180 ISOLATION OF BACTERIA IN PURE CULTURE 



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) parallel 

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

 inserted but once into the material to be plated. On the first 

 streak a large part of the bacteria are rubbed off and a 

 continuous growth results, but usually on the last of a series 

 only isolated colonies appear, which are presumably pure. 

 The ideal method for securing pure cultures is to be abso- 

 lutely certain that the culture starts from a single organism. 



Fig. 131. — From the thinnest part 

 of plate S, Fig. 129, as seen under the 

 low-power objective. X 100. Colo- 

 nies much larger than on plate Z, 

 but still crowded. 



Fig. 132. — The smallest colony 

 on plate 3, Fig. 129, 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. 133 and 134). 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. 



