SEPARATION BY AGAR MEDIA 



61 



AZt 



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 three-quarters of an inch is 

 placed. These are sterilised. The gelatin is melted and inocu- 

 lated 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 convenient form of tube for this method is one 

 with a constriction a short distance below the plug of cotton 

 wool (Fig. 19). The great disadvantage of the method is, that 

 if organisms liquefying the gelatin be present, the liquefied 

 gelatin contaminates 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 rt&f5&k 

 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 first to melt the agar, by j|| 



boiling in a vessel of water for a few minutes, 

 and then to cool it 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. Agar plates 

 in Petri dishes should be incubated in an inverted 

 position; otherwise trouble may be caused by 

 condensation water dropping off the cover on to 

 the surface of the medium. 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. Esmarch's tube 

 of a strong watery solution of pure gum arabic for roll culture, 

 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 



FIG. 19. 



