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Popular Science Monthly/Volume 74/January 1909/On the Therapeutic Action of Fermented Milk

< Popular Science Monthly‎ | Volume 74‎ | January 1909

ON THE THERAPEUTIC ACTION OF FERMENTED MILK
By C. A. HERTER, M.D.

PROFESSOR OF PHARMACOLOGY AND THERAPEUTICS, COLUMBIA UNIVERSITY

DURING the past year there has been in the United States a large increase in the use of fermented milk in the treatment of disorders of digestion and nutrition. A clearly discernible influence in bringing about this increase lies in the publications made by Professor Metclmikoff and his colleagues in reference to the fermented milk known as lacto-bacilline. The statements made by these scientific workers have been repeatedly exaggerated by persons having a commercial interest in the sale of certain kinds of fermented milks. It is apparently true that many physicians have been influenced by these statements in the direction of recommending among their patients a much wider use of fermented milk, and especially of lacto-bacilline, than was previously the case. Moreover many persons have decided without the advice of a physician to make a trial of some form of fermented milk or of some form of lactic acid ferment capable of acting upon milk sugar. It appears that this dietetic practise is still on the increase and likely to modify the habits of a not unimportant part of the community in respect to diet. In view of this fact it seems to me desirable to consider from a critical standpoint the therapeutic effects supposed to be derivable from the use of fermented milks, and more especially from milk that has been fermented through the use of the B. bulgaricus recommended by Professor Metchnikoff and now widely employed in the.production of lacto-bacilline. I believe that at the present time there exists a considerable confusion of mind as to what may or may not reasonably be expected in the way of therapeutic results from the use of milk which has undergone lactic acid fermentation. It is the object of this paper to consider briefly the elements which should enter into the formation of a judgment as to the therapeutic efficacy of lacto-bacilline and allied milk products.

In order to be able to form an unimpeachable judgment on the therapeutic action of a fermented milk, it is necessary that experiments of a very painstaking sort should be carried on in a number of individuals for considerable periods of time. Experiments of a kind calculated to furnish a firm scientific foundation for a rational use of fermented milks have not yet been made. Such experiments in order to be decisive would have to be conducted not merely on people in good health, but also on a suitable variety of digestive and nutritional disorders in which the bacterial conditions in the intestine, the state of metabolism and the general conditions of life are taken into account with the greatest care and judgment. Although I have for many years been interested in watching the influence of fermented milk on the human organism in various states of digestive derangement, and have accumulated many observations bearing on the question, my experience falls far short of what is necessary to establish final conclusions. In this communication, therefore, I do not offer any solution of the therapeutic problems pertaining to the use of fermented milks, but seek only to discuss critically, in the light of such information as now exists, some of the claims that have been made for the employment of these kinds of milk. I do this with the thought that a discussion of the various elements which should enter into the formation of a Judgment regarding the therapeutic value of milk subjected to lactic acid fermentation may prove helpful to those who have not given the subject much personal study and are therefore unable to analyze the problem in a way that is likely to serve as a practical guide.

There are five important kinds of effects referable to the action of fermented milks which must be considered in any judgment of the therapeutic effects of a milk which has undergone lactic acid fermentation. These are, first, the effects on the absorption of fats and proteins; secondly, the effects due to reduction of carbohydrates; thirdly, effects due to the presence of lactic acid; fourth, effects due to the bacteria used in lactic fermentation; fifth, effects due to a lowering of putrefactive decomposition. These latter effects, which are of the first importance in connection with any study of the action of fermented milk, are of course not entirely distinct from the others just mentioned, but stand related to each of these other factors. Owing to their prominence, however, it is desirable that they should be separately considered.

At present the influence of lactic acid fermentation upon absorption of the milk constituents is but little understood. The question relates especially to the absorption of fats and of proteins, for the carbohydrates of the milk are in large degree removed by the fermentative process, lactic acid, carbon dioxide and alcohol being the chief constituents resulting from the breakdown of the milk sugar. It is important that we should obtain exact data with regard to the absorption both of the fats and of the proteins, but, so far as I am aware, these do not at present exist. If it could be shown that the absorption of milk fat and of milk proteins is increased in health through the influence of lactic acid fermentation of any kind, this would be a distinct argument in favor of the use of such milk as an article of diet. since it would make for economy in the administration of the machinery of the body. Equally important and desirable are reliable observations on the effect of fermented milks on the absorption of milk fats and milk proteins in various types of intestinal infection with their accompanying acute and chronic catarrhal inflammation of the mucous membranes of the digestive tract. The therapeutic claims put forward by enthusiastic advocates of the use of fermented milk have in general taken a different direction and have concerned themselves much more with the question of the reduction in intestinal putrefaction than with increase in absorption. But it must not be overlooked that an improved absorption of proteins is one of the most important conditions in general for reducing intestinal putrefaction, because whatever favors prompt and complete absorption must correspondingly limit the opportunity for decomposition. In a lesser degree this statement holds true also of the fats. I have been able to show experimentally that in normal persons the butter-fat may be much increased above the usual intake—say from fifty grams to one hundred and fifty grams daily—without materially increasing putrefactive decomposition. On the other hand, such an increase in butter-fat in persons already suffering from increased putrefactive decomposition shows a pronounced tendency to still further increase the putrefaction. I attribute this tendency to the mechanical obstacle to prompt absorption of proteins arising from the presence of fat in abundance. The failure in prompt absorption of proteins from an intestine infected with putrefactive microorganisms means intense putrefaction, whereas a similar failure in a healthy intestine is far less significant owing to the relative infrequency of putrefactive bacteria.

In considering the therapeutic influence of fermented milk, it is necessary to take into account the fact that in such milk the carbohydrate material has been in a large degree replaced by the products of fermentation. Where milk is used in only small amounts in the dietary, and these small amounts are replaced by a fermented milk, the difference in quantity in respect to the intake of carbohydrates may be so small as to be negligible. Where, however, the dietary consists largely of milk and this large amount of milk is replaced by an amount of fermented milk equivalent in protein and in fat, the difference in respect to the carbohydrate material may assume considerable importance. In the case of the unfermented whole milk, there is enough milk sugar to markedly encourage fermentative decomposition in the intestine with the production of considerable gas. The gas-forming organisms especially likely to attack the milk sugar are B. lactis cerogenes, B. coli and B. cerogenes capsulatus (B. welchii, or B. perfringens). In cases where there is marked flatulence from the use of whole milk, the use of any fermented milk in which the milk sugar has been largely destroyed by fermentative bacteria introduces conditions unfavorable for intestinal fermentation. I consider that the diminution in fermentable material thus arising from the decrease in the parbohydrates of the milk is an important factor not merely in reducing intestinal fermentation, but also in reducing intestinal putrefaction, for it is true that in some intestinal infections in which we are justified in assuming that the colon bacillus or B. ærogenes capsalatus or both these organisms have extended in an upward direction toward the stomach, the abundant presence of fermentable carbohydrate pabulum leads to a great increase in these microorganisms. After the absorption of the acid produced in the course of this fermentation there may be established a neutral or even an alkaline reaction in the lower part of the small intestine and in the colon. In the absence of acid and indeed in the presence of a moderate amount of acid, the colon bacilli and B. ærogenes capsulatus are capable of making an increased attack upon the protein material. This increases intestinal putrefaction. On the other hand, the irritation arising from organic acids formed in the small intestine and stomach often leads to a fermentative diarrhœa.

Turning now to the effects attributable to the presence of lactic acid in the soured milk, it is at once apparent that we have to distinguish clearly between the action of such preformed lactic acid as may be introduced with the milk and such acid as may be formed in the course of further lactic acid fermentation after the soured milk has been ingested. The essential difference lies in the fact that such lactic acid as is preformed in fermented milk is liable to be absorbed from the upper part of the small intestine, whereas if lactic acid fermentation goes on within the digestive tract, the acid may be formed at any level of the intestine. In the former case the action of the acid is to be regarded as largely limited to the portion of the intestine in which putrefactive decompositions seldom occur; in the latter case there may be production of acid within the territory in which putrefactive decompositions are apt to take place. We should therefore expect greater anti-putrefactive efficacy from the use of soured milk containing living lactic acid producers than from the same milk after sterilization. Whether such a difference as this is actually discernible in practise I am unable to say, as I am not aware of the existence of satisfactory experiments made to test this point.

As to the efficacy of lactic acid as an anti-putrefactive agent it is necessary to speak with caution. It has been the practise of many physicians to employ lactic acid in the treatment of disorders of digestion, especially those of infancy. But I am unaware that we have adequate data for the establishment of the therapeutic anti-putrefactive value of lactic acid. Where the stomach secretes no free hydrochloric 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 persons suffering from chronic intestinal putrefaction have shown no diminution in the putrefactive products excreted in the urine where the patients have added a soured milk to their usual diet. It is, of course, clear that in cases of this sort the failure of the putrefactive process to decline may be attributable to the introduction of more than the habitual amount of protein material. The observation is, however, of interest in that it emphasizes the fact that the ingestion of lactic acid, even if probably associated with lactic acid fermentation within the intestine, may not suffice to exert any beneficial influence in reducing putrefaction.

I do not wish to be understood as maintaining that the presence of lactic acid in soured milk is of no value in checking intestinal putrefaction. I wish merely to point out that the administration of lactic acid per se can not be regarded as a significant anti-putrefactive procedure. It seems to me probable, on the other hand, that the presence of lactic acid in the large intestine would at least in a degree tend to restrict putrefactive decomposition. But I must own that positive evidence on this point seems to be at the present time entirely wanting. In my judgment only very carefully planned studies would suffice to enable us to form a final opinion on the value of lactic acid as an antiputrefactive agent. We are not justified in developing an enthusiastic attitude toward lactic acid as an agent in the inhibition of intestinal putrefaction on the basis of our present knowledge.

Let us now consider the effects derivable from the bacteria used in lactic fermentation. As an example of a strong lactic acid producer we may take B. bulgaricus, used in the production of lacto-bacilline. This organism is a powerful lactic acid ferment, forming large amounts of lactic acid from milk sugar while forming very little alcohol. The organism grows well in milk and on some media containing an abundance of soluble carbohydrates, as, for instance, in malt extracts. We may take the behavior of B. bulgaricus in the digestive tract as being typical of efficient lactic acid bacilli in general. There are two questions which we must put to ourselves regarding the therapeutic effects of such bacteria. First, to what extent do the lactic acid bacilli replace obligate normal types of bacteria or the undesirable saprophytic forms present in disease? Secondly, to what extent is it desirable that there should be a replacement of the intestinal flora by lactic acid bacilli?

It is one of the fundamental assumptions of the sour milk treatment of intestinal diseases that the lactic acid producing microorganisms establish themselves throughout the digestive tract and through their more or less aggressive growth directly or indirectly inhibit the development of putrefactive or other undesirable forms of bacteria. In some of the statements put before the public in regard to the action of the lactic acid bacilli it is claimed that they drive out other forms of bacteria from the large intestine the chief seat of intestinal putrefaction. It is desirable that we should soberly consider the known facts relating to this question. I think it safe to say that the ability of lactic acid forms to replace or dominate other types of bacteria in the large intestine is much exaggerated. I have devoted some study to this question, especially in the case of the B. bulgaricus employed in the production of lacto-bacilline. This organism, owing to its large size, morphology and cultural peculiarities is easily recognized and is cultivable, from the intestinal contents. When given to human beings in the large numbers present in lacto-bacilline it can after a few days' administration be cultivated without difficulty from the movements. Even when large quantities of the fermented milk have been taken I have not found that it becomes the dominant organism, although it may be present in moderate numbers. On stopping the administration of the lacto-bacilline, the B. bulgaricus generally disappears in the course of a few days, showing that it has not permanently established itself within the intestinal tract. There may be exceptions to this statement, but I have not yet met with any. These clinical results are quite in accord with those obtained by Dr. Kendall and myself in experiments upon a monkey fed for two weeks on lacto-bacilline exclusively. At the end of this period, when the movements were showing the regular presence of B. bulgaricus in relatively moderate numbers, the animal was killed and the digestive tract examined with care at all its levels. The lactic acid organisms were found in greatest abundance in the small intestine. In the lowest portion of the small intestine a notable falling off was observed and other types of bacteria were prominent. In the large intestine the numbers were only moderate as compared with other varieties of bacteria, thus clearly showing that in this instance, at least, the B. bulgaricus was very far from dominating other associated types of bacteria. I consider this fact noteworthy, as the experiment was carried out under conditions highly favorable to the establishment of the lactic acid bacilli in the digestive tract. The large number of microorganisms given and the relatively short extent of the digestive tract in the monkey should, it would seem, provide conditions for the adaptation of the organisms throughout the alimentary canal.

It is probable that the experience Just recounted with regard to lactic acid bacilli is not at all exceptional, or in other words that foreign bacteria in general find it difficult to gain a permanent footing in the digestive tract. The literature of experimental bacteriology shows this to be the case. Personal experiments made with a highly fermentative putrefactive organism—B. ærogenes capsulatus (B. welchii or B. perfringens of the French writers)—in feeding experiments on monkeys showed that in health these animals have the power of very quickly ridding themselves of this variety of bacteria. Experiments now under way with a microorganism described by myself and Dr. Kendall as B. infantilis and found very abundantly in some of the digestive diseases of children, show the same thing to hold true.

The fact that B. bulgaricus does not readily gain a dominant position in the digestive tract in man or in the monkey has an obvious bearing on the results to be expected from its therapeutic use. If it be indeed true that B. bulgaricus is capable by its presence in the intestinal tract of inhibiting undesirable types of bacteria and especially the microorganisms concerned with intestinal putrefaction, then it must be equally true that the difficulty in obtaining a dominant and permanent foothold in the intestinal tract is a fact with which we must reckon in any estimate of the results likely to be obtained through the administration of these organisms. The moderate representation of B. bulgaricus in the large intestine after the free administration of lacto-bacilline is surely something very different from what has been already frequently pictured by the enthusiastic upholders of the use of this form of fermented milk in the treatment of diseases of the digestive tract.

I think it has been assumed with far too little reason that the dominant presence of foreign microorganisms of the lactic acid group is necessarily a desirable thing. If it could be shown that lactic acid bacilli, such as B. bulgaricus or certain varieties of B. acidi lactici, have the faculty of replacing undesirable forms of microorganisms such as the bacilli of typhoid or of paratyphoid fever or putrefactive microorganisms, such as B. proteus vulgarus or B. ærogenes capsulatus, this would undoubtedly be cause for congratulation, especially if it could be shown at the same time that the normal flora of the digestive tract remained unchanged. I do not deny the possibility that this selective kind of anti-bacterial action may some day be proved to exist. I desire merely to point out that at present I know of no facts to justify tis in believing that such antagonistic action as the lactic acid bacteria may possess is directed solely against the disease-inciting invaders of the digestive tract. If it should prove true that the antagonism exerted by the lactic acid bacilli against injurious invaders is also exerted against the obligate bacterial inhabitants of the alimentary canal, such as B. coli communis and B. lactis ærogenes, I am by no means convinced that this could be regarded as a point in favor of the prolonged therapeutic use of lactic acid bacilli. If, as appears to be true, these obligate inhabitants of the digestive tract are especially adapted to the normal conditions of secretion and digestion in the human intestine and tend to be suppressed in some serious conditions of the digestive tract (while their reappearance and reestablishment in abundant numbers is one of the first, most definite and most reassuring signs of improvement in the clinical condition of certain kinds of patients) to make use of any mode of lactic acid bacillus therapy which will inhibit the normal development of B. lactis ærogenes or B. coli communis in the digestive tract would, in my judgment, be a profound error in principle. I do not wish to intimate that I consider B. bulgaricus or any of the common lactic acid bacilli to be capable of seriously checking the growth of B. lactis ærogenes and B. coli communis in the digestive tract, but wish to state that in so far as such modification is possible it appears to me not without undesirable features. To the validity of this statement there is one possible exception that occurs to me. In cases where there is a colon bacillus infection of the intestine, that is to say, an inflammatory state associated with a great over-growth of B. coli, the antagonistic influence of lactic acid bacilli might be useful. But I am not sure that this is more than a merely apparent exception to the general rule which I have above expressed as valid, for it is not clear that it has been proved that the colon bacilli apparently answerable for digestive infections are in reality the normal colon bacilli. It appears to me more likely that they are commonly variants of such bacilli whose fermentative characters have not yet been determined fully and precisely.

I would also mention here the fact that there are diseases of the intestinal tract associated with the presence of bacteria capable of forming lactic acid. Obviously, then, this property of a microorganism does not necessarily screen the digestive tract from injury.

One of the most important and most loudly heralded effects of the administration of soured milk is that on intestinal putrefaction. Under conditions of health the putrefactive decompositions in the intestinal tract seldom attain a considerable degree of intensity—a surprising fact when we consider the immense numbers of bacteria which inhabit the large intestine. In many pathological states the conditions of putrefaction in the intestine are very much altered in the direction of great intensification. This is shown both by the dominance of putrefactive microorganisms in the large intestine and by the appearance of products of putrefaction in the urine. It is unnecessary here to discuss the nature of these products. It should, however, be pointed out that the intensity of putrefaction as judged by the quantity of putrefactive products in the urine is notably influenced by the quantity of protein material ingested. We may say that in general a considerable increase in the protein intake is followed by a corresponding increase in putrefaction and that a marked diminution in protein intake is followed by a distinct falling off in putrefaction. This statement holds true in general in conditions of health and it is even more strikingly exemplified in cases of chronic intestinal infection associated with habitual excess in putrefaction. In view of this fact it is clear that in experiments designed to determine the influence of fermented milks upon the intensity of putrefaction it is essential to take accurate cognizance of the quantity of protein ingested. It is easy to understand that if a patient has been in the habit of eating for his midday meal an abundance of protein food and decides under advice to take a fermented milk for his lunch in place of the more elaborate meal, the mere reduction in protein will suffice to reduce putrefaction. So it is clear that a decrease in putrefaction can be effected through a variety of dietaries which have in common the fact that they contain a smaller amount of protein material than the patient has been in the habit of eating. Whole milk and various fermented milks are thus capable of influencing putrefaction in such a way that we may readily fall into the error of exaggerating their influence upon putrefactive decomposition in the intestine. Hence it is evident that the only fair test of the value of a fermented milk in respect to its influence on putrefaction is to compare it with the effects of other articles of diet containing exactly the same amount of protein material. Such careful comparisons have not, I believe, been made up to the present time. In the future they will doubtless be made and will enable us to form quite definite judgments as to the relative effectiveness of different kinds of fermented milks upon intestinal putrefaction. At present I should hesitate to say that one kind of fermented milk is more effective than another in bringing about a reduction in intestinal putrefaction. "We may regard it as well established that a diet in which milk takes the place of other kinds of food is very apt to be followed by a reduction in the intensity of putrefactive decomposition in the intestine. There are, however, clinical indications that the use of fermented milks does possess real advantages over the use of whole milk at least in some disorders of digestion. Although the exact character of these advantages is not yet firmly established, they seem to be none the less real. From what has already been said in this paper on the criteria of judgment of the action of fermented milks, it is evident that the clinical advantages which have been observed may be attributable to several different peculiarities possessed by fermented milks in general. One of these is the favorable mechanical influence on the minute subdivision of the casein, which prevents the undesirable effects associated with the presence of large clots of casein which are not easily disposed of in persons with weak digestion. The exact consequences of this advantageous mechanical state of the milk food can not now be appraised. A second point which has already been mentioned is the formation of lactic acid. Here again the precise extent of the favorable influence can not be measured; but on the other hand it can not be denied that in at least some disorders of digestion the presence of lactic acid in the intestinal tract may exert a degree of anti-putrefactive action. It should, however, be remembered that there are persons with chronic inflammatory states of the digestive tract who tolerate very badly acids of all sorts. These persons are unable to take considerable quantities of fermented milk if the milk contains a high percentage of lactic acid, the attempt to utilize such food being followed by various unpleasant sensations and diarrhœa. The possible anti-putrefactive influence of the presence of living lactic acid bacilli in various parts of the digestive tract has already been discussed at sufficient length and it has been pointed out that this factor again is one whose value can not at present be accurately estimated.

It must be plain from what has been said that the therapeutic use of fermented milks rests at the present time rather more securely on the clinical observations that have been made with it than on an adequate scientific study of the influence exerted upon digestion and nutrition and especially on the processes of putrefaction. To obtain the necessary scientific data will require elaborate and very laborious experiments covering long periods of time. With the aid of such experiments I have no doubt that the usefulness of soured milks in health and in disease will be definitely and discriminatingly established. The limitations of utility will become equally plain, and I predict that they will prove to be many. The importance of this subject for the welfare of people at large not only in respect to immediate physical comfort and efficiency but as regards the prolongation of life, would, in my opinion, amply justify a very considerable expenditure of money to acquire this knowledge.

It can not be regarded as surprising that the enthusiasm which has been aroused partly through the public exploitation of various kinds of fermented milk in the treatment of disease and partly by the undoubted successes of the treatment should have led to various abuses. One of the most important things to understand in reference to the use of fermented milk is that it should be employed in most instances as a substitute for other forms of food rather than as an addition to the usual dietary. Especially is it necessary to bear this in mind in the case of chronic disorders associated with an increase in putrefaction. The addition of a considerable amount of fermented milk to the habitual dietary has often been practised with disastrous results, and I do not doubt that this practise is still widely extended. Such bad results might be predicted, for since all fermented milks contain a large proportion of protein material capable of undergoing putrefaction and since this putrefaction is not checked, in any specific way, through the agency of the fermented milk itself, a great increase of putrefactive decomposition may follow the injudicious excessive use of such food. I have seen several instances of this error, which is not confined to laymen, but is sometimes committed by physicians also.

Another feature of fermented milk which needs to be closely scrutinized is the character of the microorganisms employed as ferments of the milk. In a few instances I have known to be used as ferments what I believe to be very undesirable types of bacteria. I think it may be said that most of the fermented milks on the market in this country at the present time contain chiefly fermentative organisms which are harmless when not excessively administered. In some cases the lactic-acid producing bacteria have become contaminated by possibly undesirable yeasts. It is only natural that accidents of this sort should occur in what is comparatively a new industry, and it is likely that with increasing experience the manufacturers of the various fermented milks will be compelled to exercise every reasonable caution in regard to the purity and quality of the ferments employed in their products.

The use of tablets of other preparations of lactic acid bacilli is now becoming widespread. The tablets are taken with some carbohydrate material which will permit the growth of the bacteria and the formation of lactic acid. I have seen good results from this method of using lactic acid bacilli, in the relief of symptoms referable to excessive intestinal putrefaction. But I do not think the data exist at present for an intelligent comparison of this use of lactic acid bacilli with their use in fermented milks. I hope before long to be able to discuss this question on the basis of experimental observations.