AGAR MEDIA. 49 



6 to 8 per cent of glycerine and sterilise as above. This 

 is used especially for growing the tubercle bacillus. 



3 (c). Glucose Agar. Prepare as in 3 (a\ but add i 

 to 2 per cent of grape sugar along with agar. This medium 

 is used for the culture of anaerobic organisms at temperatures 

 above the melting-point of gelatine. It is also a superior 

 culture medium for some aerobes, e.g. the B. diphtherias. 



These bouillon, gelatine, and agar preparations constitute 

 the most frequently used media. Growths on bouillon do 

 not usually show any characteristic appearances which 

 facilitate classification, but such a medium is of great use 

 in investigating the soluble toxic products of bacteria. The 

 most characteristic developments of organisms take place 

 on the gelatine media. These have, however, the dis- 

 advantage of not being available when growth is to take 

 place at any temperature above 24 C. For higher 

 temperatures agar must be employed. Agar is, how- 

 ever, never so transparent. Though quite clear when 

 fluid, on solidifying it always becomes slightly opaque. 

 Further, growths upon it are never so characteristic as 

 those on gelatine. It is, for instance, never liquefied, 

 whereas some organisms, by their growth, liquefy gelatine 

 and others do not a fact of prime importance. 



Agar smeared with Blood. This method was introduced 

 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 pneumo- 

 coccus. Human blood or the blood of animals may be 

 used. "Sloped tubes" (vide p. 58) of agar are employed 

 (glycerine agar is not so suitable). Purify a finger first 

 with i- 1 ooo 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. 58), 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 to two days at 38 C. before 



