METHODS OF RESEAECH : PREPARATION OF CULTURE MEDIA 105 



isms. It is best not to autoclave milk or any substance contain- 

 ing sugars. We do, however, autoclave most other substances, 

 heating them seldom higher than 110°C. or longer than 15 or 

 20 minutes. 



Next to milk, beef bouillon is one of the most commonly 

 used substances in the bacteriological laboratory. It may be 

 used alone but is generally fortified by the addition of 1 per cent 

 peptone to which sugar is also sometimes added. A number 

 of peptones are on the market and they vary so much in quality 

 and in their nutrient value for organisms, that it is best, except 

 for special experiments, to make use of only one, namely, that now 

 commonly recommended in bacteriological laboratories, and 

 known as Witte's peptonum siccum (See the observation under 

 Olive Tubercle Organism, No. XIII, concerning the inhibiting 

 action of Merck's peptone from flesh). Beef bouillon is much 

 better, in my judgment, if made from flesh of beef than from meat 

 extracts. In my experience meat extracts often contain salt and 

 bacteria which are hard to sterilize in the steamer, whereas a 

 good grade of fresh meat seldom gives any trouble. For further 

 notes on preparation of this medium and many other media 

 consult ''Bacteria in Relation to Plant Diseases," Vol. I. 



Blood Serum. — Loffler's solidified blood serum is another sub- 

 stance much used by the bacteriologist, but as it is rather dif- 

 ficult of preparation, for those who do not have access to the 

 apparatus and the animals of an animal pathological laboratory, 

 it is recommended to purchase from time to time in the open 

 market the small amount the plant pathologist needs. We buy 

 ours from Parke, Davis and Company, Detroit, Michigan. 



Gelatin and Agar Media. — The bacteriologist makes very 

 extensive use of gelatin and agar culture media in the form of 

 Petri-dish poured plates for isolation of his organisms. The 

 basis is peptonized beef bouillon to which agar is generally added 

 in 1 per cent or gelatin in 10 per cent quantities to obtain the 

 proper solidity. Of course, other substances may be employed, 

 if desired, instead of beef juice, such as simple peptone-water 

 or peptone-water reinforced with 1 or 2 per cent cane-sugar, 

 grape-sugar, milk-sugar, etc. On the whole, agar is more useful 

 for^isolation purposes than gelatin, especially in very warm 



