466 THE PLAGUE BACILLUS 



broth remained clear. " A highly characteristic appearance is obtained 

 when 1 c.c. of blood containing say 10 to 100 bacilli per c.c. is inoculated into 

 a 100 c.c. flask of neutral broth. The plasma forms a soft clot dispersed 

 throughout the broth and if the flask be kept undisturbed each bacillus 

 ultimately gives rise to a tack-like growth enclosed in a similarly shaped 



cavity." In some cases, with a virulent culture the broth 



was somewhat turbid. ] 



(b) Gelatin. The plague bacillus does not liquefy gelatin. 

 Isolated colonies appear in 2-4 days : they are rounded, 

 granular, and yellowish and are occasionally surrounded 

 by a transparent ring with irregular margins. In stab 

 culture a yellowish, semi-transparent growth forms on 

 the surface while a whitish streak marks the line of the 

 stab. 



(c) Agar. Glycerin-agar. Serum. When a scraping from 

 a bubo is sown on any of these media the plague bacillus 

 grows in the form of transparent white colonies, iridescent 

 at the margins when examined by reflected light. When 

 sub-cultured, a milky-white slimy layer forms in 24 hours. 



[The growth of the plague bacillus on agar is very like that of 

 the pasteurella group (Chap XXVIII.) and it is only by further 

 cultivation and inoculation experiments that it can with 

 certainty be distinguished from the latter bacillus.] 



(d) Milk. The growth is poor and the medium is not 

 coagulated. 



(e) Potato. The growth is slow and minimal in amount, 

 consisting of a whitish or yellowish streak. 



SECTION III. BIOLOGICAL PROPERTIES. 

 1. Viability and virulence. 



(2 a dS?). fr m a bUb In cul t' ure > the plague bacillus is a delicate organism. 

 A temperature of 58 C. for an hour or of 100 C. for a 

 minute is sufficient to sterilize the growth ; similarly, exposure to sunlight 

 for 3-4 hours and weak antiseptic solutions are equally bactericidal. 



In dried pus, the organism is more resistant and may retain both its vitality 

 and virulence for several weeks. 



In soil the bacillus remains alive for several months ; its virulence under 

 such conditions becomes lowered, but can be restored. According to Yokote, 

 putrefaction destroys the plague bacillus in dead bodies in 15-30 days 

 (p. 463). 



[The Advisory Committee showed that floors of cow-dung grossly con- 

 taminated with B. pestis remain infective for 48 hours while floors of chunam 

 (a mixture of sand and lime put down moist and allowed to set) do not 

 remain infective even for 24 hours (the infectivity in both cases was tested 

 by rubbing scrapings into susceptible animals).] 



The virulence of the organism diminishes rapidly in artificial culture. 

 Scrapings of buboes sown on agar yield colonies varying in virulence (Yersin) : 

 the larger are only slightly virulent and grow so much more rapidly than the 

 virulent colonies that the latter soon become crowded out with the result that 

 subsequent sub-cultures rapidly lose their virulence. (It has already been 

 pointed out that it is possible to raise the virulence of an attenuated bacillus.) 



[The virulence of Bacillus pestis for the rat is unaltered (neither increased 

 nor diminished) when passed through a large number of animals in succession. 



