94 
PRINCIPLES OF BACTERIOLOGY 
less we grow them, but because the different ways in 
which the different bacteria grow on media and the dif¬ 
ferent changes which they produce in the culture media 
(fermentation of sugars, the production of acid, coagu¬ 
lation of milk, liquefaction of gelatin, etc.) constitute one 
of the most valuable means of bacterial differentiation 
and identification. (Fig. 18.) 
By a culture medium (plural: culture media) we mean 
any substance on which bacteria grow outside the animal 
body. 
Before the various culture media can be prepared, sev¬ 
eral other things have to be attended to. 
Preparation of Glassware. —If the glassware is new, 
immerse it in 1 per cent solution of hydrochloric acid, 
then wash it in 1 per cent sodium hydroxide, and, finally, 
wash it in running water. Old glassware containing in¬ 
fectious material should first be autoclaved for one hour, 
then emptied, and boiled for one hour in soap suds, then 
cleaned with a brush, and sterilized for one hour in the 
hot-air sterilizer at 150° C. If it is very dirty, it may, 
after it has been autoclaved, be immersed for twelve to 
twenty-four hours in a mixture of three parts of saturated 
aqueous solution of potassium bichromate and one part 
of sulphuric acid. 
The usual glassware for culture media consists of test 
tubes. Erlenmeyer and Florence flasks and Petri dishes, 
which are shown in accompanying illustrations, should 
all be plugged with nonabsorbent cotton; the best and 
easiest way to plug the test tubes, is to take a piece of 
cotton, two inches square, put over the mouth of the 
test tube and push it in for a distance of one inch with 
a pencil or a glass rod. 
The Composition of Culture Media. —Most culture 
media contain meat, agar, peptone, salt, and water. 
