General Characters of Bacteria. 67 



which they have all been removed, is said to be 

 sterile. The germs found in milk belong to the 

 lowest orders of the vegetable kingdom. Most of 

 them are included in the bacteria, although many 

 yeasts and moulds are frequently found in milk. 



The bacteria. — The bacteria are extremely minute 

 bodies consisting of a single cell filled with protoplasm. 

 They are of three general forms,— spherical (coccus); 

 rod -like or cylindrical (bacillus), and curved or wavy 

 (spirillum). They reproduce by fission ; that is, the 

 cell elongates slightly in the direction of its longer 

 axis, and a partition is formed across the cell trans- 

 versely, and two individuals exist where there was 

 but one before. These may break away and form 

 separate bacteria, but often they are kept . together 

 in various ways. Many forms are endowed with 

 motion, and all require a liquid or semi -liquid me- 

 dium for growth and development, though many 

 may exist for long periods of time in a dry condi- 

 tion. Like other plants, in order to grow and de- 

 velop, the bacteria must have suitable food. They 

 require for their sustenance carbon, hydrogen, oxy- 

 gen and nitrogen, together with small amounts 

 of mineral matters. Organic compounds are more 

 available for food supply than simple inorganic 

 salts. Substances like sugar and the various al- 

 buminous compounds are admirably suited for their 

 food. In ordinary milk nearly all kinds of bacteria 

 find an adequate and easily available food supply 

 in a medium favorable to their growth, so that not 

 only the forms of bacteria ordinarily found in milk, 



