204 MICROSCOPIC AND CULTURAL STUDY OF BACTERIA 



Potato. New potatoes have an acid reaction, as a rule, and old 

 potatoes are slightly alkaline. 



Large potatoes are thoroughly scrubbed, the skin removed, and 

 cut into cylinders with a cork-borer. The cylinders, which should be 

 at least 1.5 cm. in diameter, are divided into equal parts by a diagonal 

 cut. The pieces are placed in running water overnight so that they 

 will not darken, and are inserted, base downward, in large test tubes. 

 It is advisable to add about 1 c.c. of water to each tube to prevent 

 drying. Sterilize in the autoclave. 



Hiss's 1 Semisolid Medium. 



FORMULAE 



Water . . -. 1000 c.c. 



Agar 8 grams 



Peptone 10 " 



Meat extract 3 " 



NaCl 5 " 



Gelatin 2 40 " 



When all the ingredients are dissolved, adjust the reaction to +0.5 

 (phenolphthalein), filter, and add sufficient litmus solution to impart 

 a clear blue. Dissolve 1 per cent, of dextrose, lactose, saccharose, 

 mannite, or other carbohydrate in the medium, and fill test-tubes 

 with it. Sterilization of lactose and saccharose semisolid media is 

 preferably carried out in the Arnold sterilizer. Dextrose and mannite 

 media may be sterilized in the autoclave. 



Semisolid media are inoculated by the stab method. A change in 

 reaction is indicated by the litmus; gas-forming organisms form bubbles 

 in the depth of the medium. 



Russell Double Sugar Medium. To 1 liter of nutrient agar, slightly 

 alkaline to litmus, add sufficient sterile 5 per cent, litmus solution to 

 impart a distinct clear blue color. Add 1 per cent, of C. P. lactose 

 and 0.1 per cent, dextrose, and distribute in test-tubes. 



Sterilize in the Arnold sterilizer for three successive days, and allow 

 to harden in a slanted position. 



Media for the Cultivation of Aciduric Bacteria. Acid Broth. Add 

 sufficient glacial acetic acid to a liter of 2 per cent, dextrose broth 

 to make the reaction equal to 50 c.c. of normal acid. A precipitate 

 forms, which will settle out, leaving a clear supernatant fluid that may 

 be removed to sterile test tubes with a sterile 10 c.c. pipette. 



Oleate Agar. The addition of 0.2 per cent, sodium oleate to dextrose 

 agar makes a favorable medium for the cultivation of aciduric bacteria. 



1 Jour. Exp. Med., 1897, ii, 677. 



2 Add after the other ingredients are in solution. 



