108 THE BACTERIOPHAGE 



species which are not homogeneous as regards the bacteriophage. 

 Typhoid bacilli may acquire, in vivo or in vitro, a resistance to 

 the action of the bacteriophage, that is, they may possess an 

 acquired immunity (this must not be confused with natural im- 

 munity) and they become inagglutinable as well as possessed of 

 an enhanced virulence for experimental animals. As has been 

 shown for B. dysenteriae, repeated culturing on agar progressively 

 lowers the resistance to the bacteriophage, and coincidently 

 restores agglutinability. 



B. paratyphosus A 



A bacteriophage showing virulence for this bacillus is relatively 

 frequent in the normal intestine. As regards the action of the 

 bacteriophage, B. paratyphosus A strains form a more homogeneous 

 species than do the typhoid bacilli. Just as with B. typhosus, 

 the paratyphoid A organisms may acquire a resistance to the 

 bacteriophage, may become inagglutinable, and may show an 

 increased virulence. 



B. paratyphosus B 



A bacteriophage for this organism is very frequent in normal 

 stools. The resistance of B. paratyphosus B places this bacillus 

 intermediary between B. typhosus and B. paratyphosus A. Re- 

 sistant bacteria are inagglutinable and are of high virulence. 

 Mixed colonies on agar, in which the bacterium has acquired a 

 high resistance for the bacteriophage, present a viscid appearance 

 resembling B. Friedldnder. 



Salmonella (hog cholera) 



One strain of bacteriophage active for this organism has been 

 isolated from a normal man. When derived from the body it 

 possessed an average virulence. 



B. typhi murium 



The anti-paratyphoid B strains of the bacteriophage are some- 

 times endowed with virulence for B. typhi murium. Very active 

 strains have been isolated from the intestinal tracts of white and 



