Selective Survival of Nonpigmented Mutants in Pasteurella Pestis Cultures

Differential death rates that occur under certain conditions in cultures of Pasteurella pestis result in dramatic population shifts. Comparable growth and death rates were observed for pigmented inocula and their nonpigmented variants in a casein digest (NZ-Amine, Sheffield, Type A) medium containing added xylose and shaken at 26 C. A change of energy source from xylose to sodium gluconate had little effect on growth, but, in the death phase, greatly accelerated the death of pigmented cells while decreasing the death rate of nonpigmented mutants. These cultures in the death phase at 48 hours were often predominantly nonpigmented. In gluconate cultures, the death rate of pigmented and nonpigmented populations could be returned toward normal (comparable) rates by substituting Na2HPO4 for the K2HPO4 (0.025 M) added during preparation of the medium. In gluconate-K2HPO4 cultures, the viability of both populations was prolonged by the addition of 0.1 M CaCl2 or, to a lesser extent, of 0.2 M NaCl to the medium.