1. Staphylococci activated by rapid growth in the presence of excess O2 and subsequently brought to a resting state by storage in Locke's solution at 5°C. produce a significant rise in [phage] when added to phage-containing solutions.

2. For satisfactory activation the staphylococci require a period of active growth in the presence of oxygen. Activation proceeds best on the acid side of neutrality although variation in pH from 5 to 9 has relatively little effect. Activated cells retain their phage-augmenting property for from 4 to 24 hours, and this property may be destroyed by heating the cells at temperatures which do not kill them. The critical thermal increment for heat inactivation is 90,000 suggesting that the reaction involves protein denaturation.

3. The reaction between activated cells and phage has the following characteristics:

A. It is complete in 1 to 2 minutes after mixing the reactants.

B. The increase in phage does not depend upon bacterial growth nor does it involve any untoward effect on the titration system.

C. Serum prepared by injecting rabbits with normal live staphylococci or with activated staphylococci when mixed with activated cells before the addition of phage will prevent the customary increase in [phage].

4. The phage-producing reaction which follows the addition of activated cells to phage can be interpreted in terms of the precursor theory. It is likely that the precursor either is a protein or contains a protein as an essential component.

5. There is no way of deciding at present whether the reaction between phage and precursor represents the hydrolytic cleavage of a protein or whether it is the final step in a synthesis catalyzed by phage.

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