The digestive tract of an infant contains no bacteria at birth, but usually some gain access during the first day of life. In the average adult it is estimated that each day's food in its passage through the digestive tract is subjected to the action of over one hundred billion bacteria, chiefly in the large intestine.

Since bacteria are regularly present in the digestive tract in such large numbers, it has been questioned whether they may not perform some essential function in connection with the normal processes of digestion. Experiments to demonstrate whether animals are independent of such bacteria are beset with many difficulties. Nuttall and Thierfelder kept sterile for several days the digestive tracts of young guinea pigs delivered by Caesarean section and fed upon thoroughly sterilized food, and as the animals thus treated lived and gained in weight, the experimenters concluded that intestinal bacteria are not essential to normal nutrition. This view has recently received strong support from the observations of Levin, who examined the intestinal contents of Arctic animals in Spitzenberg. The digestive tracts of white bears, seals, reindeer, eider ducks, and penguin were found to be in most cases free from bacteria, showing that the latter are not essential to the normal processes of digestion and nutrition. Kendall, however, in citing the evidence presented by Levin, points out that Arctic mammals, as soon as they are brought to temperate regions, rapidly acquire intestinal bacteria which do not seem to interfere with the well-being of the host.

Furthermore Schottelius claims that the conclusions of Nut-tall and Thierfelder are not justified since their experiments did not cover a long enough period. He himself experimented with chickens from bacteria-free eggs. One group kept in an absolutely bacteria-free environment and fed on sterile food, did well for ten days, but thereafter developed very slowly. When they were given "infected" food (containing common bacteria), they gained rapidly. Meanwhile a second group which had been kept in a sterile environment but had received "infected" food from the start, grew normally, as did a third group kept throughout under ordinary conditions. From these results Schottelius concluded that intestinal flora seem to be necessary for the normal development of chickens. Similar observations have been made by Madame Metschinikoff using tadpoles, and by Moro using turtles.

Notwithstanding this conflicting evidence, it would seem fair to conclude from the observations of Levin that if it were possible to exclude absolutely all bacteria from the digestive tract, the well-being of the body would be in no wise impaired; yet under such conditions as ordinarily exist, the bacteria which usually predominate in the digestive tract of the healthy man probably render an important service in helping to protect the body against occasional invasions of obnoxious species.

According to Herter, a few species, such as B. lactis aerogenes, B. coli, B. bifidus, have adapted themselves so well to the conditions existing in the human digestive tract that they are ordinarily not harmful to the host unless present in abnormally large numbers, and being able to hold their own against newcomers they may act beneficially in giving rise to conditions which check the development of other types of organisms, capable of doing injury, which under ordinary conditions man can hardly prevent from occasionally gaining ingress through food or drink.

"The presence in the colon of immense numbers of obligate micro-organisms of the B. coli type may be an important defense of the organism in the sense that they hinder the development of that putrefactive decomposition which, if prolonged, is so injurious to the organism as a whole. We have in this adaptation the most rational explanation of the meaning of the myriads of colon bacilli that inhabit the large intestine. This view is not inconsistent with the conception that under some conditions the colon bacilli multiply to such an extent as to prove harmful through the part they take in promoting fermentation and putrefaction."

Proteolytic enzymes formed by intestinal bacteria may assist in the digestion of food, and it is conceivable that bacteria may synthesize proteins or amino acids which may then be absorbed by the host, but the recent experiments of Osborne and Mendel seem to show that this cannot be an important factor in protein metabolism.

If for our present purpose we consider only the bacteria which are prominent in producing decomposition of foodstuffs in the digestive tract, and these only with reference to this one property, we may regard as the three main types: (1) the bacteria of fermentation, such for example as the lactic acid bacteria; (2) the putrefactive bacteria, such as the anaerobic B. aerogenes capsulatus (B. welchii); (3) bacteria of the B. coli type, showing some of the characters of both the fermentative and putrefactive organisms, but tending in general to antagonize the putrefactive anaerobes.

Among cases of excessive bacterial decomposition in the digestive tract the fermentation of carbohydrates with production of organic acids (and possibly also alcohol) is most likely to occur in the stomach, while the putrefaction of proteins occurs mainly in the large intestine. While it is true that in general the products of fermentation tend to restrict putrefaction, yet, since the two processes take place for the most part at such widely separated points of the digestive tract, there may be excessive fermentation and excessive putrefaction in the same individual at the same time. Among the conditions which favor excessive fermentation are: diminished tone and motility of the stomach, dilation, diminution or absence of free hydrochloric acid in the gastric juice, and excessive use of carbohydrate food - especially sucrose and glucose, which are more susceptible to fermentation in the stomach than are lactose, maltose and starch.

In the normal human stomach the conditions are quite unfavorable for the development of anaerobic putrefactive bacteria, not only because of the presence of air, but also because of the action of the gastric juice; and favorable conditions are not found in the anterior portion of the small intestine. In the lower third of the small intestine the numbers of bacteria increase and among them sometimes putrefactive forms. In the large intestine the conditions are much more favorable for the anaerobic putrefactive bacteria, and these may produce marked decomposition in any protein still remaining unabsorbed. In general the greater the amount of digestible but undigested or unabsorbed protein and the longer the material stays in the large intestine, the greater the amount of putrefactive decomposition. Not infrequently excessive fermentation in the stomach causes local sensitiveness which results in the taking of less bulky food (or such as has less indigestible residue), which in turn tends to stagnate in the intestine and thus render the conditions more favorable for intestinal putrefaction. According to Herter there sometimes results from the eating of large quantities of meat and sugar a type of fermentation in which oxalic acid is produced and which must therefore be highly injurious; but ordinarily the products of fermentation are only irritating, while putrefaction gives rise to products which are more distinctly toxic. These include indol, skatol, phenol, and cresol, which are for the most part absorbed into the system and finally excreted in combination with sulphuric acid as "ethereal" or "conjugated" sulphates. Of these the best-known is potassium indoxyl sulphate, commonly called "indican." The amounts of conjugated sulphates and of in-dican in the urine are valuable indications of the intensity of the putrefactive process in the intestine.