HOW WE GET BACTERIA FOR STUDY 255 



Where Bacteria may he found. Growths of bacteria may be 

 obtained on dishes prepared in the following ways : 



(a) exposing to the air of the schoolroom ; 



(6) exposing in the halls of the school while pupils are passing ; 



(c) exposing in the halls of the school when pupils are not mov- 

 ing; 



(d) exposing at the level of a dirty and much-used street ; 



(e) exposing at the level of a well swept and little used street ; 

 (/) exposing in a park or elsewhere among trees ; 



(g) exposing in a factory building ; 



(h) touching the medium with the fingers ; 



(i) touching the medium with fingers moistened with saliva ; 



(j) touching the medium with decayed vegetable or meat ; 



(k) touching the medium with dirty coin or bill ; 



(I) placing in dish two or three hairs from one's head ; 



(m) keep one dish, the " check " or " control," unexposed. 



This list might be prolonged indefinitely. 



Place all of the dishes in a moderately warm place (a closet in 

 the schoolroom will do) for the 

 process called incubation. After a 

 day or two, little spots, brown, yel- 

 low, white, or red, will begin to 

 appear. These spots, which grow 

 larger day by day, are colonies made 

 up of millions of bacteria ; and 

 probably each colony arose from a 

 single bacterium. 



How we may isolate Bacteria of 

 One Kind from the Other. — In 



order to get bacteria of a given Colonies of bacteria growing in a 

 T . T , , 1 .. 1 Petri dish. 



kmd to study, it becomes necessary 



to grow them in what is known as a pure culture. This is done 

 after first growing the bacteria in some medium such as beef broth 

 or gelatin, or on potato.^ When the colonies of bacteria appear 



^ For directions for making a culture medium, see Hunter, Laboratory Problems 

 in^ivic Biology. Culture tubes may be obtained, already prepared, from Parke, 

 Davis, and Company or other good chemists. 



