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Digestibility and Biological Value of Bacterial Cells

Laboratory of Bacteriology, Departments of Dairy Science and Animal Science, University of Illinois, Urbana

ABSTRACT

The biological value of bacterial protein is of interest because of the role of the rumen microflora in utilization of nonprotein nitrogen (3, 9, 15, 21) and also because a portion of the nitrogen from protein degraded in the rumen (8, 19) is incorporated into bacterial protein.

Biological values ranging from 66 (10) to 88 (16) have been reported for mixed dried rumen bacteria, while other studies (20) have indicated that the biological value of rumen bacterial protein approximated that of casein. Investigators (12, 13, 20) have reported that rumen bacterial protein is deficient in methionine. Reed et al. (20) observed that rumen bacteria from sheep fed green forage contained more methionine than those from sheep receiving dry feeds. Tryptophan also has been suggested (6) as a limiting amino acid in microbial protein formed in sheep fed casein or gelatin.

Feeding trials suggest that rumen bacterial protein is deficient in methionine, but analysis of rumen ingesta from animals maintained on purified diets containing urea (5, 14) show that the ten essential amino acids are synthesized in large amounts in the rumen.