Dynamics of Dissociated Bacterial Cultures

In order to obtain material for a study of the alleged specific racial stabilizing or antimutation hormones 1 in bacterial cultures, a 7-day broth culture of Staphylococcus aureus, grown from a single agar-plate colony, was plated out on Martin's agar. Plates thus obtained showed from 97 to 99.7% aureus colonies with a 0.3 to 3% albus dissociation (average dissociation about 1.5%). Pureline albus strains grown from these white dissociates showed no demonstrable tendency to revert to the ancestral orange type. Highly pigmented orange strains similarly selected usually underwent approximately 1.5% albus dissociation by the end of 7 days' growth in peptone-broth.

Flasks containing 50 cc. peptone-broth were inoculated with 0.5 cc. of 24-hour broth cultures of pure-line white and orange dissociates thus obtained. The resulting growth-curves of 2 typical contrasting strains are recorded in Fig. 1.

From this figure it is seen that the albus growth-curve is characterized by a shorter lag-phase than the aureus control. The albus dissociate also showed more rapid proliferation during the logarithmic phase of population increase. Moreover, there was a stabilization of the albus viable population at a lower level than in the aureus control, and an earlier and more rapid development of the senescent phase, or terminal fall in viable count.

Identically the same relative differences in “growth-vigor” and “longevity” were noted when the same white and orange dissociates were reinoculated into filtrates or centrifugates from 24-hour to 5-day albus or aureus broth cultures. Varying the volume or the age of the inoculum caused no demonstrable changes in the relative growth rate or longevity.

From the observed differences in inherent “growth-vigor” and longevity one would predict marked fluctuations in the relative percentage of albus and aureus individuals in a mixed (or dissociated) staphylococcal culture as the culture increased in age.