72 CULTURES IN SOLID MEDIA. 



deposited upon our plate, exposed as above described, a little mass 

 of organic material containing two or more different bacteria, and 

 this would serve as the nucleus of a colony from which we could not 

 obtain a pure culture. 



Koch's Plate Method. In the experiment above described, 

 colonies were obtained from air-borne germs which were deposited 

 upon the surface of our gelatin medium. By Koch's famous ' ' plate 

 method " we obtain colonies of any particular microorganism which 

 we desire to study, or of two or more associated bacteria which we 

 desire to study separately in pure cultures. Evidently, when we 

 have obtained separate colonies of different bacteria upon the sur- 

 face of a solid culture medium, we can easily obtain a pure culture 

 of each by inoculating stick cultures from single colonies. 



To obtain separate colonies we resort to the ingenious method of 

 Koch. Three test tubes containing a small quantity of nutrient 

 gelatin (or of agar) are commonly employed. The tubes are num- 

 D3red 1, 2, and 3. The first step consists in liquefying the nutrient 

 jelly by heat, and it will be well for beginners to place the tubes in 

 a water bath having a temperature of about 40 C. (104 F.) for the 

 purpose of keeping the culture material liquid, and at the same time 

 at a temperature which is not high enough to destroy the vitality of 

 the bacteria which are to be planted. We next, by means of a 

 platinum-wire loop or the platinum needle used for stick cultures, 

 introduce into tube No. 1 a small amount of the culture, or material 

 from any source, containing the bacteria under investigation. Care 

 must be taken not to introduce too much of this material, and it 

 must be remembered that the smallest visible amount may contain 

 many millions of bacteria. The reason for using three tubes will 

 now be apparent. It is usually impossible to introduce a few bac- 

 teria into tube No. 1, but we effect our object by dilution, as follows : 

 With the platinum-wire loop we take up a minute drop of the fluid in 

 tube No. 1, through which the bacteria have been distributed by 

 stirring, and carry it over to tube No. 2. Washing off the drop by 

 stirring, we may repeat this a second or third time this is a matter 

 of judgment and experience ; often it will suffice to carry over a 

 single ose (the German name for the platinum- wire loop). Next 

 we carry over one, or two, or three ose from tube No. 2 to tube No. 

 3. By this procedure we commonly succeed in so reducing the num- 

 ber of bacteria in tube No: 3 that only a few colonies will develop 

 upon the plate which we subsequently make from it; or it may happen 

 that the dilution has been carried too far and that no colonies de- 

 velop upon the plate made from this tube, in which case we are 

 likely to get what we want from tube No. 2. The next step is to 

 pour the liquid gelatin upon sterilized glass plates, which are num- 



