BACTERIOLOGY OF PLAGUE.]
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If the flask containing the stalactite formation be gently moved, the appearance of a snow-
fall is produced—each stalactite apparently rolling itself into a small mass and falling to the
bottom (vide Plate VII.)—the supernatant fluid becoming again, perfectly clear. Then a fresh
surface growth with stalactites shorter than the first is produced—again to fall in snow-
like flakes on agitation. A series of crops of stalactites may thus be observed in a culture
flask until the nutritive material is exhausted.
As mentioned above, in bouillon cultures, inseminated from animal bodies direct, or from
even fresh cultures from animals, it sometimes happens that stalactites do not readily appear :
the persistency of the zoogleic stage in such cases being remarkable. Gently agitating the
flask will have the effect in these cases of causing the stalactite growth to appear after a lapse
of from 24 to 36 hours. It must be noted that the stalactites thus produced from a recent
animal culture are finer and scantier as a rule than those which are the result of inoculating
a flask with an old and artificial growth. Very old cultures, re-transplanted, often, however,
show very ragged stalactites.
Growth in milk.
In milk the growth is not distinctive. Klein says that
after a month's incubation at 37° C. in litmus milk, curdling
and a change of reaction will occur.*
Growth on Agar.
The growth on Agar—(Peptone-Salt Agar )—is characteristic, provided the culture mate-
rial has been evenly spread over the surface of the slant, and
also that the Agar is "dry." If these conditions are fulfilled,
in 24 hours at the temperature of the air in Bombay ( 25·5—29·8°G. ), minute grey
translucent colonies cover the whole surface of the Agar—the appearance, when held
up to the light, and looked at from below, being similar to that of fine ground glass. With
age the culture deepens lightly and reaches a maximum in about 5—7 days. Here and there
more opaque colonies will often be noticed, and these eventually become very prominent
and of a faintly yellowish-brown colour. These are " Giant Colonies, " and they often
appear to contain more elongated forms than the rest of the culture. (Vide Plate VIII.)
These were originally termed "Cannibal Colonies" by Mr. Haffkine.
Isolated colonies on the surface of Agar, examined under a magnification of 75 diameters
with transmitted light, show after 24 — 36 hours' growth a raised angularly-marked centre,
with a faint marginal layer which has an uneven border. In 3 to 4 days this centre
thickens so as to become opaque and brownish in colour. It then forms a regular oval or
circular shape, surrounded by the fiat uneven clear margin, which is now plainly defined
and sometimes shows radiate markings. Occasionally such colonies go on growing until
they become 5—7cm. in breadth, and to the naked eye appear to exhibit raised concentric
rings around the nucleus-like centre. (Vide Plates IX and X.)
Growth on gelatine.
The appearances produced by the growth of the plague-bacillus on gelatine are some-
what similar to those already described on Agar-agar. Klein*
states that on gelatine-plates grey punctiform translucent
colonies occur after 24 hours' incubation at 20—21° C. They are visible to the
naked eye. Later (3—7 days) the superficial colonies become larger greyish-white
patches distinctly raised in the centre. Their margins are distinctly angular or crenate,
and, according to McFarland,† they often show a fine semi-transparent zone around
the central part, which appearance has already been noted with regard to the growth of
* 26th Annual Report of Medical Officer to Looal Government Board—1896-97,
† Pathogenic Bacteria, J. McFarland, Henry Khvipi.au, 1898.
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