The importance enzymes and microorganisms in animal digestion
Enzymes are extremely active components in the digestion systems of all animals. Each enzyme is highly specialised and made for one role only, but in general, their job is to break various components down in order to facilitate absorption. While some enzymes, such as those found in pancreatic juices (amylase breaks sugars into glucose; chymotrypsin, peptidase and trypsin break down proteins; lipase acts on fats) and the small intestine (maltase, sucrose and lactase break complex sugars into glucose, while peptidase continues the breakdown of proteins) are somewhat universal across a wide variety of animal species, others are specific only to certain types of animal. A group of enzymes collectively known as “rennet,” for example, are only present in the stomach of mammals, and is responsible for curdling milk in order to slow its passage through the digestive system. The enzyme ptyalin is present in the saliva of almost all herbivores and omnivores and begins the breakdown of carbohydrates into sugars, but is not present in the saliva of most carnivores.
Microorganisms are also present in the digestive systems of all animals (best imaged with a microconvex or linear probe with a veterinary ultrasound), but have a far greater role in some species than others. In ruminants, for example, microorganisms play a huge part in digestion, being present in the rumen and reticulum and breaking down the tough cellulose of plants walls into glucose for the animal to use. Without these microorganisms, breakdown of this plant material would be simply impossible.
Foregut fermentation is also very significant in some highly specialised non-ruminants, such as the Hoatzin bird (Map of Life, 2010), whose enlarged muscular crop contains fermentation chambers which harbour a wide variety of microorganisms.
A similar process occurs in the colon and caecum of some hindgut fermenters. The most significant of these in terms of domestic animals is the horse, which has an extremely enlarged caecum in order to house the large populations of bacteria needed to break down the otherwise undigestible cellulose. Other hindgut fermenters include herbivorous birds and omnivores such as the pig. This symbiotic relationship between the bacteria and the animal allows the animal to unlock more nutrients from the cellulose remaining in the chyme. This is not as an efficient a process as in ruminants, however, because microbes which are washed out cannot be digested, and valuable protein is therefore lost (whereas in foregut fermenters, fermentation occurs early enough in the digestive tract that the microbes themselves can still be digested further along).
While the total population and efficiency of cellulose breakdown in the pig cannot be compared with the ruminant stomach (for reasons discussed above), certain levels of microorganisms in the pig caecum are at similar levels to those found in the rumen (Varel, 1987). There are also those that argue that the abilities of the microorganisms of the pig caecum to digest fibrous materials has been underestimated (Varel, 1987) and that the efficiency of the pig digestive system to digest fibrous materials can be increased (up to a point) through a gradual increase in the fibre content of meals (and subsequent increase in the population of micro-organisms in the caecum). It is worth noting that numerous studies have shown that attempting to change the microbal composition of the pig caecum through the direct feeding of desirable bacteria has no affect. While in humans the caecum (appendix) is significantly smaller than that of a pig, this fact does cast some doubt upon the long-term effectiveness of a number of "friendly bacteria" drinks and yoghurts.