Sheep are ruminants whose nutrition is critically dependent on their highly specialised digestive system.
Adult ruminants have a digestive system that is very different to that of non-ruminants.

The most striking difference is that the oesophagus delivers food to the reticulo-rumen. Functionally this is a single, large "fermentation chamber" containing micro-organisms which convert plant carbohydrate to volatile fatty acids (mainly acetate, propionate and butyrate), lactate, carbon dioxide, methane and hydrogen.

Digestion of non-fermented food materials, and of the microbial biomass occurs subsequently and is equivalent to the digestive processes of non-ruminants. In ruminants this process starts with peptic digestion in the abomasum.

The microbial life forms that live in the reticulo-rumen include bacteria, protozoa and fungi. They proliferate under warm, dark, anaerobic conditions in a buffered aqueous medium.

They use the host ruminant's feed material as their primary growth substrates and of prime importance in this regard are plant carbohydrate polymers which ultimately provide the energy for growth.

To be used for this purpose plant carbohydrate polymers first have to be hydrolysed to their component monosaccharides.

However, not all micro-organisms can use these plant carbohydrate polymer feed materials directly.

Only a few species of bacteria and fungi can elaborate amylases, cellulases and hemicellulases which hydrolyse the feed polymers and release the monosaccharides which they need for their energy metabolism. These micro-organisms are the only ones that can use the primary feed carbohydrates.

Many of these hydrolytic enzymes are extracellular, they are secreted into the growth medium, and so some monosaccharides are released into the reticulo-rumen fluid and can be used as growth substrates by other microbial species that do not produce these extracellular hydrolases. This is the most fundamental example of the dependence of one group of organisms on the activities of another, but you should note that the benefit is all "one-way"; the organisms that make the extracellular cellulases don't require the presence of the other groups for their own growth.

In the reticulo-rumen there are other much closer associations between different species of micro-organisms where both gain from the presence of the other and these are described as syntrophic relationships.  In a syntrophic relationship one species of micro-organism (the recipient) uses, as a growth substrate, a waste product of the metabolism of another species (the donor) and as the result of removal of the waste product the growth of the donor species is improved. The benefit is not just "one-way".  Within the reticulo-rumen the waste products commonly utilised are hydrogen, carbon dioxide, lactate and succinate and we shall examine the patterns of energy metabolism in the species that produce and in the species that use these materials and live in syntrophic relationships.

Section 2.3 in the monograph by Fenchel and Finlay contains a good brief analysis of syntrophy.

It should be noted also that this concept of syntrophy as conferring mutual benefit to different organisms that live in association with each other is quite similar to the popular concept of symbiosis.   The monograph by Angela Douglas however presents a different view of symbiosis where mutual benefit is not the key feature of the association.  Douglas concludes from her analysis of many examples that symbiotic associations are associations between different species that persist for prolonged periods, whose common feature is the novel metabolic capability acquired by one organism from having a functional link with a partner organism.

The relationship between the host ruminant and the microorganisms in the reticulo-rumen is usually regarded as being symbiotic in line with the popular notion of mutual benefit but this relationship also holds up as symbiotic from the more specialised viewpoint advanced by Douglas. The patterns of intermediary metabolism seen in many of the tissues of the host ruminant are very different to the patterns of intermediary metabolism seen in the tissues of carnivores because of the metabolic activities of the microorganisms that live in the reticulo-rumen.

In this CAL we shall not be discussing the importance of the various protozoa and fungi that also inhabit the reticulo-rumen because there is relatively little that is established with certainty about their contributions. This is particularly true of the protozoa because all are phagotrophs and many of them contain endosymbiotic bacteria which makes assignment of the contributions of the individual organisms very difficult.

Lifestyles of the Bacteria that live in the reticulo-rumen
 
 
Bacterial cell bearing polar flagellae.
The bacteria that live in the reticulo-rumen can be broadly classified according to their growth characteristics as chemo-autotrophicor chemo-heterotrophic.

The autotrophic species can use carbon dioxide as the sole carbon source for growth and the most important are the methanogenic and the homoacetogenic bacteria.

The carbon doxide they use is waste from the metabolism of other heterotrophic microorganisms living in this growth environment.

The heterotrophic micro-organisms are much more diverse. There are perhaps 200 species present. They use organic chemicals as the source of carbon for growth. These growth substrates include monosaccharides (released by hydrolysis from the host ruminant's feed carbohydrate) and in some cases individual species of microorganisms use organic acids such as succinate and lactate which are waste products of the metabolism of some of the other species growing in the reticulo-rumen.

The mechanism of ATP synthesis differs in autotrophic and heterotrophic microorganisms.

The autotrophs take up hydrogen from the growth environment and use it to reduce carbon dioxide. This is achieved using an electron transport chain, which, by a chemiosmotic process, maintains a transmembrane electrochemical gradient of protons or sodium ions. Synthesis of ATP is coupled to dissipation of this gradient and the mechanism of ATP generation is described as anaerobic respiration.

The heterotrophs by contrast mainly ferment their organic growth substrates in reaction sequences reminiscent of the pathways of intermediary metabolism that you already know about from your study of animal biochemistry, and ATP is synthesised mainly by substrate level phosphorylation.

When studying the metabolism of heterotrophs, it is most useful to keep in mind the relationships between the need to maintain "redox balance" and production of individual fermentation products.

This classification of the lifestiles of rumen micro-organisms however is overly simplistic because many heterotrophs can obtain a proportion of their ATP by some variant of anaerobic respiration, and one of the autotrophic groups; the homoacetogens can adapt to a heterotrophic life style.  They can ferment monosaccharides to yield much of their ATP by substrate level phosphorylation and can still "fix" carbon dioxide using hydrogen taken up from the growth environment and obtain some extra ATP by anaerobic respiration.

These features will be analysed more thoroughly when we examine the contributions of individual bacterial species.

Some Important Species of Rumen Bacteria

Fibrobacter succinogenes is the predominant cellulolytic Gram-negativebacterial species in the rumen. It ferments glucose and produces acetate and succinate as waste products. Most ATP is derived from substrate level phosphorylation but some is also derived from anaerobic respiration coupled to the production of succinate.

Ruminococcus flavifaciens is a Gram-positive, cellulolytic bacterium. It is the most active species involved in the digestion of plant cell walls due to its high cellulase and hemicellulase activity. It produces hydrogen, acetate and succinate as end products. The host uses the acetate as an oxidisable substrate and the succinate is used as a growth substrate by some propionate producers. The hydrogen is an important source of reducing equivalents supporting the growth of the methanogens and the homoacetogens.

Megasphaera elsdenii is a Gram-negative coccus and is the predominant bacterial species in the rumen of young ruminants. It is important because it ferments glucose to propionate which is then available to the host for gluconeogenesis.

Selenomonas ruminantium is a non-cellulolytic Gram-negative species which ferments glucose and occurs in large numbers when ruminants are fed grain. This species is an important producer of  propionate which is used by the host ruminant for gluconeogenesis.

Veillonella parvula is a Gram-negativebacterium that uses lactate as a growth substrate, from which it makes acetate and propionate. Acetate is produced as the end product of a metabolic pathway that generates ATP by substrate level phosphorylation. Succinate is an intermediate in the propionate production pathway and this organism also takes up succinate produced by other bacterial species and uses it as a growth substrate. The use of succinate in this way allows Veillonellato make additional ATP by a chemiosmotic mechanism based on dissipation of an electrochemical transmembrane sodium ion gradient. The energy needed to create the gradient comes from the exergonic decarboxylation of methylmalonyl CoA produced from succinate.

Butyrivibrio fibrisolvens are Gram-negative cellulolytic bacteria, common in the rumen, producing acetate and butyrate by fermentation of glucose and other substrates.

Lactobacillus ruminis is predominant in the reticulo-rumen of young animals. It is an important glucose fermenter and produces mainly lactate. Streptococcus bovis is another important glucose fermenter also producing mainly lactate. Normally lactate is a valuable gluconeogenic substrate for the host ruminant in addition to being used as a growth substrate by propionate producers such as Veillonella parvula. These two lactate producers can however create highly acidic conditions in the rumen if the animal eats excessive amounts of readily fermentable carbohydrate. This can lead to the serious clinical condition known as lactic acidosis which is part of the cereal overeating syndrome.

Methanobacterium ruminantium and Methanosarcina barkeri are important rumen methanogens.  All methanogens are strict anaerobes and they obtain their ATP by a form of anaerobic respiration linked to a chemiosmotic mechanism. They live in syntrophic relations with other bacteria that produce hydrogen and carbon dioxide (as waste products) which the methanogens use for their growth. Uncertainty exists as to whether an electrogenic sodium ion or a proton gradient is generated by the electron transfer processes that use hydrogen to reduce carbon dioxide. Methanogens are very important  because their abundance in the reticulo-rumen contributes to determining feed conversion efficiency of the host ruminant.

In summary this circumstance arises from the fact that co-culture with methanogens improves the growth of some species of hydrogen producing heterotrophic microorganisms but alters the formation of their organic acid end product metabolites.  Although the microbial biomass is increased the availability of particular metabolites for use by other bacteria is altered and this alters the materials available to the host ruminant.

Of particular importance in this regard is the decrease in succinate production by Ruminococcus flavifaciens that occurs in co-culture with methanogens. We shall study this in detail when we address the issue of interspecies hydrogen transfer.

The second reason methanogens affect food conversion efficiency is that methane is lost from the rumen by eructation and this represents a loss of feed carbon. We address this issue when we deal with Agribusiness interventions.

Acetomaculum ruminis and Ruminococcus schinkii are homoacetogenswith growth characteristics somewhat akin to the methanogens in that they can grow as chemoautotrophs with CO2 as their source of  carbon for growth and they use hydrogen to supply reducing equivalents for its reduction.  The reduced product is acetate. In this growth mode they derive their ATP by a form of anaerobic respiration which depends on electron transport maintaining a transmembrane electrogenic sodium ion gradient which provides the free energy to drive a membrane bound ATP synthase.

As indicated previously these micro-organisms are very adaptable. They are not restricted to growing as chemoautotrophs. They can also grow as chemoheterotrophs fermenting glucose to derive some of their ATP by substrate level phosphorylation.  When they do this they produce carbon dioxide and hydrogen and then they can obtain an extra yield of ATP by anaerobic respiration using the hydrogen to reduce the carbon dioxide to acetate as in the autotrophic growth mode.

Now that we have had a brief look at the reticulo-rumen environment and the micro-organisms that live there, we look at the ruminant feedstock.