The Microbes Responsible for Fermentative Digestion Include Bacteria, Fungi, and Protozoa
The bacterial population associated with fermentative digestion is vast, with at least 28 functionally important species occurring in the rumen. Box 3l-l lists some of the major species found in the rumen and their preferred substrates.
Total bacterial numbers in the forestomach or hindgut normally range from lθ,° to I On cells per gram of ingesta. Most of these bacteria are strict anaerobes that cannot survive in the presence of oxygen, although facultative organisms arc also present. In the rumen, fungi are present, and research suggests that fungi may play an important role in the digestion of plant cell walls.There is also a large population of protozoa in the rumen as well as in the cecum and colon. Protozoal numbers average about IO3 to IO6 cells per gram of rumen contents. Although this number is considerably smaller than the number of bacteria, the relatively larger size of the individual protozoa compared with bacteria results in a total rumen protozoal cell mass approximately equal to the bacterial cell mass, under most dietary conditions. Most of the rumen protozoa are ciliated and belong to the genus Isotricha or Entodiniumt although flagellate species are also present, especially in young ruminants. As with the other organisms of the rumen, the protozoa are anaerobic.
The digestive abilities, or capacities, of protozoa and bacteria are similar; thus either type of organism can perform most of the fermentative functions of the rumen. Protozoa ingest large numbers of bacteria and hold rumen bacterial numbers in check. However, none of the actions of protozoa appears essential to rumen function because ruminants can survive well without protozoa. Thus the role of protozoa in the total ecological picture of the rumen is uncertain. One potentially important function of protozoa involves their ability to slow down the digestion of rapidly fermentable substrates, such as starch and some proteins. Protozoa are capable of ingesting particles of starch and protein and storing them in their bodies, protected from bacterial action. The starch and protein remain engulfed until digested by the protozoa, or until the protozoa die or are swept from the rumen into the lower digestive tract. Thus, protozoa may have the effect of delaying or prolonging the digestion of these substrates. Especially in the case of starch, this protozoal effect may be beneficial to the host through modulation or delay of the digestion of rapidly fermentable substrate.