BACILLUS PESTIS 375 



usually exhibit a certain degree of immunity from subsequent 

 infection. 



Nuttall^ notes that feeding-experiments have resulted 

 in fatal infection in gray and white rats, house- and field- 

 mice, guinea-pigs, rabbits, hogs, apes, cats, chickens, sparrows, 

 and flies. He also calls attention to the fact that flies may 

 live for several days after being infected with this organism, 

 and if at liberty to fly about may infect persons or foodstuffs 

 on which they alight or faU. 



Investigations in India demonstrate that the flea is the 

 most common and important of the agents of transmission, 

 carrying the disease from man to animals (rodents, rats in 

 particular) and from animals to man. 



The bacilli apparently lose their virulence after long-con- 

 tinued cultivation under artificial conditions, and it is said 

 that from slowly developing, chronic buboes non-virulent 

 or feebly virulent cultures are often obtained. Variations 

 in the degree of virulence have been observed in different 

 colonies from the same source. Virulence is said by Yersin, 

 Calmette, and Borrel^ to be accentuated by passing the 

 organism through a series of susceptible animals. 



It has been observed that in the suppurating lymphatic 

 glands of man a variety of organisms may be present, but 

 among them are always the plague bacilli. Occasionally 

 micrococci predominate. In these cases of mixed infection 

 the pest bacilli are said to stain less intensely with alkaline 

 methylene-blue than do the streptococci, and more intensely 

 than do the micrococci that are present. Also, in this event, 

 the streptococci retain the Gram stain, while the pest bacilli 

 do not and the -staphylococci may or may not. It has been 



1 Loo cit. 2 Annales de I'lnstitut Pasteur, 1895, p. 589. 



