THE EFFECTS OF METABOLITES UPON INTERACTIONS BETWEEN VARIANTS IN MIXED BRUCELLA ABORTUS POPULATIONS

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Abstract
The causes for the attainment of a population equilibrium in mixed smooth and rough (S + R) broth cultures of Brucella after a certain period of growth, regardless of the initial proportion of each variant type, have been analyzed. Such interactions in mixed populations can be attributed to differences in rate of alanine production by different variants and to the respective resistance of these variants to the inhibitory effects of alanine. Thus, in mixed S + R cultures with an initially high proportion of alanine-sensitive S cells with rapid alanine production, the proportion of R type cells will increase because metabolite levels endowing the more alanine-resistant R cells with high selective value will be reached early. In contrast, in mixed cultures with an initially high proportion of R cells with low rate of alanine production, the proportion of S type cells will increase during 10 days of incubation, since in the absence of high alanine levels S cells have a higher selective value than R cells. Supporting evidence for this role of metabolite levels in interactions between variant types has been obtained from quantitative studies on rate of alanine production, from expts. on modifications of population shifts in the presence of different alanine levels, and from comparison of viable counts of "pure" S and R cultures in the absence and presence of alanine. The results have confirmed that the selective value of any variant may differ under different environmental conditions and that, at least in the case analyzed here, this shift in selective value can be referred to environmental changes produced by the cells themselves.