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METHODS OF BACTERIOLOGICAL INVESTIGATION. 53

last part separate stocks of timothy and clover, growing singly,
or in pure cultures. To apply this method to the cultivation
of bacteria, let us suppose that we wish to obtain a pure
culture of the bacteria from the white deposit about the necks
of the teeth. We proceed in the following manner: Having
melted a tube of nutritive agar-
agar, we carefully remove the Fig. 15.
cotton stopper, hold the mouth
of the tube for a moment in the
flame of a Bunsen burner, to
destroy all germs sticking to it,
then pour the material upon a
sterilized glass plate, using the
apparatus Xo. 11. As soon as
this has stiffened, which takes
place in about one or two min-
utes, we take up some of the de-
posit upon the point of a ster-
ilized platinum needle, draw it
lightl}^ over the surface of the
culture medium ten or fifteen
times, and place the plate in a
damp chamber at the tempera-
ture of the human body. In
the course of a day or two
LixE Culture ox Xutritivk
w^e will invariably find that
Agar-Agar.
an abundant growth has taken The tiumber of colonies in each line
gradually decreases from a to h, until in
place along the lines. On the
the last line we have only twelve to fifteen
first line it will be very thick, colonies in which at least three diflferent
mlcro-orsanisms are represented, the col-
and on each succeeding line it
onies c, (/, and e plainly dittering from each
will gradually become thinner, other to the naked eye- The first colony
in the third line from a is a mould. Nat-
till on the last line the growth
ural size.
will appear only as a few iso-
lated points (Fig. 15). These points usually represent pure
cultures. If they are not pure, they can be made so by vacci-
nating from them a second plate in the manner just indicated.
A portion of a culture made in this manner is represented in
Fig. 15. In making line-cultures on nutritive gelatine we pro-