Population changes in Diplococcus pneumoniae.

Carta, G. (Wesleyan University, Middletown, Conn.) and W. Firshein. Population changes in Diplococcus pneumoniae. J. Bacteriol. 84:473-477. 1962.-Avirulent mutant strains derived from pneumococcal types I, II, III, and VII established themselves in initially virulent or predominantly virulent populations within 36 to 48 hr of incubation at 37 C in Brain Heart Infusion-blood broth. Aged blood-broth cultures or broth cultures lacking blood permitted a more rapid establishment of avirulent cells. This selective population change was due to a greater viability of avirulent than virulent cells, in the stationary phase, and to a toxic factor, produced by large numbers of avirulent cells, which inhibited the multiplication of virulent cells. Virulent cultures were stabilized, however, by the addition of mixtures of deoxynucleosides and deoxynucleotides, which prevented the establishment of avirulent cells.