TEXT-BOOK OF BACTERIOLOGY. 177 



covering', of a shining grayish-white, and which often breaks up into 

 separate flakes. Elegant salt-crystals are nearly always separated 

 from the gelatin, and hang' down from the under surface of the 

 bacterial coating, like little roots. This is a result of a change of 

 reaction, brought about in the gelatin by the action of the B^c. 

 acid, lact., causing the previously alkaline food medium to become 

 distinctly acid. 



On agar-agar the growth of Bac. acid. lact. offers nothing re- 

 markable. On potatoes it grows as a brownish-yellow greasy 

 covering. 



The peculiar action of the bacillus is best seen if we place a 

 small quantity of a pure culture in absolutely sterilized milk. The 

 latter, it is true, is by no means easy to obtain. Milk often contains 

 germs of ver}' tenacious vitality — for example, those of the potato 

 bacillus, which one can only destroy by adopting' very thorough 

 measures. One must therefore either keep the milk for several 

 hours at 100° C. in Koch's steam generator, or employ fractional 

 sterilization, warming it up to 60° C. on five or six successive days. 

 The first process is the more reliable, and the changes which it pro- 

 duces in the composition of the milk, though sometimes important, 

 are of no consequence in this case. 



If we inoculate milk thus sterilized with the Bac. acid, lact., we 

 soon perceive that the sugar of milk turns into lactic acid and car- 

 bonic acid, thereby causing an acid reaction. 



This result then leads to the separation of the casein, which is 

 here brought about by the lactic acid, just as it might be by any 

 other acid ; for example, acetic acid. At a suitable temperature, the 

 best being from 35° to 40° C, the whole process is completed in 

 eight or ten hours, and a homogeneous gelatinous coagulum, which 

 we call "curds," has been formed. 



While the bacillus itself, as already stated, belongs to those 

 which can, under some circumstances, exist without air, it seems to 

 require oxygen for the exercise of its decomposing powers, which it 

 can only employ when air is obtainable. 



According to the investigations of Grotenf elt, the sour-milk bac- 

 teria belong to those saprophytic species which, like many patho- 

 genic micro-organisms when kept for a long time apart from the 

 conditions of their natural existence, lose part of their specific 

 power. Thus on our gelatin which contains no sugar they grad- 

 ually lose more and more of their capacity for causing lactic acid 

 fermentation, they become " attenuated," and this process can only 

 be counteracted by returning them repeatedly to their natural 

 food medium. 



