Relations of rumen ciliates to their hosts
Ciliates swarm in such tremendous numbers in the rumen and reticulum that everyone who has seen them has speculated on their role in their host's nutrition. This problem has been reviewed by Hungate (1950, 1955) and Oxford (1955, 1955a), to whose papers reference is made for further details. It should be said that in these reviews the name "Diplodinium" is used for practically all the ophryoscolecids except Entodinium and Ophryoscolex, but the other genera involved can often be determined from their specific names.
The rumen ciliates are obligate anaerobes. The holotrichs (Isotricha and Dasytricha) have been cultivated by Sugden and Oxford (1952), Gutierrez (1955) and others. Diplodinium, Entodinium, Eudiplodinium, Polyplastron and Metadinium have been cultivated by Hungate (1942, 1943), Sugden (1953), and others, but Ophryoscolex has not yet been cultivated.
The holotrichs absorb soluble carbohydrates from the medium and convert them into amylopectin, which is stored in ovoid granules measuring 3 by 2 u and resembling small yeast cells. They are able to utilize glucose, fructose, sucrose, cellobiose, inulin and levans. In addition, both Isotricha intestinalis and I. prostoma rapidly ingest small starch granules and are able to metabolize them. Dasytricha ruminantium does not ingest starch. Gutierrez and Hungate (1957) found that D. ruminantium ingested small cocci and occasionally small rod-shaped bacteria; they were able to cultivate this species in a medium containing these types of bacteria, but not without them. Gutierrez (1958) showed that Isotricha prostoma feeds selectively only on certain rods among the many types of rumen bacteria, but that pure strains did not fulfill all the protozoon's growth requirements, since it divided once and then died out in a monobacterial culture.
The holotrichs produce hydrogen, carbon dioxide, lactic, acetic and butyric acids, and traces of propionic acid (Heald and Oxford, 1953; Gutierrez, 1955).
Many but not all species of Entodinium ingest and digest starch. According to Kofoid and MacLennan (1930), E. longinucleatum and E. acutonucleatum feed selectively on pollen grains. Certain species of Entodinium are the predominant starch-ingesters among the rumen protozoa and are the dominant protozoa in animals on full feed. Among those known to ingest starch are E. caudatum, E. longinucleatum, E. minimum and E. dubardi. Almost nothing is known about the products of starch fermentation by this genus. Granules of polysaccharide are stored in the outer zone of the endoplasm, but they have never been isolated and identified; it would be difficult to separate them from ingested starch granules.
It has been suggested that carbohydrate metabolism is dependent upon intracellular bacteria. Sugden (1953) was unable to cultivate E. longinucleatum in the presence of streptomycin except when streptomycin-resistant strains of bacteria were present. However, Appleby, Eadie and Oxford (1956), who found various bacteria in disintegrated Entodinium, concluded that there was so far no good reason for denying the existence of protozoan enzyme systems concerned with carbohydrate fermentation. Gutierrez and Davis (1959) found about 100 to 150 gram-positive diplococci (Streptococcus bovis) per ciliate in E. caudatum, E. minimum, E. dubardi, E. longinucleatum, E. bursa, E. nanellum, E. exiguum and E. vorax in cattle being fed a high starch ration. The ciliates sometimes ingested starch granules with adherent starch-attacking bacteria. Entodinium species could be cultivated in the presence of S. bovis but not without it. Thus, bacteria are ingested by Entodinium and appear to be necessary for its nutrition, but most likely as a source of nitrogen rather than of prefabricated enzymes.
Epidinium, like Entodinium, ingests starch and also bacteria; its metabolic products are also unknown. Gutierrez and Davis (1959) found that E. ecaudatum (syn., Diplodinium ecaudatum) ingested not only Streptococcus bovis but also other bacteria.
Diplodinium and related genera (Eudiplodinium, Polyplastron, Eremoplastron, Metadinium) ingest and digest cellulose in addition to starch and bacteria. Hungate (1942, 1943) cultured Diplodinium dentatum (syn., D. denticulatum), Polyplastron multivesiculatum and Eudiplodinium maggii in media containing dried grass and pure cellulose, but the protozoa failed to grow if the cellulose was omitted. These species and Eremoplastron neglectum all contained a cellulase. Sugden (1953)found that Metadinium medium also utilized cellulose. Gutierrez and Davis (1959) found that E. neglectum and a large unidentified species of "Diplodinium" contained gram-positive diplococci and other bacteria on different occasions, Sugden and Oxford (1955) found that a "pure", washed, living suspension of Metadinium medium had no action on glucose in the Warburg apparatus.
Diplodinium and related species were found by Hungate (1946) to produce hydrogen, carbon dioxide and volatile acids.
The skeletal plates of all ophryoscolecids which have them stain brown with iodine and are polysaccharide in nature. According to Oxford (1955), Hirst et al. extracted enough of the storage polysaccharide from Metadinium medium to identify it as of the "glycogen-amylopectin" type, but they were not sure whether it was pure amylopectin.
The mode of nutrition of Ophryoscolex has not been determined, altho it is known to ingest starch granules and sometimes cellulose fibers.
Lubinsky (1957b) reported that accidental predation on smaller protozoa is a common trait of many of the larger species of Ophryoscolecidae, particularly of Diplodinium and related cellulose-feeding genera. Predation is rare in Ophryoscolex, however. The prey of these occasional predators consists primarily of spineless smaller species. The spines are thus of value in protecting the smaller ophryoscolecids against ingestion. Lubinsky gave a table listing cases of predation among ophryoscolecids from the Canadian reindeer, goat, sheep and Indian water buffalo, which included 8 genera and 9 species of predators and 7 genera and 9 species of prey.
The role of the rumen protozoa in their host's nutrition is still not clear. Young animals on a milk diet do not have them. As they grow older and begin to feed on hay and grass, they become infected from protozoa in the saliva of faunated animals. This is the only way in which transmission occurs. There are no resistant forms or cysts, and the protozoa are killed when they enter the abomasum.
The relation between the protozoa and their hosts is not symbiotic, since the host does not need the protozoa for survival, and indeed gets along perfectly well without them. Becker, Schulz and Emmerson (1929, 1930) and Winogradow et al. (1930) killed the protozoa in the rumens of goats without harming the goats. The defaunated animals continued to break down cellulose just as actively as the normal controls, due to the action of cellulolytic bacteria. Pounden and Hibbs (1950) raised calves successfully without protozoa.
The fact that defaunation is not harmful does not mean, however, that the protozoa are of no value to their hosts. It means simply that they are not essential.
It has been suggested that the protozoa might harm their hosts by excreting ammonia which may then not be utilized by the rumen bacteria for protein synthesis and which would therefore be lost to their hosts; by robbing the host of B vitamins; by feeding on and destroying valuable bacteria; or by producing lactic acid and other undesirable intermediate products of carbohydrate metabolism which the rumen bacteria cannot cope with (see Oxford, 1955). However, there is no proof that they are actually harmful, and this is simply speculation.
Rumen protozoa form about 20% of the protein which reaches the abomasum (Hungate, 1955). McNaught et al. (1954) found that the rumen protozoan and bacterial proteins both had a biological value for rats of 80 to 81, which is higher than that of brewer's yeast (72). Furthermore, the true digestibility of the protozoan protein was 91%, much higher than that of the bacterial (74%) or yeast (84%) proteins. Hence the protozoan protein is nutritionally superior. No amino acid analyses have been carried out on it.
While many of the protozoa store reserve starch (amylopectin), this stored starch is not of much importance for the host's nutrition. About 1% of the carbohydrate required by a mature sheep is supplied from this source (Hungate, 1955).
The protozoa are an important source of volatile fatty acids. Carroll and Hungate (1954) estimated that about 2.2 kg of volatile fatty acids are produced per 100 kg rumen contents in cattle. Gutierrez (1955) calculated that the fermentation acids produced by the rumen holotrichs would constitute a little more than 10% of this amount. If the ophryoscolecids produced an equal amount, then protozoa would provide about 20% of the fermentation products available to their host (Hungate, 1955). As Hungate (1955) remarked, Gruby and Delafond, who first discovered the rumen protozoa in 1843, guessed that they supplied 1/5 of the food used by their hosts, and the results of investigations during the next 110 years have not significantly modified that estimate.
Another advantage to the host lies in the fact that the holotrichs take up soluble carbohydrates from the medium and convert them into stored starch, withholding them for a while and then fermenting them for a long time. This smooths out the fermentation process, which would proceed much more irregularly if it depended upon bacteria alone (Hungate, 1955; Oxford, 1955). Entodinium and Epidinium, too, help smooth out the fermentation process by converting starch into reserve foods. In addition, as Hungate (1959) pointed out, when animals are shifted from hay to grain, there is a period of adaptation during which lactic acid is produced explosively by Streptococcus bovis and may be extremely harmful. The adaptation period may be due to the time needed for Entodinium, Epidinium and other bacteria-feeding protozoa to multiply enough to keep the streptococci in check.