218 LECTURE X. 



tines, living at the expense of our food materials and especially the albu- 

 minous substances. They are found in the small as well as in the large 

 intestine, and occur there as aerobes and anaerobes. 1 The intestinal 

 " flora," that is, the bacterial content of the intestines, is very dependent 

 upon the nature of the nourishment, and varies with it. The anaerobic 

 bacteria, and especially the Bacillus putrificus, are largely responsible for 

 the putrefaction of albumin. The activity of this bacillus is accelerated 

 by the presence of aerobic bacteria, especially Bacterium coli and lactis 

 aerogenes. They predominate when a large amount of oxygen is present ; 

 the activity of the anaerobes then becomes restricted, as a result of which 

 the albumin putrefaction is also diminished. When, on the other hand, 

 there is a small amount of oxygen present, the anaerobic bacteria become 

 active. That the anaerobic and aerobic species of bacteria act together 

 is due to the fact that the latter consume the oxygen which restricts the 

 living processes of the former; while the anaerobes, on the other hand, form 

 products by their activity which serve as sources of nutriment for the 

 aerobes. Naturally, the action of anaerobic bacteria alone on albumin will 

 not give the same products as when the two kinds of bacteria act together. 

 The bacteria themselves are introduced into the intestines with the food. 

 The intestines of the new-born are sterile. 2 No bacteria can be found in 

 their meconium, the first excretory product from the intestine. When a 

 definite bacterial flora has settled itself in the intestines, the amount 

 present is but slightly dependent upon any further addition from the food- 

 supply. The fate of the bacteria is, then, determined by the food presented 

 to them, their nutrient medium. There is, furthermore, a vigorous 

 struggle for existence in the intestines. Those bacteria whose nutrient 

 requirements are satisfied most favorably triumph over the others. On the 

 other hand, a form of symbiosis develops by which one variety of bacteria 

 will consume the products of another, thus giving rise to quite a variety 

 of bacteria. Their development is kept within certain limits in a number 

 of ways, so that the putrefactive processes do not play any great part 

 in the intestines, and only a small portion of our food is sacrificed to 

 them. 



Great importance has been attached to the hydrochloric acid in the 

 stomach for restraining bacterial activity; in fact, for a long time it was 

 believed that this was its most important function. The amount of 

 hydrochloric acid in the gastric juice varies in different animals, as already 

 mentioned. It is sufficient, however, to restrict the activity of the putre- 



1 Cf. also Escherich: Die Darmbakterien des Sauglings, Stuttgart, 1886. A. Mac- 

 fayden, M. Nenki, and N. Sieber: Arch. exp. Path. Pharmak. 28, 311 (1891). D. Ger- 

 hardt: Ueber Darmfaulniss. Ergebnisse der Physiologie (Asher and Spiro), 3, 1, 107 

 (1904). 



3 Bienstock: Arch. Hyg. 36, 335 (1899);t7mJ. 39, 301 (1901). 



