equivalent, so far as the contained organisms are
concerned, to 200 c.c. of the original water. (Some bacteria will of
course be left behind on the walls of the filter and in its pores.)

Up to this point the method is identical, irrespective of the particular
organism whose presence it is desired to demonstrate; but from this
point onward the methods must be specially adapted to the isolation of
definite groups of organisms or of individual bacteria.

The Coli-Typhoid Group.--

1. Number nine tubes of bile salt broth (_vide_ page 180), consecutively
from 1 to 9.

2. To No 1 add 1 c.c. } of the original water sample
2 add 2 c.c. } before the nitration is commenced.
3 add 5 c.c. }

3. To the remaining tubes of bile salt broth add varying quantities of
the suspension by means of suitably graduated sterile pipettes, as
follows:

No. 4 0.05 c.c. (equivalent to 10 c.c. of the original water sample).
No. 5 0.125 c.c. (equivalent to 25 c.c. of the original water sample).
No. 6 0.25 c.c. (equivalent to 50 c.c. of the original water sample).
No. 7 0.5 c.c. (equivalent to 100 c.c. of the original water sample).
No. 8 1.0 c.c. (equivalent to 200 c.c. of the original water sample).
No. 9 2.5 c.c. (equivalent to 500 c.c. of the original water sample).

4. Put up each tube anaerobically in a Buchner's tube and incubate at
42 deg. C.

5. The subsequent steps are identical with those described under the
Enrichment method (see page 428 to 431; Steps 8 to 18).

_Alternative Methods._--

A few of the older methods for the isolation of the members
of the coli-typhoid groups are referred to but they are
distinctly inferior to those already described.

(A) The Carbolic Method:

1. Take ten tubes of carbolised bouillon (_vide_ page 202)
and number them consecutively from 1 to 10.

2. Inoculate each tube with a different amount of the water
sample or suspension, as in the previous method.

3. Incubate aerobically at 37 deg. C.

4. Examine the culture tubes after twenty-four hours'
incubation.

5. From those tubes which shows signs of growth, pour plates
in the usual manner, using carbolised gelatine (_vide_ page
202) in place of the ordinary gelatine, and incubate at 20 deg.
C. for three, four, or five days as may be necessary.

6. Subcultivate from any colonies that make their
appearance, a