110 MICRO-ORGANISMS AND FERMENTATION. 



afterwards opaque. The surface colonies develop with extra- 

 ordinary difficulty : they are transparent, and have a some- 

 what irregular edge. This species excites fermentation even 

 in complete absence of air ; in presence of a full supply of air 

 the fermentation is restricted. Both Leichmann and Weig- 

 mann consider this species to be the regular means of spon- 

 taneously curdling milk, and explain the process of souring 

 as follows : Hueppe's Bac. acidi lactici, and other species of 

 the aerogenic group, remain in the upper layers of the milk, 

 to satisfy their great demand for air, and acidify these ; whilst 

 the facultative anaerobe, Bact. lactis acidi, develops in the lower 

 layers, and acidifies them. This species is easily grown in 

 lactose- or grape-sugar-broth, and then forms long chains. 

 As already stated, it ferments these sugars as well as lactose, 

 mannite, and raffinose. It forms dextro-lactic acid, and 

 produces no evolution of gas in sterilised milk. 



A number of so-called species described by different authors 

 appear to be varieties of this species. In spontaneously 

 soured milk, Leichmann discovered another species of frequent 

 occurrence, which has great similarities with the above, but 

 may be distinguished by its production of laevo-lactic acid, 

 and by the evolution of gas (Micrococcus acidi Icevolactici). 

 He also isolated a species which thrives best at 44- 5 2 C., 

 develops laevo-lactic acid, and forms thin rods of varied length. 

 On agar it forms root-like, branching colonies. The great 

 series of interesting varieties cultivated in a pure state by 

 Weigmann deserves special attention. Their appearance at 

 first resembles that of Leichmann's Bact. lactis acidi. Biologi- 

 cally, however, they show important differences, and several 

 have found widespread industrial application. Other species 

 have been described by Marpmann, Conn, Kozai, Beijerinck, 

 etc. In addition to these typical lactic acid bacteria, other 

 species occur in the souring of milk which possess a curdling 

 and a peptonising enzyme. 



The ripening of cheese, which consists in a conversion 

 of casein (paracasein) into simpler albuminoids, and the 

 breaking down of the latter, is brought about chiefly through 

 the action of certain micro-organisms. As is well known, 

 the curd may be separated from the milk by the addition of 



