CHAPTER VI.
CULTIVATION OF BACTERIA; CULTURE-MEDIA.

ACCURACY of observation requires that the bacteria be
separated from their natural surroundings and artificially
grown upon certain prepared media of standard compo-
sition, in such a manner that only organisms of the same
kind are together.

One after another various organic and inorganic mix-
tures have been suggested, but, although almost any
compound containing organic matter, even in small
amounts, will suffice for the nourishment of bacteria,
a certain few have met with particular favor as being
most valuable.

Rather than give a complete review of the work which
has already been done, in the following pages the most
useful preparations only will be considered.

Our knowledge of the biology of the bacteria has
shown that they grow best in a mixture containing at
least 80 per cent, of water, of a neutral or feebly alka-
line reaction, and of a composition which, for the patho-
genic forms at least, should approximate the juices of
the animal body. It might be added that transparency
is a very desirable quality, and that the most gener-
ally useful culture-media are those that can be readily
liquefied and solidified.

Bouillon is one of the most useful and most simple of
the media. Its preparation is as follows : To 500 grams
of finely-chopped lean, boneless beef, 1000 c.cm. of clean
water are added and allowed to stand for about twelve
hours on ice. At the end of this time the liquor is de-
canted, that remaining on the meat expressed through a
cloth, and then, as the entire quantity is seldom regained,

124