SEPARATION BY AGAR MEDIA 



55 



and the resulting plates stacked in series on the top of a. The chamber 

 is labelled and set aside for a few days till the colonies appear on the 

 gelatin plates. The further procedure is of the same nature as with 

 Petri's capsules. 



3. Esmarch's Roll Tubes. Here the principle is that of 

 dilution as before. In each of three test-tubes 1J or 1J inch in 

 diameter, gelatin to the depth of f of an inch is placed. These 

 are sterilised. The gelatin is melted and in- 

 oculated in series 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 

 sheet of gelatin instead of a flat one. A con- 

 venient 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 liquefying the 

 gelatin be present, the liquefied gelatin contamin- 

 ates the rest of the medium. 



Separation by Agar Media. 1. Agar Plates. 

 The only difference between the technique here 

 and that with gelatin 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 much 

 above 42 C., it is necessary 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 gelatin. 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 



FIG. 20. 

 Esmarch's tube 

 for roll culture. 



