METHODS USED IN CULTIVATION OP BACTERIA 



161 



Colony Study. — Cultures are usually incubated for from twelve to 

 forty-eight hours. Considerable aid to the recognition of species is 

 derived from the observation of both the speed of growth and the ap- 

 pearance of the colonies. It is therefore necessary to proceed in the 

 study of developed co'onies in a systematic way. The development of 

 colonies should be observed in all cases both upon gelatin and upon agar. 

 In forming any judgment about colonies, the acidity or alkahnity, and 

 the special constitution of the media should be taken into account. 

 The colonies are carefully examined with a hand lens and with the low 

 power (Leitz No. 2, Zeiss AA, Ocular No. 2) of the microscope. The 

 colonies should be observed as to size, outline, transparency, texture, 

 color, and elevation from the surface of the media. Much information, 



Pig. 45. — Variations in the Conformation of the Borders of Bacterial 

 Colonies. (After Chester.) 



also, can be obtained by observing whether a colony appears dry, 

 mucoid, or glistening, like a drop of moisture. By a careful obser- 

 vation of these points, definite differentiation, of course, can not usu- 

 ally be made, but much corroborative evidence can be obtained which 

 may guide us in the methods to be adopted for further identification 

 and for a final summing up of species characteristic as a whole. 



The Counting of Bacteria. — It is often necessary to determine the 

 number of bacteria per c.c. contained in water, milk, or other substances. 

 For this purpose definite quantities of the material to be analyzed are 

 mixed with gelatin or agar and poured into Petri plates. The exact 

 dilutions of the suspected material must largely depend upon the number 

 of germs which one expects to find in it. The plates, if prepared with 

 gelatin, are allowed to develop at room temperature for twenty -four to 

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