66 LABORATORY BACTERIOLOGY 
EXERCISE XVIII 
A STUDY OF CERTAIN SAPROPHYTIC BACTERIA 
97. 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. 
98. 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 
2@) plate cultures in gelatin and one of 2 plates in agar from 
a sample of milk furnished. The milk will be either freshly 
drawn in sterile flasks or samples of market milk. 
99. Identifying species of bacteria. The genera of bacteria 
are determined by the morphology. Thus, a spherical organ- 
ism is a micrococcus ; a motile, rod-shaped one is a bacillus ; and 
a nonmotile, rod-shaped one is 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. fphosus and B. colt look so nearly alike 
that one could not be sure of a difference microscopically: 
&. coli coagulates milk, B. typhosus does not ; B. colz produces 
gas in glucose media, B. ¢yphosus does not. Knowing these’ 
properties and having these cultures we could readily tell the 
