A STUDY OF CERTAIN SAPROPHYTIC BACTERIA 69 



EXERCISE XXI 



A STUDY OF CERTAIN SAPROPHYTIC BACTERIA 



101. It is desirable to have a definite knowledge concerning 

 the characters and properties of the commonly encountered 

 species and groups of saprophytic bacteria. It is likewise 

 important to understand the method of identifying species. 

 For these reasons a few exercises on saprophytic bacteria, 

 especially from air, milk, and water, have been introduced. 



102. Work for this exercise. Examine and carefully describe 

 the cultures made by exposing agar and gelatin plates to the 

 air in the last exercise. Determine the number of different 

 colonies and carefully describe each. Make a microscopic 

 examination (hanging-drop) of the bacteria in one of each of 

 the different kinds of colonies and determine its genus. 



Make for examination in the next exercise a series of three 

 plate cultures in gelatin and one of two plates in agar from 

 a sample of milk furnished. The milk will be either freshly 

 drawn in sterile flasks or samples of market milk. 



103. Identifying species of bacteria. The genera of bacteria 

 are determined by the morphology. Thus a spherical organ- 

 ism is a microcotcus, a motile, rod-shaped one is a bacillus, and 

 a nonmotile, rod-shaped one a bacterium. Each genus has a 

 large number of species. This requires some method by which 

 bacteria which look exactly alike under the microscope may 

 be differentiated provided they are different. This method 

 consists in the study of the growth of these bacteria on the 

 different media and possibly their effect upon small animals. 

 For example, the B. typhosus and B. coli communis look so 

 nearly alike that one could not be sure of a difference micro- 

 scopically ; B. coli communis coagulates milk, B. typhosus does 

 not ; B. coli communis produces gas in glucose media, B. typho- 

 sus does not. Knowing these properties and having these 



