38 MILK AND THE PUBLIC HEALTH CHAP. 



dense, strongly acid, and frequently, though not always, surrounded 

 by minute, irregular spines on the edge. The gelatine is not 

 liquefied. 



Agar streak. No growth, or one that is scarcely visible. 



Lactose, glucose, and saccharose. Acid produced from all three, 

 but no gas. 



Bouillon. Frequently no sign of growth, but commonly a slight 

 sediment. 



Milk. Acid production and clot in from 6 hours to 2 days. 

 The curd is smooth and hard, without gas bubbles, and never shows 

 any digestion. 



Potato. Usually no growth, but sometimes a thin transparent 

 film. 



Grows at 20 and 37 C., but better at 20 C. Facultative 

 anaerobe, growing better without oxygen, and hence curdling milk 

 at the bottom first. 



" The most characteristic features of this organism are the 

 peculiar litmus gelatine colonies, the absence of surface growth, 

 and the smooth, hard acid curd in milk." 



These authors describe several varieties differing in more 

 or less important particulars from this type. The most common 

 variant only differs in the extremely minute colonies on gela- 

 tine with absence of the characteristic spines. 



The group of organisms described under this type are of 

 immense importance in economic milk problems, but are in 

 themselves of no pathological significance. 



According to Kruse, Heinemann, and others, this type of 

 organism should be spoken of as streptococci. Heinemann, 1 after 

 experimentally investigating the matter, concludes, " Bacillus 

 acidi lactici is a myth. The ordinary bacteria producing lactic 

 acid fermentation in milk are B. aerogenes var. lacticus and 

 Streptococcus lacticus!' " Streptococcus lacticus agrees in morpho- 

 logical, cultural, and coagulative properties with pathogenic, 

 faecal, and sewage streptococci." 



On the chemical side it may be mentioned that the lactic acid 

 is not directly produced from lactose. The lactose in milk is first 

 converted by the action of enzymes into glucose and galactose, and 

 the lactic acid is produced from these simpler sugars. As is well 

 known, four isomers of lactic acid exist. Three of them have an 

 identical chemical formula, but differ in their action to polarised 

 light, one, the f/-acid, rotating it to the right (dextrolactic acid), the 



1 Journ. of Infectious Diseases, 1906, p. 173. 



