METHOD OF ISOLATION 45 



remained permanently liquid. The dish or plate is 

 placed in an incubator or in a warm room, and after 

 a few hours each individual organism grows and 

 multiplies in the nutrient gelatine, its descendants form- 

 ing a small aggregation of similar bacteria termed a 

 colony (Fig. 17). The colonies produced by the various 

 kinds of bacteria vary considerably in form and colour 



* 



4 



FIG. 17. Common forms of bacterial colonies, i, Round; 2, lobed ; 

 3, amoeboid; 4, mycelioid. 



and in other ways. The differences must be carefully 

 studied, as many species are characterized by the kind of 

 colony which they produce on the various media described 

 subsequently. 



Ex. 16. In the practical isolation of bacteria the first step 

 is to clean and sterilize the test-tubes, beakers, and flasks to be 

 used. 1 



(i) Test-Tubes. New test-tubes (6 in. by fin.) should be 

 cleaned by rubbing the inside with a cotton-wool plug which has 

 been tied with strong thread to the end of a glass rod and 

 dipped in strong nitric acid. After cleaning with acid wash 

 thoroughly with water and allow them to drain until nearly dry, 

 then wash out with strong or absolute alcohol, and when dry 

 put into the mouth of each a cotton-wool plug ; place a number 



1 Slides, test-tubes, beakers, stains, and chemicals, as well as incubators 

 and other bacteriological apparatus needed for the practical exercises, can be 

 obtained from Messrs Baird & Tatlock, Cross Street, Hatton Garden, 

 London, E.G. 



