BACILLUS OF TUBERCULOSIS. 287 



length of time daring which the tubercle bacillus re- 

 tains its virulence, and whether there are any naturally 

 attenuated varieties. According to experimental in- 

 vestigations, the virulence of dried tubercular sputum 

 is not suddenly but gradually lost, a certain pro- 

 portion of it retaining its specific infective power 

 under ordinary conditions, as in a dwelling-room, for 

 at least two or three months. An instance is reported 

 by Ducor (Paris, 1890) of a healthy family having 

 been infected with tuberculosis from living in a room 

 which had been occupied by a consumptive two years 

 before, and on examining the sputum-stained wall-paper 

 not only were tubercle bacilli found in it, but upon being 

 inoculated into guinea-pigs they died of tuberculosis. 



Attenuation. Metschnikoff states that when kept 

 at a temperature of 42 C. for some time the tubercle 

 bacillus undergoes a notable diminution in its patho- 

 genic power, and that when kept at a temperature of 

 43 to 44 C. it after a time only induces a local abscess 

 when injected subcutaneously into guinea-pigs. The 

 experiments of Lote also indicate that an attenuation 

 of virulence has occurred in the cultures preserved in 

 Koch's laboratory, originating in 1882 from the lungs 

 of a tuberculous ape. A culture of ours which we ob- 

 tained from Trudeau, and which has grown now either 

 at Saranac or in our laboratory for six years, is no 

 longer capable of causing tuberculosis in guinea-pigs, 

 although originally virulent. 



Mixed Infection. Some time ago attention was drawn 

 to the fact that tuberculosis, whether of the lungs, 

 lymphatics, or cold abscesses, was often a mixed in- 

 fection. The other micro-organisms with which the 

 tubercle bacillus is most commonly associated are the 



