In recent decades several methods have been developed for determination of the proportion of nitrogen-containing substances passed from the rumen into the abomasum, or small intestine, which are of microbial origin. Recently, when examining the D-amino acid content of foodstuffs, particularly milk and milk products, it was observed that, in addition to D-alanine (D-Ala), D-glutamic acid (D-Glu) and D-aspartic acid (D-Asp) can also be detected in similar quantities, primarily in products which have links with bacterial activity. This gave rise to the idea of examining the diaminopimelic acid (DAPA), D-Glu, and D-Asp content of bacteria extracted from the rumen of cattle, and that of chyme from the same cattle, to establish whether D-Asp and D-Glu can be used to estimate protein of bacterial origin. The investigations performed have provided evidence that both D-Asp and D-Glu might be appropriate for determination of protein of bacterial origin. The results obtained using these two bacterial markers (D-Asp and D-Glu) proved to be approximately 10% lower than those obtained using DAPA; this was not because of to error attributable to the new markers but rather to the unreliability of determination using DAPA Analyses performed on samples of known bacterial protein content indicate that D-Asp and D-Glu gave almost identical results for bacterial protein content which were very close to the theoretical (calculated) values.

Several methods have been developed for the determination of the proportion of nitrogen-containing substances of microbial origin in the digestive tracts of cattle. These include assays that use nucleic acids and adenosine triphosphate as indicators, radioisotopes ³⁵S, ¹⁵N, ³²P and ³³P incorporated into bacterial protein and phospholipids, duodenal amino acid composition with amino-ethylphosphonic acid, diaminopimelic acid and D-alanine (D-Ala) contents as indicators. On the basis of the data in the literature on D-amino acid content of milk and milk products the authors came to the conclusion that D-glutamic acid (D-Glu) and D-aspartic acid (D-Asp) can be considered as markers for proteins of bacterial origin. To demonstrate this, experiments have been carried out and some of the results are reported here. © 2001 Elsevier Science B.V.