42 



Scientific Proceedin gs ( i i 7) . 



colonies. The virulence had fallen to io -4 c.c. Twenty- five 

 days later, no D colonies could be demonstrated. The mutation 

 D — * G was complete, or type G had completely outgrown type 

 D. 0.1 c.c. of culture failed to produce a fatal effect. The at- 

 tenuation of this culture is to be referred to the gradual replace- 

 ment of the primordial D by the mutant G form. It is possible 

 to predict the virulence from the relative preponderance of the 

 two types, as evidenced by colonies on serum agar plates. It is 

 possible to procure sub-cultures of very high or of very low vir- 

 ulence by selection of one type or the other, so long as any of the 

 D type remain. 



While the virulence of microbe G is very low, 1.0 to 2.0 c.c. of 

 whole culture may occasionally produce fatal infections, especially 

 in young rabbits. The organisms recovered from such animals 

 at necropsy retain their granular growth character, but may 

 gain perceptibly in virulence. After three animal passages, a 

 type G culture has been observed to reach a virulence of io~ 4 c.c. 

 But despite this increase in invasive power, the non-fluorescence 

 of its colonies persisted, its granular growth character intensified, 

 and its acid agglutination optimum rose to > Ph = 5-6. It is 

 apparent from this experiment that it is unsafe to state that low 

 virulence goes invariably hand in hand with the other characters 

 gained in mutation. Experiments are under way to determine 

 whether this artificially produced virulence of type G is permanent 

 or evanescent. 



The route of infection is important in determining the ability of 

 the low-virulent type G to gain a foothold in the animal body. 

 A culture of microbe G which produced no perceptible effect when 

 injected into rabbits intrapleurally in dose of 1.0 c.c. gave rise to 

 abscesses when injected subcutaneously in 0.1 c.c. These lesions 

 remained sharply circumscribed, but the type G organism could 

 be recovered from them for several weeks after the appearance of 

 the abscess. 



The phenomenon of vicariously greater susceptibility to sub- 

 cutaneous injection would seem to be due to the rapidity with 

 which phagocytes are mobilized against type G when this organism 

 is injected into the pleural cavity. Within 6 hours after intra- 

 pleural injection of 1.0 c.c. of serum broth culture of type G, no 



