The rate of growth of virulent and avirulent streptococci in serum media.

Abstract

In a series of experiments which we reported in 1921 1 we found that after strains of beta type streptococci had been rendered virulent for mice by animal passage cultures of the virulent streptococci grew more slowly during the first few hours after they were inoculated into broth to which horse serum had been added than did cultures of the avirulent forms of the same strains. In comparing the growth of the virulent and the stock cultures of these streptococci bacterial counts were made at hourly intervals and the amount of hemolysin produced in the cultures was titrated with standardized suspensions of erythrocytes obtained from defibrinated horse blood. The amount of hemolysin produced at these intervals in these cultures corresponded with the rate of growth of the bacteria. The virulent streptococci grew slowly after they were inoculated into the broth, then after a latent growth period of several hours grew very rapidly until the sixteenth hour, after which the bacterial count gradually fell. The maximum amount of hemolysin was produced during the period of very rapid growth. The avirulent cultures showed a much shorter latent growth period. They began to grow rapidly after they had been inoculated into the broth about three hours and reached the maximum bacterial count from one to two hours before the cultures of the virulent bacteria had attained their maximum. The maximum amount of hemolysin was produced from one to two hours earlier in these cultures than in the cultures of the virulent streptococci although the maximum titers of hemolysin were equal. The avirulent forms of the streptococci always grew more rapidly under the conditions laid out for these experiments and the curve of the hemolysin titers was a constant index of the length of the latent growth period.