214 Cultures, and their Study 



Cxxltures upon Potato.— These are made by simply stroking the 

 surface of the culture-medium, .the density and opacity of the 

 potato making it impracticable to puncture it. 



Most bacteria producq smooth, shining, irregularly extending 

 growths upon potato, that may show characteristic colors. 



Cultures in Fluid Media. — Here, as has already been stated, 

 transplantation consists in simply stirring in the bacteria so as to 

 distribute them fairly well throughout the medium. 



In milk and litmus milk one should observe change in color from 

 the occurrence of acid or alkah production, coagulation, gelatiniza- 

 tion, and digestion of the coagulum. 



Adhesion Preparations.— Sometimes it is desirable to preserve 

 an entire colony as a permanent microscopic specimen. To do this 

 a perfectly clean cover-glass, not too large in size, is momentarily 

 warmed, then carefully laid upon the surface of the gelatin or agar- 

 agar containing the colonies. Sufficient pressure is applied to the 

 surface of the glass to exclude bubbles, but not to destroy the integ- 

 rity of the colony. The cover is gently raised by one edge, and if 

 successful the whole colony or a number of colonies, as the case may- 

 be, will be found adhering to it. It is treated exactly as any other 

 cover-glass preparation — dried, fixed, stained, mounted, and kept as 

 a permanent specimen. It is called an adhesion preparation — 

 " Klatsckpraparat." 



SpeciaJ Methods of Securing Pure Cultiu-es.— Pure cultures from 

 single colonies may also be secured by a very simple manipulation 

 suggested by Banti.* The inoculation is made into the water of 

 condensation at the bottom of an agar-agar tube, without touching 

 the surface. The tube is then inclined so that the water flows over 

 the agar, after which it is stood away in the vertical position. Colo- 

 nies will grow where bacteria have been floated upon the agar- 

 agar, and may be picked up later in the same manner as from a 

 plate. 



When the bacterium to be isolated (gonococcus,.etc.) will not grow 

 upon media capable of alternate sohdification and hquefaction, 

 the blood-serum, potato, or other medium may be repeatedly stroked 

 with the platinum wire dipped in the material to be investigated. 

 Where the first strokes were made, confluent impure cultures occur) 

 but as the wire became freer of organisms by repeated contact with 

 the medium, the colonies become scattered and can be studied and 

 transplanted. 



In some cases pure cultures may be most satisfactorily secured 

 by animal inoculation. For example, when the tubercle bacillus 

 is to be isolated from milk or urine which contains bacteria that 

 would outgrow the slow-developing tubercle baciUus, it is better 

 to inject the fluid into the abdominal cavity of a guinea-pig, await 

 the development of tuberculosis in the animal, and then seek. to 

 'Centralbl. f. Bakt. u. Parasitenk.," 1895, xvii, No. 16. 



* tc I 



