CONDITIONS OF GROWTH. 127 
parasitic species have a more restricted range, which approaches the 
normal temperature of the animals in which they habitually de- 
velop. At 40°C. (104° F.) growth, as a rule, ceases, but there are 
some notable exceptions to this rule. 
Miquel some years ago found a bacillus in the water-of the Seine 
which grew at a temperature of 69° to 70° C.; Van Tieghem reports 
having observed species in thermal waters capable of growth at a 
still higher temperature (74° C.) ; and Globig has more recently ob- 
tained from garden earth several species which multiplied at 65° C. 
Some of the species found by the last-named observer were even 
found to require a temperature of about 60° for their development; 
and yet this temperature is quickly fatal to a large number of the 
best known species. 
Low temperatures, while arresting the growth of bacteria, do not 
destroy their vitality. This has been demonstrated by numerous ex- 
periments, in which they have been exposed for hours in a refrigerat- 
ing mixture at —18° C. Frisch has even subjected them to a tempe- 
rature of —87° C. by the evaporation of liquid carbon dioxide, and 
found that they still grew when placed in favorable conditions. 
Parasitism.—tThe strict parasites grow only in the bodies of liv- 
ing animals, or in artificial media kept at a suitable temperature. 
As examples we may mention the bacillus of tuberculosis, the bacil- 
lus of leprosy, the micrococcus of gonorrhcea, the spirillum of re- 
lapsing fever. There is also a large class of facultative para- 
sites which, when introduced into the body of a susceptible animal, 
multiply in it, and may continue to live as parasites so long as they 
are transferréd from one animal +o another, but which are also able 
to live as saprophytes independently of a living host. To this class 
belong the pus cocci, the bacillus of typhoid fever, the spirillum of 
cholera, and many others. 
It seems extremely probable that the strict parasites were at one 
time capable of living a saprophytic existence, and that their restric- 
tion to a parasitic mode of life has been effected in course of time in 
accordance with the laws of natural selection. This view is sup- 
ported by the fact that the tubercle bacillus, which has been regarded 
as a strict parasite, which can only be cultivated artificially under 
very special conditions, has been shown to be capable of modification in 
this regard to such an extent that when cultivated for a time in a favor- 
able medium—bouillon with five per cent of glycerin—it will even grow 
in ordinary bouillon made from the flesh of a calf or a fowl (Roux). 
Reaction of Medium.—Some bacteria grow readily in a medium 
having an acid reaction, while the slightest trace of acidity prevents 
the development of others. Asarule, the pathogenic species require 
a neutral or slightly alkaline culture medium. 
