THE PREPARATION OF CULTURE MEDIA 149 



possible. Make up the volume to 1000 e.c. Add 1 per cent pepton and 

 0.5 per cent salt. Heat over the free flame until the pepton is dissolved, 

 stirring from time to time. Solution usually takes place so rapidly that 

 the loss of water through evaporation is negligible. If desired, however, 

 the medium can be measured after this preliminary heating, and the loss 

 in volume made up by the addition of water. Titrate the medium and bring 

 it to the desired reaction using the colorimetric method. Meat infusion media 

 are usually considerably more acid than meat extract media. The addition 

 of about 20 e.c. normal NaOH per liter in the case of meat infusion broth 

 usually brings the reaction to about 7.4 after autoclaving. The change in 

 reaction after autoclaving is sometimes considerable, and must be carefully 

 checked. After the addition of the alkali, the medium is autoclaved for 

 thirty minutes at fifteen pounds pressure. The broth is now filtered through 

 paper or cotton, and usually comes through clear without any further trouble. 

 After tubing and sterilization, the medium is ready for use. 



"Hormone Medium." 9 A curious observation has been made recently 

 which seems to indicate that the filtration of media removes from them 

 certain substances which considerably enhance their nutritive value for 

 bacteria. 



These substances, for want of a better name, have been spoken of as 

 hormones, and the hormone media, which are used pretty generally, and 

 which we have found to possess unusual advantages over ordinary culture 

 media, and which, consequently, we are using almost altogether in our routine 

 work, are made as follows, the description given being that of Huntoon : 10 



The basis for "hormone media" is beef heart instead of the customary 

 beef or veal. It is important that the hearts are fairly fresh. The heart 

 muscle is cut up in the usual way ai d after the removal of fat and large 

 vessels, put through the meat grinder. The chopped meat is weighed and 

 1 liter of water added for every 500 grams of meat. One per cent pepton 

 and 0.5 per cent salt are added directly, and 1 egg (whole) added for each 

 liter of medium. If the bouillon is to be used for broth, 1 per cent gelatin 

 is added immediately. If the bouillon is to be used as a basis for agar, 

 it is not necessary to add the gelatin. The 3 per cent agar, finely cut up, 

 is added to the other ingredients. 



When all the ingredients have been placed in the same pot, the mixture 

 is heated over the free flame until it reaches a temperature of about 70 C. 

 and meat begins to turn brown. 25 e.c. of normal NaOH are then added 

 per liter. The pot is placed in the Arnold and allowed to cook for 1^ 

 to 2 hours. At the end of this time, a firm clot has usually formed and 

 the broth or agar can be decanted. 



9 Cole and Lloyd, Journal Path, and Baet. vol. 21, 1916. 



10 Huntoon, F. M., Jour, of Inf. Dis., 23, 1918, 169. 



