68 



ESSENTIALS OF BACTERIOLOGY 



exposed to the air led to the use of solid media for the 

 artificial culture of the same. It was thus learned that each 

 germ tends to form a separate colony and remain isolated, 

 and so pure cultures were first obtained. 



Esmarch's Cubes. The potato is first well cleaned and 

 peeled. It is then cut in cubes ^ inch in size. 



These are placed, each in a little glass dish or tray, and then 

 in steam-chest for one-half hour, after which they are ready 

 for inoculation (the dishes first having been 

 sterilized in hot-air oven). 



Test-tube Potatoes. Cones are cut out 

 of the peeled potato and placed in test-tubes, 

 which can then be plugged and easily pre- 

 served. 



Roux's test-tube (Fig. 19), specially de- 

 signed for potato cultures, consists of a tube 

 with a small constricted portion at the bot- 

 tom, hi which water may be kept to keep the 

 potato moist. 



Manner of Inoculating Potatoes. With 

 a platinum rod or a spatula (sterilized) the 

 material is spread upon one of the slices, 

 keeping free of the edges. The growth on 

 this first, or original, potato will be quite lux- 

 uriant, and the individual colonies often diffi- 

 cult to recognize; therefore dilutions are made. 

 From the original or first slice a small portion, including 

 some of the meat of the potato, is spread upon the surface of 

 a second slice, which is first dilution. From this likewise a 

 small bit is taken and spread on a third slice, or second dilu- 

 tion, and here usually the colonies will be sparsely enough 

 settled to study them in their individuality. 



This is the principle carried on in all the cultivations. It 

 is a physical analysis. 



Potato and Bread Mash. These pastes are used chiefly 

 in the culture of molds and yeasts. Peeled potatoes are 

 mashed with distilled water until thick, and then sterilized 



Fig. 19. Tube 

 for potato cul- 

 ture. 



