Figure 9 Generalized scheme of protein degradation in the rumen. Both bacteria and protozoa participate in the process. a-Keto acids may be used intracellularly as anabolic intermediate compounds, or decarboxylated to VFAs, which are then exported.

be Clostridium aminophilum, C. sticklandii, and Peptostreptococcus anaerobius. Classic proteolytic species such as P. ruminicola appear to be important in protein hydrolysis (Wallace et al., 1999), but they are probably less important in amino acid fermentations, as their rates of ammonia production from amino acids in vitro are one or two orders of magnitude lower. Both C. sticklandii and P. anaerobius are monensin-sensitive, which may explain the protein-sparing effect observed on inclusion of monensin in ruminant diets (Krause and Russell, 1995). Because the concentrations of peptides and free amino acids in the rumen are very low, competition for these substrates among both proteolytic and nonproteolytic microbes is probably intense.

b. Protein Synthesis Whereas the ruminal microflora is responsible for this extensive loss of feed protein, they also contribute up to half of the nitrogen requirements of the animal through synthesis of microbial cell protein, which is hydrolyzed in the abomasum and is subsequently available to the animal (0rskov, 1982). Protein synthesis by ruminal bacteria occurs primarily from ammonia and organic acids. Indeed, most ruminal bacteria will grow in vitro on ammonia as the sole nitrogen source, and many species cannot incorporate significant amounts of amino acids or peptides. Ruminal ammonia is supplied either as a direct product of the ruminal degradation of feed proteins or from urea recycled back into the rumen by the animal. The organic acids used for protein synthesis are derived from both protein and carbohydrate fermentation. Availability of these organic acids is important for adequate carbohydrate nutrition. For example, the predominant ruminal cellulolytic bacteria require isobutyrate, isovalerate, and 2-methyl-butyrate as precursors for intracellular synthesis of the branched chain amino acids valine, leucine, and isoleucine, respectively (Bryant, 1970). This provides an excellent example of both the interactions among different physiological groups of ruminal bacteria and the interaction between energy and protein metabolism in ruminant nutrition.

Because of their impact on production of microbial protein, quantitative aspects of microbial cell yield have received considerable attention. The efficiency of microbial growth (growth yield) varies among species and with growth conditions. Important determinants of growth yield include (a) efficiency of energy conservation (ATP production per unit substrate consumed), (b) ability to import and incorporate preformed organic compounds (e.g., amino acids) into cell material, (c) maintenance energy (the amount of energy that must be expended to maintain cellular constituents and function), and (d) extent to which cells carry out other non-growth-related functions such as polysaccharide storage or wasteful ''energy spilling'' (Russell and Cook, 1995). A microbe's growth rate also has an impact on cell yield. At low growth rates, yields are depressed somewhat, because a larger portion of the total energy expenditure is devoted to maintenance.

Carbohydrate-fermenting ruminal bacteria have true growth yields (cell yields not corrected for maintenance) within the range of 0.1-0.6 g cells/g carbohydrate; in some instances, these yields may be artificially high if the organisms synthesize storage polysaccharides (Table 7). Cell yields of ruminal bacteria decline when the pH of the environment decreases below 6 (Russell and Dom-browski, 1980). Nevertheless, the growth yields of ruminal bacteria are generally higher than those of anaerobic bacteria native to other anaerobic environments (Hespell, 1979).

Microbial growth yield is affected by growth rate-induced metabolic shifts that alter the ATP yield. For example, increased growth rate on sugars in some species is accompanied by a shift in fermentation products from acetate to lactate and a reduced ATP yield (because conversion of pyruvate to acetate results in formation of one unit of ATP, whereas the conversion of pyruvate to lactate does not) (see Fig. 5). In this instance, the organisms have increased growth rate by selecting a pathway with an inherently high substrate flux (rate of substrate consumed per unit time) at the sacrifice of some ATP yield. By contrast, interspecies H2 transfer reactions increase the ATP yield of the H2 producers by allowing more of the organic substrate to be converted to acetate and less to other compounds (e.g., ethanol or lactate) (Wolin, 1990).

Table 7 Growth Yields and Maintenance Coefficients for Several Species of

Ruminal Bacteria Grown in

Continuous Culture

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