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Formalin is then, in the form and the amounts used in our experiment, a perfect disinfectant
of bacillus pyocyaneus, on all the materials used.
4.—Vibrio of Asiatic cholera.
Experiment A.—This, as also the following experiment, B., were made with the vibrio
which has now been kept for many subcultures in the laboratory. It was originally
derived from a case of true Asiatic cholera, and in morphological and cultural respects possesses
all the characters of the typical cholera vibrio; except that its virulence on the guinea pig has
suffered slight diminution. The emulsion was made by distributing in separated milk a
relatively large amount of agar surface culture that had been incubated for 24 hours at 37° C.
The culture was then liberally applied to wood, cloth, linen and paper, and was allowed to dry
on these materials in the air of the laboratory, after which they were exposed to the action of
formalin as in the previous experiments. Next day the materials were transferred to peptone
salt solution, as mentioned on a former page, and incubated for 24 hours at 37° C. If the
peptone solution became turbid, microscopic specimens—stained films—alone could be relied
upon as showing the presence of the vibrio; if a positive result was hereby obtained the cholera
red reaction was tested in order to confirm the previous results.
Experiment B.—In this experiment the culture of the vibrio that was used was gelatine
(melted) culture incubated for 24 hours at 37° C.
The result of this, as also of the previous experiment, was entirely negative, that is to
say, the culture in peptone salt made with the materials after exposure to the formalin yielded
no growth of the cholera vibrio.

In order to make sure that the keeping per se of the vibrio on the materials for 24 hours,
apart from any exposure to a disinfectant, has no deleterious effect on the vitality of the vibrio,
a control experiment was made. In this the vibrio, grown in hot gelatine (i.e., at 37° C.), was
applied to the same paper as used in all these experiments and kept for 24 hours. From the nature
of the paper being denser than any of the other materials, the microbe would on it (paper) remain
more superficial, i.e., more exposed to the drying process than on any of other materials, and therefore
we would get a reliable answer to the above question, viz., whether or no the 24 hours' drying
in the air of the laboratory did or did not injure the vitality of the microbe'.
The result of this control experiment was that the control cholera-paper yielded, in peptone
salt culture, copious growth of the vibrio after 24 hours' incubation.
5.—Staphylococcus aureus.
The experiments that were made with this microbe were of exactly the same kind as
with the above, and it is not necessary to describe them in detail. There is only one point
that it is necessary to mention, it is this:—After exposure to formalin of the materials, wood,
cloth, linen and paper impregnated with the emulsion—broth emulsion of a recent (24 hours'
old) agar culture—they were placed in broth, and after 24 hours' incubation of this at 37° C.,
agar surface cultures were made; these would, after incubation for 24 hours at 37° C., infallibly
show the golden yellow colonies of the microbe, if such were present, in a living condition. The
experiments made showed that the formalin as used had completely killed the microbe.