1950] 



GROWTH AND VARIATION OF BRUCELLA ABORTUS 



297 



alanine per ml of medium was reached (tables 3 and 4, figure 4). The resistance 

 of the rough variant to the toxicity of alanine was, therefore, approximately 10 

 times that of the smooth type. 



It was also observed that during prolonged incubation of cultures in which 

 rough variants had become established alanine accumulation continued that 

 led to an eventual second decline of the viable count. This decline was then 

 followed, approximately 50 days after the start of the originally smooth cultures, 

 by a renewed increase in viable cells and a simultaneous establishment of another 

 smooth variant. This highly alanine-resistant variant, labeled S', proved to be 

 similar antigenically to the original smooth type (acriflavine test and aglutinin 

 absorption test), but when S' was plated in mixtures with the original S a distinct 

 difference in colony morphology was detectable. 



Growth of rough B. abortus 



TABLE 4 

 in synthetic medium containing different concentrations 

 DL-alanine* 



of 



* No variants were observed in these cultures up to 15 days, 

 t Averages of 6 plates. 



DISCUSSION 



These data demonstrate that the accumulation of a specific metabolite, alanine, 

 limits the increase in viable smooth cells and at the same time creates an en- 

 vironment favorable for the progressive establishment of nonsmooth (e.g., R) 

 types with greater resistance to this toxic metabolite. It is interesting to note 

 that prior to population changes in a closed system (test tube) the parent type 

 creates conditions that may be termed suicidal and thereby sets up specific 

 environmental effects that will favor the establishment of metabolite-resistant 

 mutants. In a way, then, the parent type actually influences the direction in 

 which population changes may occur since its metabolic products may determine 

 which of the many possible spontaneously arising mutants will have the greatest 

 survival value at any given time of a culture's growth. It is easy to visualize 

 how the continual quantitative and qualitative alteration of metabolite con- 

 centration may create a constantly changing environment that successively 

 favors different mutants and thereby apparently causes progressive population 

 changes as long as reproduction continues. 



139 



