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Scientific Memoirs by
In all other cultivations of boiled milk periods of from 18 to 24 hours elapsed
between the boiling of the milk and the establishment of the cultivation, but in
this series the first cultivation was set within 10 minutes from the boiling, and,
although the amount inoculated and the subsequent treatment were identical
with those which in other cases were invariably followed by an enormous crop of
Bacillus subtilis, the cultivation in this case proved absolutely sterile. This,
however, was not the case with a similar cultivation set 24 hours later, for here the
usual result followed and the plate became occupied by innumerable discrete
colonies and an abundant, diffuse, superficial growth of Bacillus subtilis. It would
appear, thus, that only a relatively small number of the spores of Bacillus subtilis
are normally present in ordinary samples of milk, and that these are incapable
of successfully holding their own in the struggle for existence with the other
schizomycete organisms with which they are normally associated, but that, if
relieved from the struggle by the destruction of their rivals by means of boiling,
they find the medium well adapted to their development and multiplication. The
sample of boiled milk from which the above results were obtained also afforded
an excellent demonstration of the series of changes which occur in specimens of
boiled but not sterilised milk—in other words, in specimens in which the organisms
whose multiplication is ordinarily associated with the acid fermentation and com
-
mon coagulation of the fluid have been destroyed, but in which rapid multipli
-cation of Bacillus subtilis goes on. After a lapse of 24 hours from the period of
boiling, the milk remained perfectly fluid, but showed a slight change in reaction,
its previous neutrality having been replaced by feeble acidity. Forty-eight
hours after the commencement of the experiment the reaction was neutral at
first, becoming faintly alkaline on drying, and coagulation and partial separation
had already occurred. The coagulum was of the peculiar, finely pulverulent
character normal to specimens of boiled milk. On the following day separation
of coagulum and fluid had advanced considerably and the reaction was now
very faintly acid. The alternation of feebly developed acidity and alkalinity
which occurred in this case is a phenomenon which frequently manifests itself in
connection with samples of milk which are undergoing change in connection with
the growth and multiplication of Bacillus subtills within them.
The variations which presented themselves in regard to the persistence of
Bacillus subtilis, in specimens of milk which were repeatedly boiled, at first
sight appear very remarkable ; two successive boilings in some cases securing
sterilisation, as in specimens Nos. IV and V of the present table, whilst in others,
such as that of No. XIII, six successive boilings completely failed to get rid of
or diminish the numbers of Bacillus subtilis appearing in subsequent cultivations.
This holds good of ordinary brief exposure to the influence of boiling over an
open flame only, for . a single prolonged exposure of several hours' duration in a
boiling water-bath, although in all probability the actual temperature in such
circumstances is not so high as in the previous set of cases, appears invariably