132 MILK BACTERIA AND HEALTH. 



This conclusion is dependent partly upon the effect of the 

 growth of lactic bacteria in the milk, as described in Chapter 

 IV. The growth of the common lactic bacteria checks the 

 development of other bacteria. We have seen that, after the 

 lactic bacteria begin to become abundant, the other forms 

 gradually become less abundant, and in the course of 

 time practically disappear. The great class of peptonizing 

 bacteria, characterized by their power to liquefy gelatin, are 

 almost universally prevented from developing in milk as soon 

 as the lactic bacteria become abundant. If sterilized 

 milk is inoculated with these putrefactive forms it will in 

 a few days become badly decomposed and putrefied. If, 

 however, lactic bacteria are present the putrefaction of the 

 milk by these peptonizing bacteria is quite prevented. Indeed, 

 milk which contains lactic bacteria may be kept for many 

 days or even weeks after it has curdled, without any further 

 appreciable change taking place. The development of the 

 acid not only checks the growth of the lactic bacteria, but 

 also prevents the other forms of microorganisms from grow- 

 ing. In short, the lactic bacteria actually serve to protect the 

 milk from the decomposition changes which the class of 

 putrefying bacteria will produce if the lactic organisms are 

 not present. Pasteurized milk, in which the lactic bacteria 

 ,have been destroyed by heat, will putrefy, while unpasteur- 

 ized milk sours but does not commonly putrefy. 



Somewhat similar evidence has been obtained in regard to 

 the relation of lactic bacteria in the intestine (154). The 

 presence of lactic bacteria or other acid germs in the intes- 

 tine seems to be needed for carrying on the normal digestive 

 processes. The intestine normally contains a certain number 

 of putrefactive bacteria, which, if allowed to act by them- 

 selves, produce a putrefaction of the intestinal contents dis- 

 tinctly disturbing the digestive functions. This putrefaction 

 develops in the intestine if it does not contain acid bacteria. 

 The presence of lactic organisms in the intestine has appar- 



