Zoo-Microbial Interactions and Nitrogen Metabolism of Herbivorous Mammals
Nitrogen balance of adult mammals is always close to zero; the total nitrogen content of the body practically does not vary in the midterms. However, significant amount of amino acids are deaminated in the liver and muscles daily, and it is necessary to compensate these losses. Carnivores, consuming high-protein forage easily recover amino acids losses, but herbivores feeding on low-protein forage may lack dietary nitrogen. Paradoxically, the intensity of amino acids deamination in herbivores is on the same level in comparison with carnivores. To deal with the lack of nitrogen they can only by the symbioses with microorganisms inhabiting their gastrointestinal tract. Some bacteria are capable to fix nitrogen. Urealytic bacteria carry out the urea recycling process; that is, they reuse nitrogen, convert it into amino acids, and return to the host organism. For a long time it was assumed that mammals are able to regulate this process themselves. However, analyzing literature as well our own data, we found that the intensity of nitrogen recyclization is poorly regulated by the host organism and almost completely depends on urease activity of the symbiotic microorganisms. Thus, nitrogen balance of herbivorous mammals is a consequence of close zoo-microbial interactions that have developed as a result of co-evolution.