ii8 BACTERIA AND THE CARBON DIOXIDE CYCLE 



of danger (89), experiments have been made to determine their behaviour 

 in milk. The bacteria of typhoid, anthrax, glanders, tubercle, diphtheria 

 and cholera grow well, without causing any change in the appearance of 

 the milk that would suggest contamination ordinary market milk does 

 not look as though it contained millions of bacteria and not until some 

 time has elapsed does the milk begin to coagulate in consequence of the 

 acid produced. The anthrax bacillus produces acetic and caproic acids. 



Whether the milk of diseased cows contains the specific bacteria 

 of the infection is not certain in all cases. In tuberculosis, however, 

 tubercle bacilli have been repeatedly found in the milk. 



The acidification of milk for the preparation of cheese made from the 

 ' acid curd ' is the work of the lactic bacteria already mentioned. The 

 acid they produce precipitates the casein the milk ' coagulates.' In 

 the preparation of ' rennet curd ' the same end is attained without acid by 

 the use of rennet, an enzyme prepared from the stomach of calves. In 

 both cases, the casein freed from the serum of the milk (whey) constitutes 

 the ' curds ' from which cheese is prepared. 



Numerous ' diseases ' of milk are caused by bacterial action. Not 

 infrequently it coagulates without turning acid ; this is the work of species 

 (Tyrothrix, found in cheese) which secrete an enzyme resembling rennet. 



Pigment bacteria often make their appearance spontaneously and 

 cause the milk to have an unnatural colour. Red is produced by B. pro- 

 digiosits and some Sarcinae, blue by the harmless B. cyanogcnns^ a small 

 motile rod that grows on agar in dark-blue or bluish-grey crusts according 

 to the food-stuffs available. From yellow milk also several pigment 

 bacteria have been isolated. These coloured milks have generally become 

 more or less acid. ' Ropy ' milk is the work of certain mucigenous bacteria 

 that will be described later on. Bitterness in milk is produced especially 

 by peptone-secreting bacteria that possess very resistant spores. 



Butter is always rich in bacteria. A specimen of Munich butter 

 contained from six to twenty-five million per gram. Since the butter 

 contains from 0-5 to 1-5 per cent, lactose and other food-stuffs, changes 

 may be set up by the formation of lactic and butyric acids. The butter 

 is thereby rendered rancid, but it must be remarked that the rancidity 

 of butter is in most cases the result of the oxidation of the butter fat 

 to fatty acids (butyric and lactic) by the free oxygen of the air accelerated 

 by exposure to light. The peculiar aroma, too, that renders some butters 

 so tasty, is the product of certain bacteria that are now cultivated in 

 dairy laboratories and added to the fresh butter (90). 



A process of great complication, the details of which are difficult 

 to follow, is the ripening of cheese. This is the work of many kinds 

 of bacteria, which are therefore present in cheese in enormous numbers (91). 

 In a gram of German cheese from five to six million germs have been 



