52
plague in most respects, but which differed from it in the rapidity with which it
formed involution forms on agaragar. Mr. Hankin found that the plague mic-
robe could be changed into a microbe having these aberrant characters by culti-
vation on agaragar containing three or four per cent. of salt, and that further
the microbe from Siwri was pathogenic to a rabbit that had not been protected
by Haffkine's plague vaccin, but had no effect on a rabbit that had been so pro-
tected. Consequently it was supposed that this microbe was the plague microbe
altered by its sojourn in salt water. This observation has led to the discovery
of a rapid method of identifying the plague microbe by its cultivation on agar-
agar containing a certain percentage of salt. It has been further worked out
by Surgeon-Captains James and Thomson under Mr. Hankin's supervision.
Mr. Hankin has investigated the relation of insects to the dissemination
of the disease. He has found the microbe in bugs from the infectious Diseases
Hospital. He has also found it in ants that had recently been eating rats dead
of plague. A long series of experiments showed that ants do not retain the
plague infection long, and further that no trace of infection is present in ants
from localities in which rats have not been dying recently. Hence these
insects cannot be regarded as important agents for the spread of the disease.
Experiments were carried out on different kinds of grain and other articles
of export. They were infected with plague microbes under many different
conditions. At stated intervals extracts of specimens of the infected grain were
prepared and injected into mice. By this means it was found that the infec-
tion usually died out within 4 to 6 days. The possibility was indicated that it
might in exceptional cases survive for so long as 13 days, but of this no definite
proof was obtained. Mr. Hankin points out that grain, when it becomes rotten,
usually acquires a well marked acid reaction, owing to the production of various
fatty acids from the decomposition of carbohydrates. He has found that such
fatty acids act as disinfectants for the bubonic microbe. Hence it is difficult to
believe that the microbe can exist in rotten grain, as has been frequently
assumed. The matter was put to an experimental test by Dr. Srinivasa Rao
while working in Dr. Hankin's laboratory. It was found that the microbe
always dies out in rotten grain within 2 to 24 hours.
The microbe, though usually dying out, when placed in sterilised water,
within three days, appears to be able to survive in sterilised cowdung for several
days. The observation appears to have some bearing on the question of the
persistence of the infection in houses having cowdung and mud floors. Through
lack of time these experiments could not be continued sufficiently for any
conclusion to be arrived at as to whether any particular kind of filth is more
suitable than another to sustain the life of the microbe. But the general trend
of Mr. Hankin's observations is to indicate that the chief source of infection is
not likely to be in a saprophytic form of the microbe in the outside world, but
more probably in the recently passed excreta of men or animals suffering from
the disease.