14 BACTERIOLOGICAL DIAGNOSIS. 



the fluid is slightly alkaline.* Boil the fluid for half an 

 hour to coagulate any albumen 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 sterilised cotton- 

 wool. 



If the broth is to be used for the manufacture of 

 gelatin or agar it is next sterilised in the flask, while 

 if it is to be used as it is as a culture medium, it is 

 decanted into tubes and then sterilised. 



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 contami- 

 nating the culture. Ordinary non-absorbent (brown) 

 wool is better than the white absorbent wool, as it is 

 less easily wetted. 



The broth (and other culture medium after being 

 melted) may be poured into the tubes in the following 

 way. A sterilised funnel is united by a short length 

 of india-rubber tubing to a piece of glass tubing drawn 

 out to a point : the rubber tube is clipped by a spring 

 clip or a pair of pressure forceps. The funnel is now 

 mounted on a retort stand, filled with the medium, and 

 covered over with a piece of glass. The cotton-wool 

 plug is removed from a test-tube and the latter placed 

 so that the glass tube attached to the funnel reaches 

 nearly to the bottom. The clip is released and the 



* If during the neutralizing process too mnch alkali is added, then 

 it is necessary to re-acidify with dilute hydrochloric acid and re- 

 neutralize. The sodium chloride formed makes no practical differ- 

 ence in the medium. 



