58 PURE CULTURE OF BACTERIA 



If separated milk cannot be obtained, take ordinary fresh 

 whole milk; leave it to stand in a beaker in a cool place 

 all night, and in the morning pipette or siphon off the " skim- 

 milk " from below the cream layer. 



Ex. 32. Litmus Milk. Before tubing the milk, add 

 enough Kubel-Tiemann's neutral litmus to tinge it a purple 

 colour ; then sterilize as advised in previous exercise. 



When inoculated with an acid-forming organism, the purple 

 colour becomes changed to pink. 



Ex. 33 Whey as Medium. Take a litre of separated or 

 skim-milk in a dish or bowl, and pour into it 10 c.c. of distilled 

 water, to which 8 to 10 drops of commercial rennet .'extract have 

 been added. Stir briskly for a few seconds, so as to mix the whole 

 and allow the milk to coagulate. When the curd is formed, cut 

 it with a knife and leave it for an hour so that the whey may 

 separate. Then filter the whey through muslin. Add the white 

 of an egg whipped up with 100 c.c. of water. Heat in steam 

 sterilizer for an hour. Filter into test tubes and sterilize in the 

 usual way. 



Ex. 34. Potato. Procure some long sound tubers. Wash 

 them in water, and scrub off all soil with a nail brush. Cut off 

 the two ends, and with a cork-borer cut cylinders from the 

 potatoes. Divide the cylinders diagonally with a sterile knife, 

 and wash the pieces twelve hours in running water, or wash 

 them in several changes of water until no milkiness is observed. 



Place each of the washed halves, thick end downwards, in 

 sterilized Buchner or potato tubes (Fig. 25), the bulbs of which 

 are half-filled with distilled water. The tubes can be tilted so that 

 the water touches the bases of the slices and keeps them moist 

 when bacteria are being cultivated upon them. If no special 

 tubes are at hand, place a piece of wet cotton wool at the bottom 

 of ordinary wide test-tubes before putting in the potato slices. 



Sterilize on three successive days in steam for thirty minutes 

 each time. It is very necessary to observe great cleanliness in 

 preparing the potato slices, as the soil upon the tubers frequently 



