38 METHODS OF CULTIVATION OF BACTERIA 



tournesol) may be added to show change in reaction during 

 bacterial growth. The litmus is added, before sterilisation, as 

 a strong watery solution (e.g. the Kubel-Tiemann solution, vide 

 p. 42) in sufficient quantity to give the medium a distinctly 

 bluish tint. During the development of an acid reaction the 

 colour changes to a pink and may subsequently be discharged. 



Use of neutral red. This dye has been introduced as an aid 

 in determining the presence or absence of members of the b. coli 

 group, especially in the examination of water. The media found 

 most suitable are agar or bouillon containing '5 per cent of 

 glucose, to which *5 per cent of a one per cent watery solution 

 of neutral red is added. The use of these media and their 

 probable value are described below (vide Typhoid Fever). 



Blood Agar : Serum Agar. The former medium was intro- 

 duced by Pfeiffer for growing the influenza bacillus, and it 

 has been used for the organisms which are not easily grown on 

 the ordinary media, e.g. the gonococcus and the pneumococcus. 

 Human blood or the blood of animals may be used. " Sloped 

 tubes " (vide p. 48) of agar are employed (glycerin agar is' not so 

 suitable). Purify a finger first with 1-1000 corrosive sublimate, 

 dry, and then wash with absolute alcohol to remove the sublimate. 

 Allow the alcohol to evaporate. Prick with a needle sterilised 

 by heat, and, catching a drop of blood in the loop of a sterile 

 platinum wire (vide p. 49), smear it on the surface of the agar. 

 The excess of the blood runs down and leaves a film on the 

 surface. Cover the tubes with india-rubber caps, and incubate 

 them for one or two days at 38 C. before use, to make certain 

 that they are sterile. Agar poured out in a thin layer in a 

 Petri dish may be smeared with blood in the same way and 

 used for cultures. A medium composed of one part of fresh 

 blood (drawn aseptically) and two parts of fluid agar at 40 C., 

 has been used for the cultivation of the bacillus of soft sore. 



Serum agar is prepared in a similar way by smearing the 

 surface of the agar with blood serum, or by adding a few drops 

 of serum to the tube and then allowing it to flow over the 

 surface. 



Peptone Solution. 



V A simple solution of peptone (Witte) constitutes a suitable 

 culture medium for many bacteria, The peptone in the propor- 

 tion of 1 to 2 per cent, along with '5 per cent NaCl, is dissolved 

 in distilled water by heating. The fluid is then filtered, placed 

 in tubes and sterilised. The reaction is usually distinctly 

 alkaline, which condition is suitable for most purposes. For 



