Learning About Genetic and Environmental Factors Affecting Rumen Bugs

Diversity of rumen bugs
The rumen microorganisms are exceedingly diverse, consisting of bacteria, protozoa and fungi. Bacteria are by far the most numerous of these organisms at 10,000,000,000 cells per ml of rumen fluid. A feedlot steer may produce from 1 to 1.5 kg of rumen microbes per day. Over 200 species of bacteria have been isolated from the rumen, but only about 20 of these species occur in numbers greater than 10,000,000 per ml. Other species form a small part of the total population, but they may still be an important component of the cooperative "team" of bacteria required for feed digestion. It is this bacterial team that is responsible for the majority of feed digestion in the rumen.

Protozoa are the second most populace microorganisms in the rumen with 100,000- 1,000,000 cells ml-1 of rumen fluid and over one hundred different species identified. Protozoa are responsible for one-quarter to one-third of the fibre digestion in the rumen. However, the number of protozoa in the rumen fluctuates inversely with the number of bacteria and in fact ruminants can survive without any protozoa in the rumen. Protozoa can negatively influence protein utilization in ruminants.

Ruminal fungi are the most recently recognized group of ruminal microorganisms. Although the motile zoospores of these organisms occur in relatively low numbers (1000 to 10,000 per ml of rumen fluid) fungi may contribute up to 8 % of the total microbial biomass. Fungi are thought to be involved in the digestion of the most resistant forages such as barley straw.

Limitations in the techniques used to isolate and identify ruminal microorganisms make it almost certain that several species of the ruminal bacteria are yet to be isolated and identified. Advanced techniques of microbial identification using recombinant DNA technology are needed to identify new species of ruminal microorganisms.

Rumen environment
The volume of the rumen contents in mature beef cattle ranges from 68 litres to 86 litres with increasing levels of forage in the diet. Solid feed particles undergoing digestion by rumen microorganisms may account for 7 to 14% of the total rumen weight. Ruminal fermentation requires strictly anaerobic (without oxygen) conditions and in fact oxygen is toxic to most rumen microorganisms. Oxygen that enters the rumen either across the rumen wall or with the feed is quickly consumed by microbes adherent on the rumen wall or by oxygen tolerant anaerobes in the rumen. During fermentation, rumen microorganisms produce volatile fatty acids (i.e., acetate, propionate, butyrate) which are subsequently used as an energy source by the ruminant. Production of these acids tends to lower the pH (acidify) of rumen fluid. Generally, rumen fluid pH is high (6.0 -7.2) when forages are fed but can decline below 5.0 when the percentage of concentrate in the diet is increased. If the pH stays at 5.0 for an extended period the animal can develop acidosis or exhibit laminitis. Most often the regulatory systems in the rumen return the pH of the rumen to 6.0 - 6.2 and this cycling of pH levels can occur in the rumen on a daily basis. Thus, the rumen is a dynamic system, in which all resident species are required to adapt to a continuously changing environment.

Changes in the rumen environment
The ruminant's diet is the major influence on the nature of the rumen environment. Factors such as composition of the feed, the degree of physical processing and the presence of feed additives all affect the numbers, proportions and digestive activity of rumen microorganisms.

The most profound change in the rumen environment occurs during the transition from a forage to a grain based diet. During this transition, the principal substrates for microbial fermentation change from the components of plant cell walls (ie. cellulose, hemicellulose, pectin) to cereal starch. Fermentation of concentrates is often extremely rapid and the excessive production of acid can cause the rumen pH to decline to below 5.0. Cellulolytic bacteria and protozoa are inhibited at pH values below 6.0 and consequently, feeding mixtures of grain and forage often causes a decline in the ruminal digestion of fibre.

Since the microorganisms that digest cell wall components are different from those that digest starch, one might expect a substantial difference in the species composition of the rumen bacteria between grain and forage diets. Although this is true during transition from forage to grain, the major species in the climax microbial population are remarkably similar between these two types of diets. In one experiment using sheep as a model, cellulolytic bacterial species present were unchanged after the diet was shifted from roughage to a 70 % corn diet. Conversely, despite the absence of starch in the diet, 50 % of isolated bacterial species are still capable of digesting starch in steers fed alfalfa. Thus, the component species of the adapted microbial population are inherently stable. This stability ensures that ruminants receive a continuous, uniform supply of volatile fatty acids and microbial protein even with moderate changes in diet.

The development of a stable microbial population upon transition from a forage to a grain diet is not an immediate process. Usually the numbers of bacteria which produce lactic acid (the acid involved in lactic acidosis) increase with the introduction of grain into the diet. Simultaneously, the numbers of bacteria which metabolize lactic acid also increase and the accumulation of lactic acid in the rumen is avoided. With time, the numbers of lactic acid producing bacteria decrease and the rumen ecosystem returns to a stable condition. However, if the transition from a forage to a grain diet is too abrupt or if the particle size of the grain is too small, the microbial population may become unstable. Under these conditions lactic acid accumulates in the rumen and acid tolerant bacteria predominate. The pH of the rumen drops below 5.0 and the ruminant suffers from lactic acidosis. Additionally, the rumen contents may become viscous with the formation of a stable foam in the rumen. The foam prevents eructation, gas accumulates in the rumen and feedlot bloat develops. These conditions are largely avoided if coarse particle size grains are fed and microorganisms are given time to adapt to concentrate over a 3 to 4 week period during which increasing amounts of grains are substituted for forage at 5- to 7- day intervals.

The inherently stable nature of rumen populations makes it extremely difficult to alter the rumen environment through the use of feed additives. Probiotics, methane inhibitors, proteolysis inhibitors, buffers, microbial enzymes and ionophores have all been used in an attempt to manipulate the rumen environment. Although some of these additives may cause short term changes in the rumen environment, microbial adaptation often results in the rumen environment reverting back to pretreatment conditions. Ionophores are an exception, in that their introduction into the diet usually improves feed efficiency from 5 to 10 %. All bacterial species can be classified into two distinct groups based on the structure of their cell walls, and ionophores are found to selectively inhibit the growth of one group (Gram-positive species), which leads to a corresponding enrichment of the other (Gram-negative) group. As a result propionate production is increased, methane and lactic acid production are depressed and protein degradation is decreased in the rumen. These combined responses are likely responsible for the positive effect that ionophores have on beef cattle production.

Genetics of the ruminant
The effects of animals genetics on the rumen microorganisms is difficult to assess because of the confounding effect of diet on the composition of rumen populations. Microbial species isolated from wild ruminants (e.g., reindeer, elk, moose) are largely the same as those isolated from domesticated cattle. Studies of Svalbard reindeer have shown that changes in the rumen populations follow seasonal variations in diet, a phenomenon that has been observed in grazing cattle. Even after twenty years of selection for post weaning gain in Hereford and Angus cattle fed high and low-energy diets, differences in the population were due mainly to diet and not a result of genetic differences between breeds. Furthermore, the enhanced ability of some ruminants (North American buffalo, Asian water buffalo) to digest poor-quality feeds results from a reduced rate of feed passage within the digestive tract and increased recycling of nitrogen to the rumen rather than superior rumen bacteria. Current evidence suggests that rumen microorganisms are not unique to a given ruminant species. This line of thinking may change as molecular techniques continue to identify new species of ruminal microorganisms. It would appear that diet and geographic location have a larger influence on the composition of rumen populations than the genetics of the host animal.

Genetics of rumen microbial populations
The myriad of microorganisms within the rumen provides the rumen ecosystem with an abundance of genetic resources capable of effectively digesting the variety of feedstuffs consumed by ruminants. The complexity of the rumen ecosystem makes it difficult to measure genetic adaptations in individual species of microorganisms. Undoubtedly, genetic adaptation by rumen microorganisms is what enables ruminants to acquire increased tolerance to toxins such as mycotoxin, mimosine and nitrate. Presumably, microorganisms shift their metabolism from substrates that are commonly available in the rumen towards toxic substrates which are occasionally available for metabolism. We have recently observed an environmentally responsive adaptation of a predominant ruminal species, Fibrobacter succinogenes, to the presence of condensed tannins. This was manifested as the production of protective surface carbohydrate and the concentration of cellulolytic enzymes at the site of cell attachment. The ability to adapt is essential if a particular microorganism is to remain prevalent in a variable rumen ecosystem. The highly competitive nature of the rumen environment is such that organisms that fail to adapt to a particular environment are quickly overcome by those which are more "fit" for the digestive process. Only rarely, however, are these less "fit" microorganisms completely eliminated from the rumen. Thus, the genetic diversity of rumen populations is maintained in anticipation of future changes in the rumen environment.

Conclusion
Direct manipulation of rumen microbial populations is notoriously difficult because it is the feed that is by far the most powerful factor in determining the nature of the microbial species in the rumen. Although genetic adaptation to a specific environment is essential for ruminants to thrive, their genetic makeup appears to have little effect on the rumen microorganisms. Consequently, selection of ruminants on post-weaning gain or other performance criteria is unlikely to result in the development of a superior rumen microbial population. On the other hand, the rumen microbial populations react quickly to changes in the diet and thus short-term improvements can be achieved by manipulation of the feed.

The emerging science of genetic engineering may enable researchers to improve the genetic makeup of individual microbial species. However, if these manipulations are to result in long-term improvements in the rumen microbial populations they must be conducted with an appreciation for the competitive nature and complexity of the rumen ecosystem.

Dr. Tim McAllister
Agriculture and Agri-Food Canada, Research Centre, Lethbridge

Southern Alberta Beef Review - January, 2000. Volume 2, Issue 1