124 THE AMERICAN MONTHLY [June, 



luvestigatioii of Bacteria by Means of Cultivation.* 



By R. a. foster, M. D. 



WASHINGTON, D. C. 



The materials employed in cultivating the different species of bac- 

 terium are of two classes, namely, fluids and solids ^ the former being 

 commonly designated as " wet cultures," and the latter as " dry cul- 

 tures." 



Before commencing the cultures, the fluid or solid medium employed 

 must be sterilized — that is. rendered free from all bacteria and bacterial 

 germs. This may be accomplished by raising it to a high temperature. 

 All bacteria and their spores are killed by steam or boiling hot water, after 

 exposure for a short time. A jet of steam will kill all kinds of germs 

 in from ten to fifteen minutes ; besides, it is more potent than steam in 

 a closed chamber. 



Of the many fluid media, an infusion of meat is the one most fre- 

 quently used at the present time. It is made by putting a pound of 

 lean beef, preyiously cut up into small pieces, in a yessel containing a 

 litre [quart] of water. The yessel is then placed in an ice safe, \yhere 

 it is allowed to remain not less than twenty-four hours. At the end of 

 the twenty-four hours it is taken out and all the juice of the meat 

 squeez-ed out by means of a press. If an ice safe and press are not at 

 hand, it is suflicient to keep the material simmering for two or three 

 hours. In either case, the resulting fluid must be well boiled — that is, 

 long enough to precipitate the albumen. After filtration it may be at 

 once introduced into suitable vessels. As the meat is acid in this con- 

 dition, and as an acid medium is a poor soil for many forms of bacteria, 

 it can be neutralized with carbonate of soda ; but this should be done 

 before it is boiled. To make the infusion still more serviceable, about 

 one per cent, of peptone and a little chloride of sodium should be added 

 to it. 



An infusion of cucumber is a very good medium for cultivating micro- 

 cocci. Cow's milk, blood and urine are also good cultivating fluids. 



Artificial cultivating materials are seldom if ever used now. The 

 two solutions that have been most frequently employed are Pasteur's 

 and Cohn's. 



Pasteur's solution is composed of candy sugar. lo grammes: tar- 

 trate of ammonia, i gramme : ashes of yeast, i gramme : distilled 

 water, loo c.cm. 



Cohn's solution is a modification of Mayer's ; it contains phosphate 

 of potash, 0.5 gramme ; crystallized sulphate of magnesia, 0.5 gramme ; 

 tribasic phosphate of lime, 0.05 giamme : tartrate of ammonia, i 

 gramme ; distilled water, 100 c.cm. 



Gelatinized materials and potato are the chief solid cultivating media. 

 The former may be inade by adding from five to ten per cent, of gela- 

 tine to the diftbrent fluid media mentioned above. 



" For potato cultivations." Chevne savs. •' ripe potatoes are best, the 

 small new potato not being nearly such a good medium. As the earth 

 on the outside of the potato is full of bacteria and spores, it must be 

 washed ofl'as much as possible, and the eves of the potato cleaned out. 



* Read at the Washington Microscopical Society at its SSth Regular Meeting, Feb. 26, 1889. 



