66 METHODS OF CULTIVATION OF BACTERIA. 



inch in diameter, gelatine to the depth of f of an inch is 

 placed. These are sterilised. The gelatine is melted and 

 inoculated with the bacterial mixture as in making plate 

 cultures, but instead of being poured out it is rolled in a 

 nearly horizontal position under a cold tap or on a block 

 of ice till it solidifies as a uniformly thin layer on the inside 

 of the tube. Practically we deal with a cylindrical plate 

 of gelatine instead of a flat one. A convenient form of 

 tube for this method is one with a constriction a short 

 distance below the plug of cotton wool (Fig. 20). The 

 great disadvantage of the method is, that if organisms lique- 

 fying the gelatine be present, the liquefied gelatine con- 

 taminates the rest of the gelatine. 



Separation by Agar Media. i. Agar Plates. The only 

 difference between the technique here and that with gelatine, 

 depends on the difference in the melting-points of the two 

 media. Agar, we have said, melts at 98 C, and becomes 

 again solid a little under 40 C. As it is dangerous to 

 expose organisms to a temperature above 42 C, it is neces- 

 sary in preparing tubes of agar to be used in plate cultures 

 to first melt the agar, by boiling in a vessel of water for a 

 few minutes, and then to cool them to about 42 C. before 

 inoculating. The manipulation must be rapidly carried 

 out, as the margin of time, before solidification occurs, is 

 narrow ; otherwise the details are the same as for gelatine. 

 Esmarch's tubes are not suitable for use here, as the agar 

 does not adhere well to the sides. If to the agar 2 per 

 cent of a strong watery solution of pure gum arabic is 

 added, Esmarch's tubes may, however, be used. 



2. Separation by Stroking Mixture on Surface of Agar 

 Media. The bacterial mixture, instead of being mixed in 

 the medium, is spread out on its surface. The method 

 may be used both when the bacteria to be separated are 

 in a fluid, and when contained in a fairly solid tissue or 

 substance, such as a piece of diphtheritic membrane. In 

 the case of a tissue, for example, a small portion entangled 

 in the loop of a platinum needle is stroked in successive 

 parallel longitudinal strokes on sloped agar, the same 



