12 
BACTERIOLOGY. 
Influence 
of envi¬ 
ronment 
The influence of temperature is most important. Most 
bacteria thrive best at 37.5 0 C., and as the temperature 
varies above or below this point growth is retarded. 
A temperature of 62° C. will kill most bacteria. Low 
temperatures are not so destructive, for by experi¬ 
ments it has been proven that a temperature of 200° 
below zero (centigrade) will not kill all bacteria. 
Moisture is essential for the growth of bacteria, 
as the food material upon which bacteria thrive must 
be in solution. The reaction of the food material is 
of considerable moment, for the bacteria will not grow 
if too much acid or alkali is present. A neutral or 
slightly acid reaction gives the best growth. 
In order to cultivate bacteria, substances may be 
made artificially, called culture media, and may be 
solid or fluid. The common kinds of solid media are 
agar-agar, agar-agar with some kind of sugar added, 
gelatin, and coagulated blood serum. Solid media 
are employed when it is desired to observe either the 
surface growth or the growth in the depth of the 
medium. Fluid media are used for the determination 
of motility, acid formation, fermentation, and coagu¬ 
lation. Those most often used are litmus milk, bouil¬ 
lon, and peptone broth. The media are prepared in 
the laboratory. After the ingredients have been dis¬ 
solved by boiling, the whole is filtered, run into test- 
tubes, plugged with cotton, and finally sterilized by 
steam under 15 pounds pressure for 20 minutes on 3 
successive days in order that no bacteria may develop 
in it except those introduced for the purpose of study. 
