Author
 |
WELLS, JAMES - CORNELL UNIVERSITY |
|
Russell, James |
|
Submitted to: Applied and Environmental Microbiology
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 1/29/1996 Publication Date: N/A Citation: N/A Interpretive Summary: Providing adequate protein is a major cost of production by ruminant farm animals (cattle, sheep, goats). Microbial proteins, which are synthesized in the rumen, are the primary source of amino acids for the animal. However, much of the microbial protein turns over before it leaves the rumen, i.e. some of the microbial protein is digested to its component amino acids and peptides and then reincorporated into new protein. This turnover of microbial protein in the rumen is wasteful and decreases amino acid availability to the animal. Bacterial turnover in the rumen has been largely blamed on protozoal predation, but other workers have noted that bacterial protein also turns over in animals that are protozoa-free (defaunated). Our work showed that the lysis or turnover rate of the cellulolytic bacterium, Fibrobacter succinogenes, was a growth-rate- independent process that occurred even in rapidly growing cultures. F. succinogenes uses an extracellular proteinase (protein-digesting enzyme) to regulate its lysins (compounds that cause the bacteria to rupture), but this regulation is not good enough to prevent lysis. It may be possible to decrease the impact of bacterial turnover by increasing the fluid dilution rate of the rumen. An increase in fluid dilution rate, would wash the bacteria out of the rumen before they lyse. One can increase the fluid dilution rate by feeding salt. These results contribute to development of techniques for improving the feed-use efficiency of ruminant animals. Technical Abstract: Several metabolic inhibitors induced lysis of Fibrobacter succinogenes and lysis was proportional to the rate at which cultures had been growing. Iodoacetate, tetrachlorosalicylanilide, & dicyclohexylcarbodiimide decreased both membrane potential & intracellular ATP, but monensin only decreased ATP. Based on these comparisons, it appeared that ATP was a more important regulator of lysis than membrane potential. Because only cultures entering the stationary phase were sensitive to phenylmethyl- sulfonyl fluoride, an inhibitor of serine proteinases, it appeared that F. succinogenes was degrading & inactivating autolysins when the cell wall was no longer expanding. When F. succinogenes was grown in a medium containing only ammonium as a nitrogen source, 80% of ammonia utilization could be accounted for as cell nitrogen components (protein, ribonucleic acids, & deoxyribonucleic acids). As dilution rate of continuous cultures decreased, F. succinogenes retained less nitrogen & ammonia consumption was twice as high as cell nitrogen production. Nitrogen-containing compounds accumulated in cell-free culture fluid & much of the amino nitrogen was present as peptides. When 1/yield (mg cellobiose fermented/mg cell nitrogen) was plotted against 1/dilution rate (h), the relationship was linear & the apparent maintenance coefficient (slope) was 2.4 mg cellobiose/mg cell nitrogen/h. If yield was based on ammonia utilization, the maintenance coefficient decreased to 0.38 mg cellobiose/mg nitrogen utilized/h, but theoretical maximal growth yield was the same (33.3 mg cell nitrogen/mg cellobiose fermented). Based on differences in the apparent maintenances, the cells were turning over & lysing at a rate of 0.006 h-1, irrespective of growth rate. |
