PfcUSfCtPLES OP BACtEfclOLOGY 
34 
ture; this difference in the optimum temperature of the 
two strains of the same organism is due and proportionate 
to the difference in the normal temperature of human 
beings and the birds. 
By gradual and persistent variation of the temperature 
it is possible to adapt certain bacteria to grow abundantly 
at the temperature several degrees higher or lower than 
their normal optimum temperature, but this is usually 
accompanied by the loss of certain of their characteristics, 
as in the case mentioned above—that of anthrax bacillus 
grown by Pasteur at the temperature of 42° C.—the 
bacilli finally grew well but lost their property of spore 
formation. 
Some common pathogenic organisms, such as colon bacil¬ 
lus, exhibit a very wide variation of temperature at which 
they may develop—from 10° C. to 40° C. The sapro¬ 
phytes exhibit an even higher variation. 
Ten to fifteen minutes’ exposure to a temperature of 55° 
to 60° C. usually destroys the common pathogenic bacteria 
which are nonspore-bearing; the spores confer upon bac¬ 
teria enormous resistance to heat and the spore-bearing 
bacteria, therefore, are a much more formidable enemy 
than the other kind. 
Low temperatures are much less destructive than the 
high ones, and in the case of certain bacteria are useful 
in keeping cultures alive for a long period of time, e. g., 
pneumococcus and streptococcus. 
Moisture. —The presence of water is absolutely neces¬ 
sary for bacterial life. The effects of complete drying 
vary greatly with the different bacteria; thus gonococcus 
and cholera organisms die within a few hours while the ty¬ 
phoid and tubercle bacilli may withstand complete drying 
for two to three months. 
Light.—Most of the pathogenic bacteria are inhibited 
