In order to investigate whether defective phages of killed sensitive bacteria by a lysis from without mechanism, the minimal number of phages required for killing was determined. This figure was found to vary with the m.o.i., giving a value of 1 on extrapolation to an m.o.i. of 0. This excluded lysis from without as the only killing mechanism, although it might play a role at high m.o.i.s. This was confirmed by experiments on leakage of ATP and u.v.-absorbing material, the uptake of oxygen and the effect of the phages on the membrane potential. Apart from a short, initial leakage of ATP, the cell membrane was not affected at low m.o.i.s. These results lead to the conclusion that at low m.o.i.s the phages acted on a cytoplasmic component. Treatment of defective phages for 10 min at pH 2.5 resulted in breakdown of the phages without complete abolition of the killing activity. The active component, which was shown not to be DNA, could not be isolated from the mixture, but SDS gel electrophoresis of PBS X and a non-killing mutant of this phage suggested that a protein with a mol. wt. of 85000 was involved in killing.


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