VIL. 
CULTURES IN LIQUID MEDIA. 
PRIOR to the introduction of gelatinous media by Koch in 1881, 
cultures were made in various organic liquids, and these are still 
largely used, being for certain purposes preferable to solid media. 
The method of preparing and sterilizing the flesh infusions and 
other organic liquids commonly used has already been given. We 
are here concerned with the various modes of using these nutritive 
liquids in cultivating bacteria. 
Flasks and tubes of various forms have been employed by differ- 
ent investigators, but the most useful receptacle for liquid as well as 
for solid culture media is the ordinary test tube. These are care- 
fully cleaned, plugged with a cotton air filter, sterilized in the hot-air 
oven at 150° C., and are then ready to receive the filtered liquid. 
Usually the tube should not be filled to more than one-third to one- 
half of its capacity. Sterilization of the culture liquid is then effected 
by placing the tubes in the steam sterilizer for half an hour on three 
successive days. Before using, the tubes should be placed for a few 
days in an incubating oven at 30° to 35° C. to test the sterilization. 
This is especially important with liquid media, for if a single living 
spore is present it may give rise to an abundant progeny, which will 
be distributed through the liquid in association with the species 
which has been planted. In solid cultures, on the contrary, such a 
spore would give rise to a colony, which by its locality and characters 
of growth would probably be recognized as different from the species 
planted, and consequently accidental. This is the great danger in 
the use of liquid media ; imperfect sterilization, or accidental contami- 
nation by atmospheric germs, may lead the inexperienced student 
into serious errors resulting from the assumption that the micro- 
organisms present in his cultures are all derived from the seed he 
planted. 
On the other hand, liquid media are more convenient than solid 
when it is the intention to isolate by filtration the soluble products of 
bacterial growth; for injection into animals to test pathogenic power; 
for experiments on the germicidal or antiseptic power of chemical 
agents, etc. 
