4O PRACTICAL DAIRY BACTERIOLOGY 



stabbed into a gelatin tube, in a day or two it will be found 

 that the bacteria thus inoculated are liquefying the gelatin, 

 as shown in figure 21. These bacteria do not liquefy agar, and 

 can, therefore, be best studied in gelatin. 



If these bacteria be inoculated into milk, they will usually 

 curdle it in the course of a day or two ; but they do not produce 

 any acid, hence they act differently from the lactic acid bacteria. 

 The curdling is actually brought about by a chemical ferment 



^ secreted by the bacteria, which is very similar to rennet secreted 

 by glands in the stomach of mammals, the material used by 

 cheese makers to curdle their milk. Such chemical ferments 

 are called enzymes; and it is an enzyme secreted by this class 

 of bacteria which causes sweet curdling. 1 



But these bacteria produce a second action, since liquefaction 

 of gelatin is also produced by an enzyme, though not by ren- 

 net. The enzyme in question is quite similar to another di- 

 gestive ferment in the stomach, and also somewhat like pepsin. 

 It has the power of converting casein and other proteids into 

 soluble products, prominent among which is peptone, which is a 

 sort of soluble proteid. Hence, this class of bacteria is fre- 

 quently called the peptonizing bacteria. They are also 

 called liqueners. Since many of them are putrefactive in 

 their action, they may, in the end, fill the milk with putre- 

 factive products. 



Thus, this class of bacteria actually secretes two different 

 enzymes, or ferments. The first one curdles milk, much as 

 rennet does, the second softens the casein and converts it into 

 rennet-like bodies. Hence, when they are inoculated into 

 sterilized milk, the milk first curdles, the curd being rather soft 

 and usually not acid. After a few days the curd begins to show 



Fermi. Cent. f. Ract. x., p. 401, 1891; xii., p. 713, 1892. 

 Conn. Cent. f. Bact. xii., p. 223, 1892. 

 Lubbert. Zeit. f. Hyg. xxii., p. 1, 1896. 

 Duclaux. Le Lait., 1887. 



