112. Intestinal Bacteria

So far, in presenting the relation of ferments to digestion, only the unorganized ferments or enzyms have been considered. While these are chiefly concerned in normal digestion, organized ferments are present throughout the entire intestinal canal and play a part in food changes. They are most abundant and active in the lower part of the small intestine and the upper part of the large. They act upon the proteins, causing putrefaction, dissolve cellulose, and cause a decomposition of the carbohydrates. The products of these fermentations may be in part the same as those produced in pancreatic digestion, but these include also indol and skatol, which have the characteristic fecal odor, volatile fatty acids, and gases, some of which are carbon dioxid, hydrogen, marsh gas, and hydrogen sulfide.

Under certain conditions fermentations of this character, which up to a certain extent are normal and may be beneficial, proceed so far as to be deleterious to health. Anything which retards digestion, such as imperfect mastication, excessive eating, abnormal amounts of meat in the diet, and failure of the organs secreting the digestive fluids to supply these fluids in sufficient abundance, gives these bacteria a better opportunity to act on the food residues, and increases their effect. Some foods, especially vegetables of the leguminous class, appear to be provocative of excessive intestinal fermentations. Flatulence, and even toxic poisoning may be the result of great bacterial activity in the digestive tract. It is hardly possible to check this by administering septics, but purging is of value in removing the fermentative material. At one time it was held that the bile has a specific antiseptic effect, but later researches throw doubt on this conclusion. Probably the bad results of a restricted flow of bile are indirect, the less perfect digestion giving the bacteria a greater opportunity. Free hydrochloric acid restrains bacterial fermentation and has this effect in the stomach, but this influence can hardly extend to the intestines, for the free acid is neutralized before it reaches that point. Particular foods, especially milk and kephir, have been shown to have a preventive action on putrefaction.

113. Digestion Of Food As A Whole

From what has preceded we learn that several liquids and certain organisms participate in producing the complex changes that food undergoes during digestion. Some of these liquids have certain common functions, as for instance, proteins are dissolved both in the stomach and by the pancreatic juice. Moreover, the various digesting fluids appear to act cooperatively. This is made plain by following the course of the food changes. After the food has remained in the stomach for a short period of time, it is gradually discharged into the small intestine, the rate of discharge varying with the kind of food, that is, with the promptness and rapidity of digestion, which differs with different foods. The progress made up to this point in food transference, so far as we have definite knowledge, is chiefly the cleavage of the proteins into various stages of hydrolysis, the resulting bodies being proteoses and peptones. All proteins appear to be acted on in the stomach, but to different degrees and probably at different rates. It seems probable that the simple proteins are as fully" dis-solved as any, while some of the conjugated and derived proteins, such as the nucleo-proteins and those that are coagulated by heat, are at least more slowly, and in some cases less perfectly broken up. Starch, already somewhat dissolved by the saliva, is not further acted upon by the stomach enzyms, neither are the solid and liquid fats affected to any discoverable extent. Simple sugars are not acted upon by the gastric juice, but it seems probable that the di-sugars may be split into simple ones by the hydrochloric acid. It appears then, that in the intestines protein digestion must be completed, the larger part of the starch transformed to sugar and the digestion of the fats wholly accomplished or mainly so. As a matter of fact, the partial solution in the stomach of the proteins and the swelling of the undissolved part to a gelatinous mass may be considered as a preparation of the food for intestinal digestion, for through these changes the proteins present a larger surface to the attack of trypsin and other intestinal enzyms and digestion proceeds more promptly than would be the case with the freshly ingested food. Moreover, the compounds in the chyme, especially the acid, react on the liver and pancreas, and cause an abundant flow of digestive fluids from these glands.

As soon as the chyme mixes with the bile and pancreatic juice, the mass is changed from an acid to an alkaline condition. This seems to be essential to the effective operation of the pancreatic ferments. While the pancreatic juice will carry on digestion by itself, this is not satisfactory in the absence of bile, for when the latter is not permitted to enter the small intestine, the digestion of fats is very imperfect. It seems essential that these two liquids act together. The bile aids in rendering the digesting mass alkaline, contributes to the formation and solution of the fatty acids and soaps, and in these ways and others not altogether explainable promotes the activity of the pancreas enzyms. The juices that flow from the small glands in the intestinal walls appear to essentially supplement the work of the bile and pancreatic juice. In the first place, they probably contain a substance that makes active the mother substance of trypsin, in the second place, they aid in splitting the peptones into simpler bodies, and lastly, they convert the sugars into the final form (dextrose) in which they are absorbed into the blood circulation. If we consider the digestion of the food compounds by classes, the following is a summary of the ways in which they are acted upon: pepsin, trypsin, and terepsin secreted by the stomach, pancreas, and intestinal glands act on the proteins; ptyalin in the saliva, amylopsin from the pancreas, and lactase, maltase, and sucrase in intestinal secretions act on the carbohydrates, and the fats are acted on mainly by the lipase of the pancreatic juice. The bacteria are not surely known to have necessary specific.functions, unless it be their solvent action on the cellulose. The various enzym activities finally prepare the food for absorption into the blood circulation, not merely by solution, but by such rearrangement of the ingested food compounds as to fit them for constructive purposes in the animal body or for supplying the energy that is required for internal and external work.