392 THE ETIOLOGY OF TROPICAL DISEASES 



contagion." * It should be added that Doriga's views are not universally 

 held, and were not fuUy accepted by the Indian Plague Commission. 



Quite recently, attempts have been made in Paris, with the con- 

 sent of the Prefect of the Seine, to exterminate rats wholesale, in 

 order to protect the city from an epidemic of plague.f 



The Bacteriology of Plague is one of the latest additions to the 

 science. During the Hong Kong epidemic in 1894, Kitasato, of 

 Tokio, demonstrated the cause of plague to be a bacillus. This was 

 immediately confirmed by Yersin, and further proved by the isola- 

 tion in artificial media of a pure culture of a bacillus able, by means 

 of inoculation, to produce the specific disease of bubonic plague. 



The bacillus was first detected in the blood of patients suffering 

 from the disease. It takes the form of a small, round -ended, oval 

 cell (0"7 fjL broad by 1"5 ij. long), with marked polar staining, and 

 hence having an appearance not unlike a diplococcus. In the middle 

 there is a clear interspace, and the whole is surrounded with a thick 

 capsule, stained only with difficulty. The organisms are often 

 linked together in pairs or even chains (especially in fluid cultures), 

 and exhibit polymorphic forms. In culture the bacillus may be 

 coccal or bacillary in form. Involution forms occur in old cultures, 

 and also, more rapidly, when 2-5 per cent, of sodium chloride is 

 added to the medium. On such salt-agar the involution forms are 

 very marked. The bacillus is non-motile (Plate 30, p. 398). 



The plague bacillus grows readily on the ordinary media at blood- 

 heat, producing smooth, shining, circular cream-coloured colonies, 

 with a wavy outline, which eventually coalesce to form a greyish 

 film. The colonies slip about on the agar when touched with the 

 platinum wire. If melted butter (or ghee) or oil be added to 

 bouillon, this bacillus grows in " stalactite " form, that is, the growth 

 starts on the under surface of the fat globules, and extends down- 

 wards in the form of pendulous string-like masses which readily 

 break off if the tube is slightly shaken. The following negative 

 characters help to distinguish the bacillus : There is no growth on 

 potato; milk is not coagulated; gelatine is not liquefied; Gram's 

 method does not stain the bacillus ; and there are no spores. The 

 bacillus is readily killed by heat or by desiccation over sulphuric 

 acid at 30° C. Both in cultures and outside the body the bacillus 

 loses virulence. To this may be attributed possibly the variety of 

 forms of the plague bacillus which differ in virulence. But it has 

 great powers of resistance against cold.| 



On gaining entrance to the human body the bacillus affects in 



* For a general discussion of the subject of plague in the lower animals, see Brit. 

 Med. .Tour., 1900, i. pp. 1141 and 1216. 



i" Brit. Med. Jour., 1900, vol, i., p. 722. 



X For further particulars as to cultured characters of B. pestis, see Brit. Med Jour, , 

 1902, ii., 956, 



