THE RELATION OF MICROORGANISMS TO MILK 449 



group, the vegetation range being 15 to 45. They are to be classed as 

 facultative anaerobes. 



The conditions for development are less narrow than for the Bact. 

 lactis acidi group, growth occurring on all the ordinary culture media 

 and in the absence of carbohydrates. Indol and hydrogen sulphide 

 are often formed and nitrates are reduced. The growth is usually pro- 

 fuse, the colonies large and surface growth occurring in stab cultures. 

 Gelatin is not usually liquefied. 



Lactose, dextrose and saccharose are fermented, with the production 

 of varying amounts of gas in which have been found carbon dioxide, 

 hydrogen and methane. The maximum amount of acid produced in 

 any culture medium is quite similar to that formed by the members of 

 the previous group. The relative proportions between the non-volatile 

 and volatile acids are far different, lactic acid comprising less than 30 

 per cent of the total acid formed while volatile acids, such as acetic 

 and formic, make up the remainder. Traces of succinic acid 

 (C 2 H4(COOH) 2 ) and alcohol have also been found. The lactic acid is 

 of the laevo-form. 



Milk is usually curdled, although some members of the group do not 

 produce enough acid to cause curdling. The amount of gas produced 

 varies widely. In the case of those forms that cause curdling, the 

 presence of gas is made evident by rents in the curd. If consider ble 

 gas is produced, the curd will be very spongy. When the acid formed is 

 not sufficient to curdle the milk, the gas produced is likely to pass off 

 unnoticed. The curd shrinks to a greater or less extent and thus 

 becomes so firm that it is impossible to emulsify it again. The odor of 

 the fermented milk is often offensive and the taste disagreeable and 

 sharp. The organisms of this group are to be classed as undesirable 

 and. the fermentation produced by them cannot correctly be called 

 a lactic fermentation. 



Representatives of these two great groups of acid-forming 

 bacteria are to be found in every sample of market milk in varying 

 proportions. Both find in milk favorable conditions for growth, and 

 the normal souring is produced conjointly by them, each producing 

 its own specific products, the relative amounts of which are largely 

 dependent on the number of each group that is originally introduced 

 into the milk and on the temperature at which it is kept. The higher 



temperatures tend to favor the growth of members of the B, coli- 

 29 



