Prevotella ruminicola is a fibrolytic rumen bacterium that degrades complex carbohydrates and ferments the resulting hexoses and pentoses. Cultures of strain AR29 grew more rapidly on glucose than on xylose, but produced almost the same metabolite profiles on both carbon sources. Glucose in glucose-plus-xylose medium was used preferentially. The production of cell dry matter, growth yields of dry matter and protein, and cell composition (cell carbohydrate and protein) were not significantly different in glucose-and xylose-grown cultures. High aldolase (EC 4.1.2.13) activity was found both in glucose-and xylose-grown cells of the strain AR29. A very low phosphoketolase (EC 4.1.2.9) activity was detected in cells cultured on xylose. In contrast, enzymes of the EntnerDoudoroff pathway (6-phosphogluconate dehydrase and 2-keto-3-deoxy-6-phosphogluconate aldolase) could not be evidenced. KEY WORDS: Prevotella ruminicola, metabolism, carbohydrates INTRODUCTIONAnaerobic bacteria belonging to the species Prevotella ruminicola represent one of the most numerous bacterial groups inhabiting the rumen (Stewart and Bryant, 1988). The predominance of this organism can be explained by its ability to use a variety of substrates and also by its efficient energy metabolism. Strains that have been isolated from the rumen ferment soluble sugars including soluble cellodextrins, polysaccharides, and decompose protein, peptides and amino acids. Several studies attempted to elucidate carbohydrate metabolism of P. ruminicola. Experiments by Joyner and Baldwin (1966) and Mountfort and Roberton (1978)

The rumen anaerobic fungus Orpinomyces joyonii A 4 was grown in a complex medium with glucose, xylose, microcrystalline cellulose or waste paper. The fungus produced significantly (PO.05) more formate, acetate and ethanol, but less lactate from xylose than from glucose. Protein content and yields of dry matter and protein in cells grown on glucose and xylose were not significantly different. Fungal cells contained 64.8% of crude protein, but only 34.9-35.2% of true protein determined according to Lowry. In dual-substrate fermentation both sugars were utilized simultaneously. O. joyonii A 4 efficiently metabolized microcrystalline cellulose and milled office paper (up to 41.1 and 38.0 mmol of fermentation end-products per 1, respectively). Its growth on newspaper was poor (13.4 mmol of fermentation end-products per 1). Glucose accumulated in fungal cultures with cellulose (2.55 g/1) and office paper (0.60 g/1) at the end of the incubation.