BACTERIOLOGICAL DIAGNOSIS OF PLAGUE 15 
broth. If the flask is gently shaken, the stalactites fail down to the bottom like a fall of snow ; 
after a time the broth becomes clear again, and in a day or two another lot of stalactites will be 
found hanging from the surface. 
It is most important to keep the flasks still, for the slightest movement or vibration is 
suflScient to cause the stalactites to drop. A shelf is most convenient to rest the flasks on, because 
the stalactites are best seen by placing a small candle-end lighted behind tlie flask and looking 
through it. If an incubator is used, the flask should be placed on the front of the shelf so as to 
allow room for the light behind, and the door should be opened very gently ; an incubator would 
have to be kept specially for the purpose. The film of oil probably acts merely as a pohit (Fappu'i ; 
it is by no means necessary, but it hastens the formation of stalactites. 
In making broth cultivations the writer always inseminated the flasks by means of a thin 
drawn-out glass rod long enough to project above the mouth of tlie flask ; one end is held in the 
fingers, and the rod passed seven times slowly through the flame. The other sterilized end is 
drawn over the culture, tlie rod is then passed into the flask between the cotton wool plug and the 
glass till the end touches the bottom, the unsterilized part of the rod projecting above the plug. If 
the flask is observed day by day another characteristic phenomenon may be seen : the microbe 
growth creeps up the glass rod till it reaches the surface, and then gradually spreads out till tlie 
surface of the broth is covered with numbers of small round colonies, which grow down into the 
substance of the broth as ' stalactites.' A few whitish colonies may be seen grov/ing on the 
bottom of the flask and up the sides ; the broth itself remains perfectly clear. 
This curious form of growth on the surface of the glass seems to be characteristic of the 
plague microbe ; it proves a useful test, because some idea can be formed before the ' stalactites ' 
have had time to grow. If the microbe is found to be growing in this way, and if the broth itself 
remains clear, so far as the writer's experience goes it has always tin-ned out to be plague, and 
shows a ' stalactitic ' growth in a few days more. No fat or oil was used in these cultivations. 
Microscopic Examination 
The plague microbe stains readily with aniline dyes, but not by Gram's method. The 
bipolar staining in stained specimens, when the ends are found to be deeper stained than the centre, 
is by no means a constant phenomenon and cannot be relied on for diagnosis. Mr. Watkins- 
PiTCHFORD considers it due to a greater collection or condensation of the protoplasm in tliese 
regions ; he notes that tlie spots are readily visible in the living and unstained specimens, so they 
cannot be due to staining. The detection of the spots he considers to be a most valuable aid to 
the rapid identification of the germ. Dr. E. L. Marsh drew my attention to the fact that plague 
microbes suspended in water dry as a bluish film on a microscopic slide ; if he does not get this he 
considers it unnecessary to go on with the staining. 
The bacillus is polymorphic ; it is sometimes seen as a very short bacillus, almost like a 
coccus or diplococcus ; at other times as a short stumpy bacillus with rounded ends. Very often it 
is found in pairs as a diplobacillus, and sometimes in distinct short chains. I have found this latter 
form chiefly in broth, especially in the unneutralized broth. 
