﻿STUDIES IN PLAGUE IMMUNITY. 309 



ARTIFICIAL ATTENUATION OF THE PLAGUE BACILLUS. 



In s"pite of the facts which have been brought out in the previous dis- 

 cussion, nevertheless it is true that the virulence of different pest strains 

 as they occur in nature varies greatly, the numerous cultures isolated from 

 different epidemics and different cases of human plague frequently exhibit- 

 ing marked variation in pathogenicity in animal experiments. It is also 

 true that there are a number of references in the literature which point to 

 the fact that certain strains of pest bacilli, after growing for a few genera- 

 tions on artificial culture media, unaccountably lose their virulence. It 

 therefore is not my purpose to argue that under certain conditions the viru- 

 lence of the pest bacillus may not vary from time to time and that it may 

 not frequently, under some conditions in nature, become attenuated and 

 under others, such as its passage from animal to animal, even be increased. 

 However, any such change as the latter is very unusual and it does 

 not occur under ordinary circumstances. The important point which I 

 wish to emphasize in the behavior of this organism in its passage from 

 animal to animal is its stability of virulence, and this quality is also 

 possessed to a remarkable degree by the organism on artificial culture 

 media. 



Virulent strains of the pest bacillus when grown upon artificial media 

 usually suffer no change in their pathogenicity even after long periods of 

 time and after numerous generations. Cultures sealed in test tubes and 

 protected from light and heat have been found to retain their full 

 virulence for several years. Thus Maassen found no change of virulence 

 in cultures after two years and jST. K. Schultze 110 observed that cultures 

 in bouillon were fully virulent even after four years. Attempts to 

 attenuate the virulent organism in cultures have also demonstrated its 

 stability in pathogenicity and have usually resulted unsuccessfully. 



Albrecht and Gohn 111 reported that the pest bacillus could resist a continued 

 temperature of 36° C. for only about fourteen days without loss of virulence, 

 and that after this period a rapid diminution occurred. However, these results 

 have not been confirmed by any other observers. The German Plague Com- 

 mission " 2 suspended agar cultures of the organism in bouillon and exposed them 

 to a temperature of 51° C. for varying periods of time. They found that while 

 many of the organisms were killed by this treatment, those which remained 

 alive had retained their full virulence. They also attempted to secure attenua- 

 tion of the organism by exposing it to carbolic acid, but these experiments were 

 also unsuccessful. 



Kolle and Otto" 3 found that by growing the organism at a continuous tem- 

 perature of 40° to 41° C. a further attenuation was sometimes possible in 



™GentrU. f. Balderiol. (1001), 29, AM. I, 100. 



111 Loo. tit. 



112 hoc. tit. 



m Deutsche mod. Wchnschr. (1003), 29, 403. 



