39 



lution" fonns. These involution forms must not be confused with 

 the so-called "involution" (coccoid) forms of the organism found in 

 smears from animal tissues. 



We think it worthwhile to call special attention to the great diag- 

 nostic value of involution forms developed when Bacillus pestis 

 is grown on salt agar. No other organism that we have had the 

 opportunity of working with gives forms that are at all likely to be 

 mistaken for those of Bacillus pestis, except B. mallei, and of course 

 the other points of difference would at once serve to distinguish the 

 latter organism. 



B. pestis is Gram negative, though this point is of no great value 

 except to distinguish the "coccoid'' forms from pus cocci. 



The appearance and character of the culture should be as follows: 



Agar. — Smooth, glistening, round whitish colonies which are 

 found to be sticky when touched with an inoculating needle. 



BrotJi. — ^A scanty surface growth which falls, often in globular 

 masses, when the tube is gently agitated; and a fine flocculent pre- 

 cipitate. 



Litmus milk. — Generally rendered slightly acid. 



Glucose broth. — Eendered slightly acid. Gas is not formed. 



Lactose hroth. — ^Unchanged in reaction. Gas is not formed. 



The other cultural reactions are of no material assistance in the 

 identification of the organism. Indeed, in routine work the appear- 

 ance of the growth on agar and in broth, together with the involution 

 forms on salt agar, are sufficient for identifying the organism. 



The plague bacillus is a nonmotile organism, a point worth bearing 

 in mind. 



A culture answering the above description when rubbed into the 

 shaven skin of a guinea pig or a white rat should cause the death of 

 either of these animals of plague within ten days, and an organism 

 must be isolated from their tissues after death corresponding to the 

 one inoculated. 



If one wishes to be doubly certain, one may inoculate a series of 

 laboratory animals, giving to half of them a sufficient dose of anti- 

 pest serum. The protected animals should recover, or markedly 

 outlive the controls, which should die in the usual time. 



As to the virulence of cultures of the bacillus from cases of rat 

 plague Klein (17) states "that B. pestis bred in the rat is of decidedly 

 less virulence than that bred in the human subject; moreover, the 

 former is liable, outside the animal body, to a much greater extent 

 to rapidly lose its virulence." It is evident that in any given epi- 

 demic it will be very diSicult to say just which strain, rat or human, 

 one is dealing with. 



In the case of the strains of B. pestis recovered from rats in San 

 Francisco we have seen nothing to justify such an opinion as Klein 



