74 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 microédrganism 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 stab 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- 
bered 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 stab 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. Itis 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 dse (the German name for the platinum-wire loop). Next 
we carry over one, or two, or three dse 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 fromit; 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- 
