122 [CHAP. III., PT. II.
isolated colonies on agar-agar. The deeper colonies are, as in agar, small and spherical,
white in reflected and brown in transmitted light.
Gelatine-streak cultures, according to Klein, are not unlike those of diphtheria, consist-
ing of a whitish band, increasing in thickness with an irregular-knobbed margin.
As in Agar, the stab-culture growth in gelatine is not remarkable to any degree. No
liquefaction of the gelatine occurs, nor does this phenomenon occur in any other solid media.
On blood serum a rich faintly yellowish-grey moist scum is developed.
On potato and banana the plague-bacillus grows very scantily, and the appearances
produced are not noteworthy.
Growth in water.
According to the German Plague Commission, in ordinary tap water, after five days
the virulence of the microbe is lost, and, similarly, in sterile
tap water after ten days.
Effects of disinfectants on the
plague-bacillus.
By drying, exposure to direct sunlight or by heating at 60° C. for 5 minutes (Klein)
the vitality of a plague culture is destroyed. Live plague-
microbes, spread as a thin layer on a slip of glass, and exposed
directly to the sun's light, are killed within an hour. With a thicker layer the bacilli
remain alive and virulent for about two hours, but not longer than four hours. Cultures
growing on Agar live for several hours, but a day's exposure kills them outright.
In the words of the German Plague Commission (loc. cit. p. 281):-"The bacilli are
very little resistant to external influences, e. g., drying, disinfectants, sunlight and ordinary
water; and as no spores are found, their propagation through the dust of the air, especially
in tropical countries, where they are quickly killed by sunlight and drying, certainly does
not usually occur; only dried organisms being capable of dissemination by the atmos-
phere. But, as the conditions for the inimical action of such physical agencies as sunlight,
drying, etc., are not always so favourable under natural conditions as in actual experiments,
resort to artificial disinfectants is made necessary in practice."
The effects of various antiseptics and disinfectants on plague cultures have been
thoroughly studied by Messrs. Hankin, Pitchford and Marsh and the German
Commission. The former found that Corrosive Sublimate is the most reliable for general
use, and that a solution of 1-5000 was sufficient to destroy the vitality of the plague-
bacillus in 5 minutes. 1-1000 is stated by the German Commission to destroy a culture
at once. To Carbolic Acid and its allies and alkalies the bacillus was more resistant
than to acids and various oxidising agents, particularly Chloride of Lime and Permanganate
of Potash.
The Incidence of Bubonic
Plague in Animals.
Animals of several species whose habit of life brings them into contact with human
habitations, are naturally more susceptible to plague than
others. Such are rats, monkeys, and squirrels.* When plague
was at its height in Bombay, guinea-pigs and rabbits in confinement were attacked by the
disease in an epidemic form, and there is little doubt but that rats were instrumental in
originating the outbreak, Rats are particularly affected by the disease, and they are, in all
probability, the chief disseminators of infection. The ordinary brown rat of Bombay is
the most liable to be attacked, while the musk rat appears to be immune. Mice, too, are
less susceptible than rats.
* Corthorn and Milne in the Indian Medical Gazette for May 1899.