10 CLINICAL BACTERIOLOGY AND HiEMATOLOGY 



Broth is very easily made, and, as it is the foundation of many 

 other media, the practitioner is strongly advised to prepare it for 

 himself. 



Requisites. — i. Liebig's Extract of Meat. 



2. Peptone. 



3. Common salt. 



4. A dilute solution of sodium carbonate — about i per cent,, 

 but the exact strength does not matter. 



5. A large flask, a stirring-rod, and a large glass funnel. 



6. Test-tubes and cotton-wool plugs. The exact size of the 

 tubes is unimportant, but 6 inches by | inch is convenient. The 

 plugs are best prepared from wool which has been previously 

 sterilized by dry heat, and should be fairly firm. The tube, with 

 the plug in situ, must be sterilized by dry heat ready for use. 



7. Litmus-paper. 



Method. — Take i litre of tap-water in the flask, and add 

 5 grammes of Liebig's Extract, 10 grammes of peptone, and 

 5 grammes of common salt, and boil until all are dissolved. Test 

 the reaction by withdrawing a drop of the fluid on the stirring-rod 

 and applying it to a piece of litmus-paper. You will probably 

 find that it is slightly acid. Now add some of the solution of 

 soda, drop by drop, testing after each addition, until the reaction 

 of the fluid is slightly alkaline.* Boil the fluid for half an hour, 

 to coagulate any albumin which may be present. 



Next filter the broth into a sterile flask, passing it through a 

 double thickness of white filter or blotting paper, and plug the 

 flask firmly with sterilized cotton- wool. 



If the broth is to be used for the manufacture of gelatin or 

 agar, it is next sterilized in the flask ; while if it is to be used 

 as it is as a culture medium, it is decanted into tubes and then 

 sterilized. 



In decanting media into tubes be very careful not to get the 

 plug wet, and not to let any of the medium get on to the upper 

 part of the tube ; otherwise the plug will stick to the tube, and 

 there will be some danger of bacteria from the air " growing 

 through " the fluid contained in the interstices of the plug and 

 contaminating the culture. Ordinary non-absorbent (brown) wool 



» If during the neutralizing process too much alkali is added, then it is . 

 necessary to reacidify with dilute hydrochloric acid and reneutralize. The 

 sodium chloride formed makes no practical difference in the medium. 



