TVI'KS OF BACTERIA FOUND IX MILK 







The most interesting characteristic is its effect upon milk. 

 When inoculated into sterilized milk, it grows readily and, in a 

 short time, begins to produce acid. The rapidity of the acid 

 production is variable with different 

 cultures. It is so rapid in some cases 

 that, if the specimen be placed at body 

 heat, it will curdle in six hours. With 

 other cultures, the curdling under 

 similar circumstances would not oc- 

 cur for twelve hours, or three days, 

 and with some cultures curdling never 

 occurs. All specimens of milk, how- 

 ever, become acid, although not al- 

 ways sufficiently to precipitate the 

 casein. Between these extremes every 

 conceivable grade may be found in 

 cultures that are, in other respects, 

 identical ; and they represent, doubt- 

 less, one type, differing in its power 

 of producing acid. 



The power of producing acid not 

 only varies in the different samples of 

 this type, but in different cultures 

 from the same original source. It 

 increases if the bacteria are cultivated 

 for a while in milk. If, for example, 

 a culture is taken directly from sweet 

 milk, it will usually curdle the milk 

 slowly; but if daily transferred from 

 one culture of milk to another for a 

 few days, and then taken out and 

 tested, its power of producing the acid is found to have much 

 increased. Since this power thus varies in successive cultures 

 from the same original source, it would seem that the fact that 



FIG. 17 GELATIN STAB 



CULTURES 



a. Bad. lactis aerogenes 

 b. Bact. lactis acidi 



