red in subculture.

_d._ Place the stock dishes in the ice chest to await the results of
incubation. (If any of the subcultures fail, further material can be
obtained from the corresponding emulsion; or if it has dried, by
moistening it with a further drop of sterile distilled water.)

_e._ Incubate all the subcultures and identify the organisms picked out.

4. Differential Media.--

(a) _Selective._--Some varieties of media are specially suitable for
certain species of bacteria and enable them to overgrow and finally
choke out other varieties; e. g., wort is the most suitable
medium-base for the growth of torulae and yeasts and should be employed
when pouring plates for the isolation of these organisms. To obtain a
pure cultivation of yeast from a mixture containing bacteria as well, it
is often sufficient to inoculate wort from the mixture and incubate at
37 deg. C. for twenty-four hours. Plant a fresh tube of wort from the
resulting growth and incubate. Repeat the process once more, and from
the growth in this third tube plant a streak on wort gelatine, and
incubate at 20 deg. C. The resulting growth will almost certainly be
a pure culture of the yeast.

(b) _Deterrent._--The converse of the above also obtains. Certain
media possess the power of inhibiting the growth of a greater or less
number of species. For instance, media containing carbolic acid to the
amount of 1 per cent. will inhibit the growth of practically everything
but the Bacillus coli communis.

~5. Differential Incubation.~--

In isolating certain bacteria, advantage is taken of the fact that
different species vary in their optimum temperature. A mixture
containing the Bacillus typhosus and the Bacillus aquatilis sulcatus,
for example, may be planted on two slanted agar tubes, the one incubated
at 40 deg. C., and the other at 12 deg. C. After twenty-four hours'
incubation the first will show a pure cultivation of the Bacillus
typhosus, whilst the second will be an almost pure culture of the
Bacillus aquatilis.

6. Differential Sterilisation.--

(a) _Non-sporing Bacteria._--Similarly, advantage may be taken of the
varying thermal death-points of bacteria. From a mixture of two
organisms whose thermal death-points differ by, say, 4 deg. C.--e. g.,
Bacillus pyocyaneus, thermal death-point 55 deg. C., and Bacillus
mesentericus vulgatus, thermal death-point 60 deg. C.--a pure cultivation
of the latter may be obtained by heating the mixture