108 MICRO-ORGANISMS AND FERMENTATION. 



others at 20-22 C. and 40-48 C. In general the largest 

 amount of acid is generated at temperatures somewhat below 

 the given optimum. 



In 1903, Herzog proved that Bac. acidi lactici contains an 

 enzyme which can be isolated from the living cell, and is capable 

 of producing a lactic fermentation. He treated a pure culture 

 mixed with kieselguhr with methyl alcohol, and afterwards 

 with ether. The mass was then dried, and the resulting white 

 powder, which contained no living cells, could convert minute 

 quantities of lactose into lactic acid. 



Buchner and Meisenheimer subsequently proved that if 

 a culture of one of the species growing in a distillery mash, 

 Sac. acidificans longissimus (Bac. Delbrucki, Leichmann) is 

 treated with acetone, whereby it is killed, and the mass is then 

 dried, a powder is obtained which can bring about a lactic 

 acid fermentation in a sugar solution. 



It may, therefore, be assumed that all bacteria of this 

 group contain enzymes that can bring about fermentation 

 independently of the living cell. 



To cultivate the lactic acid bacteria of milk, a preparation 

 of peptonised milk made by 0. Jensen may be used : To a 

 litre of sterilised milk, 10 c.c. of pure concentrated hydro- 

 chloric acid and 2 grammes of pepsin (P. granulatum) are 

 added. The mixture is kept in an incubator, and frequently 

 shaken. When the casein has dissolved, the acid is neutralised, 

 and the liquid sterilised at 115- 120 C. Gelatine or agar 

 may be added before neutralisation. 



In addition to the proper lactic acid organisms, there are 

 a large number of bacteria, and, amongst them, some patho- 

 genic forms which develop this acid. 



We are indebted to Pasteur for the first important work 

 on the subject of lactic acid bacteria. In 1858 he described 

 the species which appears when milk spontaneously ferments. 

 In his Etudes sur la biere he depicts certain bacteria growing 

 in wort or beer in which lactic fermentation has begun (Fig. 22) ; 

 he describes them as short rods slightly narrowed in the middle, 

 and commonly occurring singly, rarely united in chains. In 

 1877 Lister prepared a pure culture of a lactic acid bacterium 

 from sour milk, which he called Bacterium lactis. 



