Dissociation in Brucella abortus : A Demonstration of the Rôle of Inherent and Environmental Factors in Bacterial Variation

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Abstract
Inherent differences between clones of B. abortus, Strain 19, in regard to dissociation percentages have been demonstrated under standardized environmental conditions. Clones with statistically significant differences in dissociation indices (= dissociation percentages after 10 days in broth) were systematically selected and remained stable if stored at low temp. These inherent dissociation percentages are modified by environmental changes which affect growth-rates and viability, e.g., changes in pH, daily transfers, changes in temp., differences in nutrients, or reduced O/R potentials. Actual counts revealed that environmental conditions which decrease the growth- rate lower dissociation percentages; under standard environmental conditions, clones with different dissociation indices have inherently different growth-rates. It is demonstrated that inherent or environmentally induced differences in growth rate and/or viability can cause the differences in dissociation indices, even at equal rate of appearance of new variants. Counts of number of total and viable bacteria, respectively, showed that during population growth the number of total cells increases steadily, whereas the number of viable cells regardless of the size of inoculum, soon reaches a maximum which is retained as long as growth persists. At the point of population growth where the total number of cells becomes steadily greater than the viable number of cells, population pressure starts. This population pressure acts as selection pressure and will permit the establishment of spontaneously arising variants (mutants) with positive selection value. Growth-rate and/or viability of the original members of a population determine the point at which population pressure starts and the growth-rate and/or viability of a variant determine its chances to establish itself within a population after population pressure has started. Therefore, any changes in growth-rate and/or viability can produce differences in dissociation indices. Bacterial dissociation is thus interpreted in terms of spontaneous appearance of variants (mutants) and their subsequent establishment under the control of inherent and environmental factors governing population dynamics.

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