64 CULTURES IN LIQUID MEDIA. 
tube for the purpose of starting a new culture. Or we may start a 
pure culture from a drop of blood taken from the veins of an animal 
which has been inoculated with anthrax, or any similar infectious 
disease in which the blood is invaded by a bacterial parasite. 
But if we have not a pure culture to start with our liquid media 
do not afford us the means of obtaining one; and if two or more 
bacteria which resemble each other in their morphology are associated 
in such a culture we cannot differentiate them, and are likely to infer 
that we have a pure culture of a single microédrganism when this is 
not really the case. 
But if we have pure stock to start with we may maintain pure 
cultures in liquid media without any special difficulty. 
Various characters of growth, etc., are to be observed in culti- 
vating different microédrganisms in liquid media. Thus some grow 
at the surface in the form ofa thin film or membranous layer—‘‘ my- 
coderma ”—while others are distributed uniformly through the liquid, 
rendering it opalescent or more or less milky and opaque; others, 
again, form little flocculi which are suspended in the transparent 
Fig. 35. 
fluid. Usually, when active growth has ceased, the bacteria fall to 
the bottom of the tube as a more or less abundant, white or colored, 
pulverulent or glutinous deposit. In some cases the liquid is colored 
with a soluble pigment formed during the growth of the bacteria, 
and usually this is formed most abundantly at the surface, where 
there is free access of oxygen. The reaction of the medium is often 
changed as a result of the growth of bacteria in it. From being neu- 
tral it may become decidedly alkaline or acid in its reaction. These 
changes may be observed by adding a litmus solution before sterili- 
zation of the culture medium, and observing the change of color 
when an acid-producing bacterium is under cultivation. The re- 
ducing power of bacteria upon various aniline colors may also be 
studied ; also their power to break up various organic substances, as 
shown by the evolution of gas or other volatile products which 
may be collected, or by substances which remain in solution and 
can be studied by ordinary chemical methods. 
Drop Cultures.—When we desire to study the life history of a 
microédrganism and to witness its development from spores, for ex- 
ample, its motions, etc., the method of cultivation in a hanging drop 
