22 INFECTIONS OF THE DIGESTIVE TRACT 



acids were at least somewhat higher in the case of the 

 cultures in the sugar-bouillon-carbonate media than 

 in the corresponding growths in sugar bouillon. This 

 fact suggests a relatively greater activity of putrefactive 

 bacteria in the neutral media, for it is known that such 

 bacteria tend to form the higher rather than the lower 

 volatile fatty acids. This view was confirmed by the 

 study of the bacteria dominant in the two series of flasks. 

 Secondly, it was found by Dr. Wakeman that in almost 

 every instance the proportion of non-volatile acids 

 (mainly lactic) was greater in the sugar-bouillon cultures 

 than in the neutralized cultures. The most reasonable 

 interpretation of these results is that a neutral medium 

 favors the development of putrefactive bacteria (especi- 

 ally of the anaerobes), whereas an acid medium (such as 

 is maintained in the sugar-bouillon flasks) restrains the 

 growth of these bacteria and favors the growth of the 

 lactic acid makers. It is probable that in the case of 

 the neutral flasks some of the lactic acid formed is con- 

 verted into butyric acid. 



These observations and similar ones with pure cultures 

 indicate that in the digestive tract the growth of putre- 

 factive anaerobes must be favored by a neutral reaction 

 and restrained by the presence of acid. The favorable 

 influence of milk food, containing lactic-acid formers, 

 in controlling putrefactive decompositions in the digest- 

 ive tract finds its explanation partly in the inhibitory 

 action exerted by such bacteria and in part in the 

 presence of preformed lactic acid in the food at the 

 time it is ingested. 



