported via the phosphoenolpyruvate phosphotransferase system, while arabinose and xylose are transported by proton symport. High cell yie lds and the presence of electron transport components in Selenomonas s trains has been documented repeatedly and this may partially account f or the energy partitioning observed between energy consumed for growth and maintenance functions. Most strains can utilize ammonia, protein, and/or amino acids as a nitrogen source. Some strains can hydrolyze u rea and/or reduce nitrate and use the ammonia for the biosynthesis of amino acids. Experimental evidence suggests that ammonia assimilatory enzymes in some strains may possess unique properties with respect to other presumably similar bacteria. Little is known about the genetics of ruminal selenomonads. Plasmid DNA has been isolated from some strai ns, but it is unknown what physiological functions may be encoded on t hese extrachromosomal elements. Due to the predominance of S. ruminant ium in the rumen, it is an ideal candidate for genetic manipulation. O nce the genetics of this bacterium are better understood, it may be po ssible to amplify its role in the rumen.