CULTURE MEDIA 9 1 



evidenced by the reddening of the litmus. Glucose litmus gela- 

 tin is also a useful medium for anaerobes. It is best to sterilize, 

 the litmus separately and add it from a sterile pipette at the time 

 the medium is used. 



Agar is modified by the addition of 5 to 7 per cent of glycerin, 

 and such glycerin-agar is used extensively for cultivation of the 

 tubercle bacillus and several other pathogenic bacteria. Various 

 sugars, supplemented by the addition of litmus, are 

 dissolved in agar to test the fermentation properties 

 of bacteria. Glucose agar is extensively employed as 

 such for the cultivation of anaerobes. Agar also forms 

 the gelatinizing base for a number of more or less com- 

 plex special media. 



STERILIZABLE SPECIAL MEDIA. 



Potato. Potatoes were perhaps the first solid med- 

 ium employed in the cultivation of micro-organisms. 

 Boiled or steamed potatoes kept in a moist place, such 

 as a large covered glass dish, may well be employed as 

 an illustration of primitive technic, and excellent cul- 

 tures of the common chromogenic bacteria may be ob- 

 tained in this way. For most purposes it is better to 

 put pieces of potato in test-tubes where they are more 

 perfectly protected from contamination, as suggested 

 by Bolton. 1 The potato is carefully washed, a slice Potato 4 in 

 removed from each end, and a cylinder is cut out with culture 

 a cork-borer or with a test tube cut off near its bottom. 

 This cylinder is divided diagonally into two pieces. The pieces 

 are washed in running water for twelve to eighteen hours. 

 They are placed in test-tubes containing a little water to keep 

 the potato moist, and are supported from the bottom on a piece 

 of glass tubing about i to 2 cm. in length (or on cotton, or in a 

 specially devised form of tube with a constriction at the bottom) . 



1 Bolton, The Medical News, Vol. I, 1887, p. 318. 



