BACTERIA OF MILK 73 



Make stains from pure cultures, using carbol-fuchsin and 

 Gram's method. Examine for spores. Make and describe cul- 

 tures on agar plate, agar slope, gelatin stab, and potato. 



Exercise 107. Study of Bacillus denitrificans 



This organism shows the ability to reduce nitrates, usually 

 with the production of more or less free nitrogen. Studies of 

 this organism should be made in pure cultures on gelatin stab, 

 agar slope, potato, and Giltay and Aberson's solution in fermen- 

 tation tubes. After two weeks the solutions should be tested 

 for nitrites and ammonia.. 



Comparison of the denitrifying power of B. pyocyanem, 

 B. Hartlebii, and B. fluoreseens liquefaciens should also be made 

 (compare Exercise 95). 



Exercise 108. Study of Bacillus radicicola 



Continue the study of this organism, begun in exercises 99 

 and 100, by making stained preparations from pure cultures. Use 

 carbol-fuchsin, methylene blue, and Gram's stain. Study the 

 growth of the organisms in pure culture on gelatin stab, agar 

 slope, potato, and nitrogen-free media. After three weeks test the 

 cultures on nitrogen-free media for nitrogen as nitrites or nitrates. 



SECTION XIII 

 BACTERIA OF MILK 



Milk is a favorable medium for the growth of many micro- 

 organisms. It contains sugar (a food constituent for many bac- 

 teria), proteins represented by casein and lactalbumin (which 

 furnish organic nitrogen in suitable combination), potassium and 

 calcium largely as phosphates, and fats which may be used by 

 various mold fungi. These substances are in solution and at a 

 dilution suitable for nourishing bacteria. 



The bacteria in their growth processes bring about more or 

 less extensive changes in the character of the milk, and soon 

 alter its value for human consumption. These changes are almost 



