58 



PHYSIOLOGY OF NUTRITION 



highly improbable that two cells might have settled together after the shaking. 

 If, on the other hand, two or three cells have been introduced into the flask, 

 then two or three colonies, respectively, develop. 



In order to secure pure yeast cultures, solid substrata may also be em- 

 ployed, which make it possible to foUow, under the microscope, the development 

 of a colony from a single cell. ' For this purpose a drop from a young yeast cul- 

 ture—previously shaken— is introduced into a small flask of steriUzed water. 

 From this is inoculated, by means of the tip of a platinum wire, another flask 

 containing beer-wort and gelatine, warmed to 4S°C. The latter is vigorously 

 shaken and then a drop of the liquid is transferred to a circular cover glass (30 mm. 

 in diameter), which has been marked off into numbered squares, and the cover 

 is laid over a glass ring to form a moist chamber or van Tieghem cell. The 

 yeast cells are held immovable in the hardened gelatine so that it may now be 



Fig. 38. — Pasteur flask; 

 a slightly different form from 

 that of Fig- 32, P- 52. 



Fig. 39. — Petri dish. 



Fig. 40. — Showing insertion 

 of needle into solid medium in 

 inverted tube, to make stab 

 inoculation. 



noted in which squares single ones lie, and the development of colonies from these 

 may be readily followed. When the colonies become clearly visible to the un- 

 aided eye, one of them is removed from the cover glass and placed in a flask of 

 nutrient solution. The colony is lifted on the end of a bit of flame-sterilized 

 platinum wire, held by means of forceps, and the wire, with its colony, is dropped 

 into the flask. During this operation the cover glass must be held with the drop 

 on its under side, to prevent infection from the air. If a large quantity of pure 

 culture is desired, a portion of a young culture a day old, obtained as just de- 

 scribed, is transferred with a pipette to a Pasteur flask (capacity about 200 cc.) 

 of sterilized beer- wort (Fig. 38). After a day the contents of this flask are 

 poured into second flask (capacity about 500 cc.) also filled with sterile beer- 

 wort. 



Solid as well as liquid nutrient media are used for pure cultures of bacteria. 



