16 



DAIRY BACTERIOLOGY 



microorganisms ; on cultivation at temperatures approaching the 

 maximum, acetic acid bacteria may be induced to undergo a 

 striking transformation. In the case of acid-producing bacteria, 

 the high concentrations of acid produced in the medium will also 

 tend to cause the appearance of abnormal forms, known as 

 involution forms, and in old cultures of lactic acid bacteria strange 

 elongated, swollen or even branched cells may often be observed. 

 Prolonged cultivation under adverse conditions is the best method 

 of producing new varieties ; thus Emil Christian Hansen induced 

 yeasts permanently to lose their capacity of forming spores by 



FIG. 17. Involution Forms of Bacterium aceti produced by cultivation at 

 39 to 41 0. (After Hansen.) x 1,000.* 



cultivating for several generations at temperatures above the 

 maximum for sporulation. 



METHODS OF CULTURE 



It is not possible to compound a universal nutrient medium, for 

 some substances may be essential for the well-being of certain 

 organisms and poisonous to others. While certain water bacteria 

 will only thrive in very dilute solutions of organic nutrient matter, 

 yeasts and moulds thrive best in wort and fruit juice, milk bacteria 

 in milk, and pathogenic bacteria in meat broth or blood serum. 

 Such natural media are of variable and partly unknown composi- 

 tion, and, therefore, do not lend themselves so well to the study of 

 fermentations from the chemical point of view. For this purpose 

 artificial media containing as far as possible only substances of 

 known composition in definite proportions are the best ; for 

 bacteria which require proteins, a preparation of peptonised fibrin 

 known as Witte peptone is used ; unfortunately this involves the 



