56 



METHODS OF CULTIVATION OF BACTERIA. 



same is laid on its bottom. Glass benches on which the plates 



may be laid are similarly purified. 



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



are inoculated as in using Petri's dishes. The hands having 



been washed in perchloride of 



mercury i-iooo 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. 20, 21). 



The bell-jar of the leveller be- 

 ing 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 solidi- 

 fies, the plate is transferred to 



the moist chamber as rapidly 



as possible, so as to avoid atmos- 

 pheric contamination. In do- 

 ing 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 in the gelatin plates. 

 The further procedure is of the same nature as with 

 Petri's dishes. 



3. Esniarch's Roll Titbes. — Here the principle 

 is that of dilution as before. In each of three test- 

 tubes i^ or \\ inch in diameter, gelatin to the depth 

 of \ 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, as devised by Booker, till it solidifies as a 

 uniformly thin layer on the inside of the tube. 



Practically we deal with a cylindrical plate of gelatin instead of 



a flat one. A convenient form of tube for this method is one 



Fig. 21. — Koch's levelling apparatus, 

 Hands shown in second position just as ths 

 plate is lowered on to the ground-glass sur 

 face. By executing the transference of tht 

 plate from the box in this way, the surfact 

 which was undermost in the latter is upper 

 most in the leveller, and thus never meets t 

 current of air which might contaminate it. 



Fig. 22. — 

 Esmarch's 

 tube for roll- 

 culture. 



