542 J. 0RSKOV 



emplojdng a good method, a most tedious procedure, invoh-ing 

 several hours' close work for each organism isolated, if the re- 

 sults are to be relied on." 



I beheve the difficulties to have become considerably reduced 

 in the procedure described in the following pages, and, the simpler 

 a method is, the more reUable will it generally be. The prin- 

 ciple upon which the method of isolation described below has 

 been based is, as mentioned, the fact that a bacterium — possibly 

 the minutest ones excepted — can readily be distinguished on the 

 surface of a clear transparent medium, such as for instance agar, 

 gelatin, or ascitic agar. The mode of procedure is, briefly stated, 

 as follows: A young bacterial culture, such for instance as 

 a twelve-hour old broth culture of colon bacilli, is inoculated 

 on the agar plate in a Petri dish. The agar had better not be 

 more than a few millimeters thick (bacteria can also be dis- 

 tinguished on very tliick agar, but less sharply). The upper 

 and lower surfaces of the agar should be parallel so as to ensure 

 that the excised bits shall be of equal thickness everywhere, 

 partly in order to obtain plane images, partly to avoid the risk 

 of running down with the objective into the agar, which is in 

 its immediate vicinity during examination. 



It is important, in inoculating the culture, to be fairly clear 

 at the outset as to the density of bacteria desired on the plate. 

 In the diagram, figure 1, some dotted Unes show how I am ac- 

 customed to proceed. A big droplet of the broth culture is 

 deposited in the center of the circle, and, by means of a glass rod, 

 bent at a right angle, the drop is pressed down between the 

 parallel dotted Unes. Now the glass rod is moved from side 

 to side across the first inoculated area, and, finally, the remain- 

 ing half of the dish is inoculated and it is placed in the incubator 

 for about one hour at 37°C. (Inoculation can of course also be 

 performed in a streak, which some will perhaps find more to 

 the purpose, and in this way an appropriate difference in the 

 density of the bacteria can likewise be obtained.) Tliis measure 

 is taken because the development in the case of the colon bacillus 

 begins just after the expiration of one hour, and because bac- 

 teria are more readily discernible when in development, o^\'ing 



