ENVIRONMENTAL CONDITIONS AFFECTING THE POPULATION DYNAMICS AND THE RETENTION OF VIRULENCE OF PASTEURELLA PESTIS: THE ROLE OF CARBON DIOXIDE

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Abstract
Studies on the growth characteristics of virulent and avirulent strains of P. pestis at 37[degree] C in air and under N and in the presence and absence of CO2 (bicarbonate), aspartic acid or glutamic acid, yielded the following results: (a) under aerobic conditions the avirulent cells in a population are capable of outgrowing the virulent members; (b) in the absence of O2 (under N2) the virulent cells in the inoculum can grow at least as rapidly as the avirulent cells, and the culture retains virulence; (c) CO2 suppresses the initial rate of growth in air of both virulent and avirulent cultures, but in such a manner that the virulent cells of the "virulent" inoculum continue to proliferate, with the end result that the initial ratio of virulent to avirulent cells is held approximately constant, and the culture retains virulence for the test animal; (d) aspartic or glutamic acids contribute toward the retention of virulence in a manner which may or may not be related to the CO2 effect. An interpretation of the favorable effect of CO2 (bicarbonate) on the retention of virulence is given in terms of CO2 inhibition of terminal oxidative mechanisms with the consequent selective inhibition of the avirulent forms and the competitive outgrowth of the virulent members of the population within the initial inoculum.