Structure, Growth and Distribution. 11 



sistant to the influence of an unfavorable environment,, 

 such as heat, cold, drying, and even chemical agents. 

 It is this property of the spores which makes it so dif- 

 ficult to destroy the bacterial life in the process of steril- 

 izing milk. The property of spore-formation is fortu- 

 nately confined to a comparatively small number of dif- 

 ferent species of bacilli. 



Movement. Many of the bacteria are provided with 

 vibratory organs of locomotion, known as cilia (singular 

 cilium) which are variously distributed on the surface 

 of the cell. By the movement of these relatively long, 

 thread-like appendages the individual cell is able to 

 move in liquids. It must be remembered, when these 

 moving cells are observed under the microscope, that 

 their apparent rate of movement is magnified relatively 

 as much as their size. 



Conditions for growth. All kinds of living things 

 need certain conditions for growth such as food, mois- 

 ture, air and a favorable temperature. The bacteria 

 prefer as food such organic matter as milk, meat, and 

 vegetable infusions. Those living on dead organic mat- 

 ter are known as saprophytes, while those which are 

 capable of thriving in the tissues of the living plant or 

 animal are known as parasites. Certain of the parasitic 

 forms are capable of causing disease in plants and ani- 

 mals. In the first' group are embraced most of the bac- 

 teria that are able to deve^p in milk or its products, 

 such as those forms concerned in the spoiling of milk or 

 its fermentation. It is true that milk may contain dis- 

 ease-producing bacteria coming either from a diseased 

 animal or from a diseased human being. It is also true 

 that some of such harmful forms are able to grow in 

 milk, such as the organisms causing typhoid fever and 

 diphtheria. 



