60 METHODS OF CULTIVATION OF BACTERIA 



To separate organisms by this method, three tubes, a, b, c, are inocu- 

 lated as in using Petri's capsules (p. 57). The hands having been 

 washed in perchloride of mercury 1-1000 and dried, the plate box is 

 opened, and a plate lifted by its opposite edges and transferred to the 

 levelled ground glass (as in Figs. 18, 19). The bell jar of the leveller 

 being now lifted a little, the gelatin in tube a is poured out on the 

 surface of the sterile plate, and, while still fluid, is spread by stroking 

 with the rim of the tube. After the medium solidifies, the plate is 

 transferred to the moist chamber as rapidly as possible, so as to avoid 

 atmospheric contamination. In doing this, it is advisable to have an 

 assistant to raise the glass covers. Tubes b and c are similarly treated, 

 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. Esmarctis 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 

 inoculated 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. 20). 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 melting-points of the two media. Agar, 

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

 E FlG 'h?tube a am s lid a little under 40 C. As it is danger- 

 for roll culture, ous 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 

 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. Esmarch's tubes are not suitable for use 



