On the factors influencing the appearance of placques of bacterial lysis.

Abstract

If the activity of “bacteriophage” is studied upon agar cultures spots or so-called placques of lysis are seen to appear, instead of a homogeneous growth. As a rule, these clear spots stand out quite distinctly against the background of agar, but occasionally one observes among them a certain number of spots which may be considerably less distinct than others. The variations among the spots are even more marked as regards their size. The placques may vary in the same plate from the size of a pin-prick to that of a size larger than the head of the pin. A number of authors have succeeded in demonstrating that by isolating and propagating each of the variants one can obtain pure strains of “bacteriophage”—showing very little if any variation in the size or distinctness of the placques (or spots). In their opinion the co-existence in one plate of spots of different size is indicative of impurity of the lytic strain.

Having in our possession a strain of “bacteriophage” which showed a tendency to give rise to spots of lysis of various sizes, we considered it impure and attempted to isolate the several component strains by the usual method of daily transfers. When we found that pure strains of “bacteriophage” thus secured would unexpectedly show marked changes in respect to the average size of spots or their distinctness, we attempted to ascertain the cause of the changes.

1. Within certain limits, the average size of the placque changes inversely with the number of bacteria susceptible to lysis present in each cc. of agar.

2. The average size of the placque depends upon the relative concentration of young and old individuals in the culture—the greater the number of old bacteria, the smaller the spots.