STUDIES ON THE NUTRITION AND PHYSIOLOGY OFPASTEURELLA PESTIS

Virulent strains of Pasteurella pestis frequently undergo rapid attenuation during growth at 37 C in liquid media (Wats and Puduval, 1940; Devignat and Schoetter, 1942; and Fukui et al., 1957). This change in the bacterial population was related to a characteristic nutritional difference between virulent and avirulent cell types and was prevented by the presence of adequate amounts of calcium ions in the growth medium (Higuchi, Kupferberg, and Smith, 1959). In contrast to virulent cell types, the few avirulent cells initially present or arising by mutation, proliferate at 37 C in calcium deficient media, and thus rapidly outgrow the original virulent population. It was observed that an unusually high mutation rate in P. pestis from virulence to avirulence was an important factor in these processes. The present work considers the development of a differential plating medium for the detection of avirulent cell types and its application for the estimation of the mutation rate (virulence -avirulence) in P. pestis. A preliminary report on a part of this work has been presented previously (Higuchi and Smith, 1959).