LECTURE ON BACTERIOLOGY. 
367 
borne tuberculous matter, I draw a line along the gelatine, I have 
deposited at intervals along this line specimens of T. bacilli. If 
this plate be now kept at a proper temperature, after a few days, 
wherever the bacilli have been caught, a grayish spot will appear, 
which, easily seen with the naked eye, gradually spreads and be¬ 
comes larger. These spots are colonies containing thousands of 
T. bacilli. It is not probable, however, that we have been fortu¬ 
nate enough to have avoided depositing other germs along the 
line. If putrefactive bacteria are present, they will liquefy the 
gelatine. Various appearances are thus afforded, even to the 
naked eye, according to the particular bacterium present, and we 
soon become familiar with the characteristics of particular germs. 
Cultures of bacteria are usually made in test-tubes containing 
peptonized gelatine, coagulated blood-serum, etc. Let us return 
to our gelatine-plate. We find a spot which answers the descrip¬ 
tion of a colony of tubercle bacilli. We now take a minute par¬ 
ticle from this colony on a wire and convey it to the surface of 
some hardened blood-serum in a test-tube. We plug the tube so 
that no air-germs may drop in, and place it in an incubator at the 
proper temperature. After several days, if no contamination be 
present, a colony of bacilli will appear around the spot where we 
sowed the spores. Let us repeat the process; take a particle 
from this colony and transfer to another tube; this is our second 
culture. This must be repeated until we are satisfied that we 
have secured a “ pure culture.” If this be carried to the twenty- 
fifth generation, we may be assured that there remains no pus, 
no ptomains , nothing but the desired bacilli. It is a proper 
material now for inoculation. 
Practically, many precautions have to be thrown around every 
step of our work. You can see that spores might accidentally 
be attached to our tubes. They must be sterilized. This is 
accomplished by exposing them to prolonged high temperature in 
an oven. Similar precautions are taken with the plug of cotton 
used to stop the mouth of the tube. The wire must be heated to 
redness always just before using. We are in constant danger of 
contamination, and sometimes fail even with the utmost care. 
Fortunately, we can determine whether everything is as it should 
