ISOLATION OF BACTERIA IN PURE CULTURE 193 



ified in a Petri dish with a sterile heavy platinum needle, 

 glass rod or cotton swab. If the bacteria are not too numer- 

 ous, pure cultures may frequently be obtained. A modifi- 

 cation of this method is to make a series of (4) imraUel 

 streaks on a slope tube or plate of medium with a needle inserted 

 hut once into the material to be plated. On the first streak 

 most of the bacteria are rubbed off and a continuous growth 

 results, but usuall}^ on the last of a series only iso- 

 lated colonies appear, which are presumably pure. The 

 ideal method for securing pure cultures is to be absolutely 

 certain that the culture starts from a single organism. 



"Fig. 130. — From the thinnest 

 part of plate 2, Fig. 128, as seen 

 under the low-power objective. 

 X 100. Colonies much larger than 

 on plate 1, but still crowded. 



Fig. 131.— The smallest colony 

 on plate 3, Fig. 128, as seen under 

 the low-power objective. X 100. 

 Large, single, isolated colony. 



This may be accomplished by means of the (5) apparatus 

 and pipettes devised by Professor Barber of the University of 

 Kansas (Figs. 132 and 133). With this instrument a single 

 organism is picked out under the microscope and isolated in 

 a drop of culture medium and observed until it is seen to 

 divide, thus proving its viability. Transfers are then made 

 to the proper media. The method requires much practice 

 to develop the necessary skill in the making of pipettes, 

 determining the proper condition of the large cover-glasses 

 used over the isolating box, and in manipulation, but the 

 results fully compensate. 

 13 



