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CHAPTER III.
Part II.-The Bacteriology of Plague.
Plague is an acute disease caused by a bacillus belonging to a class of specifically
pathogenic micro-organisms, whose morphological and other characters show considerable
resemblance. Such organisms are those of swine-fever, chicken-cholera, &c., (Klein).* To a
Japanese investigator-Kitasato-belongs the honour of having first identified the organism of
plague. This he did in 1894, when plague was raging in Hongkong, and his results have
since been verified and amplified by numerous observers throughout the world, but more
particularly in India, which has now been for 3 years the most important focus of the disease
THE BACILLUS OF PLAGUE-'BACILLUS PESTIS ORIENTALIS' (Kitasato, Yersin).
Morphology.
The plague-bacillus, as it appears in microscopical specimens from the blood and
tissues of animals, is a short rod with rounded ends, 0·8µ. to
1·6µ. in length, and less than half that in breadth. It is
sporeless, and non-motile in the ordinary sense of the word.†
The bacillus is stained readily by the ordinary basic dyes. A weak solution of
Carbol-Fuchsin or of Aniline-Gentian-Violet applied for 1-2 minutes will be found satis-
factory for ordinary purposes. On examination, such stained specimens taken from tissues
show an intensification of colour at the poles (vide Plate I.) which is very characteristic
and careful focussing discloses also an appearance like a capsule. But it is doubtful if this
capsule is a true one.
The bacillus does not retain the stain by Grams' method.
In ordinary bubonic plague in man the micro-organism is found in the bubo. Just
before death it is often found in the blood, but in septicćmic cases it appears in that fluid
in the early stages of the disease. It is noteworthy that the presence of the bacillus in.
the blood does not necessarily indicate a fatal attack. In pneumonic cases plague-bacilli
may be demonstrated in the sputum,
In post-mortems of ordinary bubonic plague cases the bacilli are found not only in
the bubo, but in great abundance in the blood and in the liver and spleen. Sometime these
organs may be swarming with bacilli, while the blood is almost free. This occurs more
frequently in rats than in man. Dr. Bitter says there are relatively more bacilli in the
spleen than in the other organs or tissues.
Bacilli grown artificially on solid media show in young cultures, after staining,
appearances similar to those found in the tissues, except that they take the dye
uniformly and seldom show the intensification of the stain at the poles mentioned above.
Elougated forms are also to be noted, and are very characteristic. (Vide Plate II.)
If an agar culture be examined daily for some ten days, gradual changes inform and
size and in effects of staining will be observed. The bacilli become swollen, assuming,
after the first 2 or 3 days, lemon and pear shapes, and later exhibiting polymorphic
* Metschnikoff, however, classifies the plague-bacillus, in a group of cocco-bacilli, along with bacillus typh-
abdominalis, bacillus coli, etc., all characterised by the facility with which, at some period of their existence, they
can be stained to show the bi-polar appearance.
† Mr. Morvyn-Gordon has stained flagella in specimens from an Agar-culture grown for 24 hours at 37° C.