VIII. n. 6 



Barber: Dysentery Bacilli 



555 



rows of hanging drops are now brought into the field, and a 

 small droplet ejected from the pipette in the immediate neigh- 

 borhood of each (fig. 1). Each smaller droplet must be made 

 to contain a single bacillus, and, if a proper dilution has been 

 used, this is easily done. The small droplets must be made 

 very small so as to be easily examined and the presence of a 

 single cell determined. If the first droplet contains no bacilli, 

 others may be made near it at the margin of the larger drop 

 until one is obtained as desired. If two or more bacilli come 

 out, all but one may be picked up and carried to the next large 

 drop. 



When as many as desired of the larger drops are supplied 

 with the one-bacillus droplets, the pipette is withdrawn and a 

 new sterile one introduced. 



1 



ooooooooo 



n 



000 00 00 OO 



ooooo oooo 



y 



OOO OO OO OO 

 ooooo OO OO 



\3 



ooo oo oooo 



-o 



Fig. 1. Diagram of a microscopical slide to illustrate the method of single-cell isolation. 

 c Larper droplet shown on larger scale with smaller droplet near it. 



This may be filled with sterile broth from drop b or from a 

 test tube. With this pipette, broth is applied to each small 

 droplet until, with its contained bacillus, it merges with the 

 larger drop, or, broth is supplied to the larger drop until it 

 overflows the smaller one- 



The larger drop affords sufficient broth for good growth, and 

 the purpose of the smaller droplet is, obviously, to permit of 

 thorough examination in order to be sure of the presence of a 

 single cell. 



The cover is now removed from the box and sealed over a 

 shallow moist chamber. The next day, transfers to test tubes 

 may be made from each drop with a bent capillary pipette that 

 is made new for each drop or by means of a fine platinum wire 

 bent at the tip. 



By this method one can quickly obtain a series of pure cultures 

 from one source. If plump, actively growing cells are selected, 

 practically all will grow. It is best to use a young culture 

 grown two or three hours in the same broth as that used in the 

 drops. Agar may be used in place of the broth in making the 

 rows of drops. Isolations from two or more sources may be 



