Submitted to: Applied and Environmental Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/9/1997
Publication Date: N/A
Interpretive Summary: Ruminant animals such as dairy cattle harbor a bacteria that digests the carbohydrate cellulose. This symbiotic, or animal & bacteria, relationship fosters the breakdown of cellulose to smaller molecules that the animal or bacteria can utilize for nutritional value. Fibrobacter succinogenese is one of the most active cellulose-digesting bacteria ever isolated from the rumen, but our understanding of its growth and survival is very limited. Previous studies indicated that F. succinogenes cultures often died very quickly in the laboratory, even if they still had carbohydrate as a nutrient. Our experiments indicated that F. succinogenes was killed by carbohydrate when the availability of other selected nutrients such as ammonia were restricting growth. Based on these results, ruminant nutritionist should carefully design feed rations for animals that provide all of the nutrients needed by cellulose digesting ruminal bacteria. If the ediet is imbalanced, cellulose digestion and animal performance could be severely impaired.
Technical Abstract: F. succinogenes S95 cultures that were limited by ammonia with an excess of cellobiose lysed at a rapid rate (0.06 h-1) & had very low numbers of viable cells (3.3 x 10 3 cells per ml), ATP (0.13 mM) & membrane potential (45 mV). Growing cells produced acetate & succinate, but no increase in fermentation acids was noted after the ammonia was depleted, & the cells stopped growing. Ammonia-limited cells continued to utilize cellobiose, & the cellobiose was converted to cellotriose & glucose. The maximum velocities of cellotriose & glucose formation were similar (approximately 450 nmol hexose/mg protein/min), but the affinity constant for glucose formation was 15-fold lower (10 versus 0.7 mM cellobiose). Cells growing on cellobiose could not utilize glucose, & excess-glucose did not cause as dramatic a decrease in viability as excess-cellobiose. Based on this comparison, glucose formation appeared to be a mechanism of protecting cells from cellobiose toxicity. The mechanism of cellobiose toxicity could not be precisely defined. Methylglyoxal, a potentially toxic end-product of carbohydrate metabolism could not be detected (less than 0.1 mM), & F. succinogenes S85 was able to grow when as much as 2 mM methylglyoxal was added to the medium. Because the non-growing F. succinogenes S85 cells had a very high polysaccharide content, excessive intracellular glycogen deposition could be responsible for the decrease in viability.