220 ILLINOIS STATE DAIRYMEN'S ASSOCIATION. 



Another form of reproduction of bacteria is by spores. These 

 correspond to seeds of plants, and are usually formed under cir- 

 cumstances not favorable to the continued development of the 

 bacteria and their multiplication by division. Like the seeds of 

 wheat, the spores can endure conditions which would be fatal 

 to the growing form, and after surviving such conditions they 

 quickly develop when more favorably situated. Some spores 

 have been found to retain their powers of germination for more 

 than ten years. 



CONDITIONS AFFECTING BACTERIAL GROWTH. 



Three things are essential for the growth and development 

 ■of bacteria; they are food, warmth, and moisture, and when 

 these are furnished, as they are to a greater or less extent in 

 every dairy, the multiplication of bacteria takes place. Some 

 species require other conditions besides those named; certain 

 ones must have access to air, while others cannot thrive in the 

 open air; some require to be in an acid medium, but to most 

 species a medium having a neutral or alkaline reaction is neces- 

 sary; darkness is requisite to some and preferred by most species; 

 their growth is checked by bright light, and direct sunlight is 

 fatal. 



The food elements required by bacteria are present in the 

 constituents of milk, and they are in a readily available state. 

 Nitrogen, carbon, oxygen, and mineral matter are essential and 

 are furnished by the casein, albumen, milk sugar, and mineral 

 salts. The butter fat is of little importance as a food for germs. 



Bacteria thrive within wide limits of temperature. The de- 

 gree of heat has an important effect on their growth. Some 

 species do best at a high temperature, near blood heat, while 

 others prefer a lower temperature. Every person who has hand- 

 led milk knows that if kept a long time in a moderately warm, 

 place it undergoes quite a different change from that which takes 

 place at a high temperature. The reason for this is that differ- 

 ent degrees of heat are favorable to different species of germs. 



