76 PHARMACEUTICAL BACTERIOLOGY 



much of the fat as possible. The so-called round steak of beef is usually 

 employed. 



Use only the best gelatin; the so-called best French gelatin is usually 

 employed, although much of the " French gelatin" comes from Berlin, 

 Chicago, Omaha, or other places equally remote from France. Do not 

 attempt to use old friable gelatin. 



The milk requirements have already been referred to. The milk must 

 be fresh, placed on ice at once, and sterilized within twenty-four hours 

 after it is taken from the cow. If the milk is obtained from an unknown 

 dealer, test it for the presence of added water, preservatives, and other 

 foreign matter. 



Agar does not deteriorate readily, and may be kept in good condition 

 for a long time. Other highly gelatinous seaweeds may be used, although 

 this is not permissible in the preparation of any of the standard culture 

 media . 



'Serum, egg albumen, peptone, various indicators, etc., must be pure. 

 Too much caution cannot be observed in this regard. Secure the blood 

 for serum personally whenever possible, from healthy animals. Use egg 

 albumen from fresh eggs, not from cold-storage eggs Dried egg albumen 

 may be used. Before doing so, it should be examined microscopically 

 and if it contains excessive bacteria, 100,000,000 or more per gram, it 

 should not be used. Much of the dried egg and dried egg albumin of the 

 market is highly contaminated by bacteria. Peptone and other chemicals 

 should be secured from reliable dealers. 



B. Suggestions on the Preparation of Culture Media. First of all, some 

 experience is necessary before a neat article can be prepared. Do not 

 expect to prepare a medium which meets all of the requirements the very 

 first time. In preparing gelatin media, remember that these are injured 

 by excessive heating, and in preparing agar media, remember that they 

 are very difficult to filter. Both must be filtered hot, using hot-water 

 funnels; or the ordinary filtering device can be used by keeping the un- 

 filtered portion hot and pouring into the funnel from time to time. Cover 

 funnel with filter paper to keep out dust, and keep in the heat as much as 

 possible. In so far as possible filter all media through filter paper (one 

 thickness, properly folded), but it is practically impossible (for reasons of 

 time) to pass agar through filter paper. This medium is usually filtered 

 through cotton upon which a neatly folded and perforated sheet of filter 

 paper has been placed. Puncture the filter paper several times with a 

 small knife blade. Filtering through cotton is quick, but the media are 

 much less clear than when filtered through filter paper. The filtering 

 process may also be hastened by means of pressure (suction) ; connect fun- 

 nel with aspirator bottle and pump, but see to it that the connections 



