August 13, 1915] 



SCIENCE 



209 



THE BACTERIA OF THE INTESTINAL TRACT 

 OF MAm 



It has been stated that the average 

 healthy adult on a normal mixed diet ex- 

 cretes daily in the feces a number of bac- 

 teria, which have been variously estimated 

 from 128 billion to 33 trillion. This truly 

 enormous number of bacteria would weigh 

 approximately 5.5 grams when dried, and 

 the nitrogen in this dried mass would be 

 about 0.6 gram, corresponding to from 46 

 to 50 per cent, of the total fecal nitrogen. 

 It is very certain that this number of bac- 

 teria is not taken in the food, and, further- 

 more, the fecal organisms are not neces- 

 sarily the same as those found in the food. 

 Hence the conclusion is reached that there 

 must be a very great daily proliferation of 

 bacteria in the intestinal tract, and in this 

 sense the intestinal tract is the most efficient 

 and active combined culture medium and 

 incubator with which science is familiar. 



The question naturally presents itself, 

 why is there such a tremendous growth of 

 bacteria daily, and why is it that the bac- 

 teria taken in with the food are not those 

 which appear in the fecal contents? A 

 rapid survey of the life history of the intes- 

 tinal bacteria will explain at least some of 

 the facts. At birth the intestinal content, 

 the meconium, is sterile. This would be 

 expected, because the uterine cavity is 

 sterile. Very shortly after birth bacteria 

 make their appearance in the mouth of the 

 new-bom, and organisms appear in the 

 meconium from four to twenty hours post 

 partum, depending upon environmental 

 conditions. This initial infection of the 

 meconium is a mixed one. Various adven- 

 titious organisms, even pathogenic bacteria, 

 may appear in it. This is a period of mixed 

 infection, and the number of organisms in 

 the meconium increases rapidly after the 



1 From the Bacteriological Laboratory of the 

 Northwestern University Medical School. 



first food enters the intestinal tract. After 

 two to three days post partum, when the 

 intestinal tract has become thoroughly per- 

 meated with milk, the organisms observed 

 in the feces — for the meconium has largely 

 disappeared by this time — begin to assume 

 a monotony of form and a regularity of 

 type, which contrasts sharply with the pre- 

 ceding period of mixed infection. This is 

 a transitional period during which the per- 

 manent characteristic nursling bacteria ap- 

 pear and soon become dominant. 



The types of bacteria which constitute 

 the normal fecal flora of the nursling are 

 few in number and definite in their chem- 

 ical characters. The most prominent of 

 these, B. bifidtts, so-called because of its 

 developmental peculiarities in artificial 

 media, is a strict anaerobe. Other organ- 

 isms, the so-called Kopfehen bacillus, B. 

 coli, B. lactis aerogenes and Micrococcus 

 ovalis, are, as a rule, very much fewer in 

 numbers than B. hifidus, and, under normal 

 conditions, apparently less important. The 

 question arises, why should an obligate 

 anaerobe, as B. Ibifidus, dominate the nurs- 

 ling's intestinal flora? It must be remem- 

 bered that breast milk, which is the normal 

 diet of the nursling, consists monotonously 

 of about 7 per cent, of lactose, about 3 per 

 cent, of fat, and but 1.5 per cent, of pro- 

 tein. Consequently, the intestinal tract of 

 the infant under ordinary conditions is 

 practically continuously bathed in a nu- 

 trient medium containing at all times at 

 least a minimal amount of sugar. The 

 normal infantile feces is always slightly 

 acid in reaction, and this acid is lactic acid 

 chiefly. It is a significant fact that the 

 dominating organism, B. bifidus, is a lactic 

 acid-producing microbe. It is also a signif- 

 icant fact that the reaction of the normal 

 nursling feces is acid enough to inhibit the 

 growth of practically all putrefactive bac- 

 teria ; there are few or no putrefactive bac- 



