acid it is reasonable to suppose that the use of lactic acid in weak concentration exerts some anti-fermentative action, especially against such microorganisms as do not readily grow in acid medium. But there are many kinds of microorganisms in the digestive tract which are resistant to the action of lactic acid in the low concentration which can be tolerated by a somewhat irritable mucous membrane. Most yeasts and some important intestinal bacteria, such as B. lactis ærogenes, B. hifidus, B. infantilis and various organisms classed at acidophiles, have this property. It is a fact little known that some of the coccal organisms of the intestine resist the action of acid in a remarkable measure. It is therefore quite clear that anything approaching a significant modification of the activities of organisms of the types just mentioned is not to be looked for through the use of lactic acid. Moreover, I have shown that a considerable grade of acidity in the intestinal tract is consistent with very active fermentative growth of B. ærogenes capsulatus. This organism forms butyric acid during the fermentation of carbohydrates, together with only small quantities of lactic acid, and there is no reason to suppose that its development in the intestine is materially inhibited by any concentration of lactic acid which is likely to be obtainable in the lower part of the small intestine or in the colon, either as the result of administering lactic acid or in consequence of the use of soured milk.
That a considerable or high degree of putrefactive decomposition in the intestine is not controllable in man by the administration of moderate doses of lactic acid has become plain to me as the result of clinical observation. And that even very large doses of lactic acid are unable to restrict intestinal putrefaction is rendered highly probable from experiments made in my laboratory by Dr. Helen Baldwin. In dogs taking a meat diet and excreting urine characterized by abundant indican and high ethereal sulphates there was no falling off in putrefaction as a result of administering doses of lactic acid as large as five grams daily. It seems to me doubtful if under these circumstances enough lactic acid could reach the large intestine to exert even a moderate anti-putrefactive action. The experiments just mentioned represent an extreme case, since they were made on animals living exclusively on meat. The results obtained can not, therefore, be regarded as strictly applicable to man. Nevertheless these experiments are instructive as indicating the inefficacy of large doses of lactic acid in controlling intestinal putrefaction where the conditions for such putrefaction are favorable and where the acid is given under conditions rendering likely its absorption in the upper part of the digestive tract.
That the presence of lactic acid in soured milk does not necessarily exert a significant anti-putrefactive action in the large intestines is clearly shown by the observation which I have several times made that
