The Sites of Fermentative Digestion Must Be Conducive to Microbial Growth
Fermentative digestion occurs in specialized compartments that are positioned either before or after the stomach and small intestine. Fermentative compartments positioned before the stomach are called forestomachs and are most highly developed in the ruminants and camel ids.
The size and development of the forestomach fermentation compartments vary greatly among species; many species have distinct forestomachs that are less developed than those of ruminants. Some species, including the horse and rat, have no anatomically distinct forestomach; however, some fermentative digestion may occur in a non- glandular portion of the proximal stomach.Fermentation compartments positioned distal to the small intestine are the cecum and colon, often collectively called the hindgut. As with the forestomach, great anatomical differences exist in the hindgut of various species. This variation can be so extensive that the cecum and colon may appear to be functionally different organs in different species; however, when the variations are evaluated critically, important similarities can be seen in hindgut function among species.
The forestomach and hindgut can support fermentative digestion because their pH, moisture, ionic strength, and oxidation-reduction conditions are maintained in a range compatible for the growth of suitable microbes. In addition, the flow of ingesta through these areas is comparatively slow, allowing microbes time to maintain their population size. The importance of these factors can be illustrated through comparison of the forestomach and colon to the stomach and small intestine. In the stomach, bacteria) numbers are kept low by the acid pH, whereas in the small intestine, bacterial numbers are kept in check by the constant flushing action of ingesta and secretions. In contrast, the pH in the forestomach and large colon is close to neutral, and the flow rate is comparatively slow.
In general, the fermentative patterns of the hindgut appear to be similar to those of the forestomach, although forestomach fermentation, especially that of the rumen, appears to be the better studied of the two. I he following discussion
Box 31-1
Grouping of Rumen Bacterial SpeciesAccording to Type of Substrates Fermented
| Major cellulolytic species Bacteroides Succinogenes Ruminococcus flave faciens Ruminococcus albus Butyrivibrio fibri so I ve ns | Major methane-producing species Methanobrevibacter ruminantium Methanobacterium formicicum Methanomicrobium mobile |
| Major hemicellulolytic species Butyrivibrio fibrisolvens Bacteroides ruminicola Ruminococcus species | bgcolor=white>Major sugar-utilizing species |
| Major pectinolytic species Butyrivibrio fibrisolvens Bacteroides ruminicola Lachnospira multiparus Succinivibrio de?trinosolvens Treponema bryantii Streptococcus bovis | Major acid-utilizing species Megasphaera elsdenii Selenomonas ruminantium Major proteolytic species Bacteroides amylophilus Bacteroides ruminicola Butyrivibrio fibrisolvens Streptococcus bovis |
| Major amylolytic species Bacteroides amylophilus Streptococcus bovis Succinimonas a my Iolytica Bacteroides ruminicola | Major ammonia-producing species Bacteroides ruminicola Megasphaera elsdenii Selenomonas ruminantium |
| Major ureolytic species Succini vibrio de?trinosolvens Selenomonas species Bacteroides ruminicola Ruminococcus bromii Butyrivibrio species Treponema species | Major Iipid-Utilizing species Anaerovibrio Iipolytica Butyrivibrio fibrisolvens Treponema bryantii Eubacterium species Fusocillus species Micrococcus species |
From Church DC, editor: The ruminant animal, digestive physiology and nutrition, Englewood Cliffs, NJ, 1988, Prentice-Hall.
focuses on rumen digestion but includes comments on hindgut digestion. Digestion in the equine cecum and colon is discussed at the end of the chapter.