GENERAL DISCUSSION AND SUMMARY 123 



flora is both desirable and possible. But it is possible only when living 

 cultures of properly qualified organisms are employed, or when such 

 specific diets are administered which stimulate and favor such a trans- 

 formation. B. bidgaricus is incapable of accommodating itself to intes- 

 tinal conditions. B. acidophilus, on the other hand, being of intestinal 

 origin lends itself readily to implantation in the intestinal canal. 



That the value of sour milk feeding does not lie in the lactic-acid- 

 producing bacteria in the milk, nor in the acids which they produce, has 

 been conclusively demonstrated by Rettger (1915) and his associates 

 in their extensive experiments with several thousand chicks. Practically 

 the same results were obtained whether sweet or sour milk was fed, and 

 no difference could be observed in the growth- and health-stimulating 

 properties of ordinary sour milk and of milk soured with B. bulgaricus. 



The feeding of B. bulgaricus, without due regard to the use of milk, 

 can have little or no importance attached to it. The beneficial results 

 which have been attributed to yoghurt and other Oriental sour milk 

 products have in all probability been due to the milk as such, rather 

 than to the acid-producing bacteria which they contain. 



Milk by virtue of its milk sugar stimulates proliferation of B. 

 acidophilus in the intestine. This important fact was apparently over- 

 looked by Metchnikoff and his pupils. What they observed as B. bul- 

 garicus in the feces after the feeding of milk soured with B. bulgaricus 

 was in reality not this organism, but B. acidophilus. That such an 

 error of interpretation could have been made should not be at all sur- 

 prising in view of the fact that these two organisms resemble each other 

 so closely and are in fact indistinguishable in practically all of their 

 various aspects. The marked resemblance of B. bulgaricus and B. 

 acidophilus in cell morphology and orientation and in colony formation 

 on lactose or whey agar plates is well shown in the accompanying 

 photographs (Plates 1 and 2). It should be borne in mind that in 

 many of the experiments conducted by Metchnikoff and his enthusias- 

 tic followers large quantities of milk were consumed, and hence the con- 

 ditions for the development of B. acidophilus were more favorable, so 

 that instead of the acclimatization oi B. bulgaricus, as maintained by 

 them, B. acidophilus gained prominence and was the organism seen by 

 them in the feces. This assumption is strengthened by the announced 

 observations of Belonowsky (1907b) that the intestinal flora of nurs- 

 ing mice was of the same character as that of the older mice receiving 

 the B. bulgaricus milk culture. 



It was demonstrated in the present investigation that the amount 

 of lactose or dextrin required to cause practically complete simplifica- 

 tion of the fecal flora could be reduced by one half, providing living 

 cultures or suspensions of B. acidophilus were given along with the 

 carbohydrate. 



B. bifidus is also an inhabitant of the large intestine, and may be 

 looked upon as a logical candidate for simplifying the flora, but this 



