Effect of incubation with bacterial extract on the survival of X-irradiated bacteria.

X-irradiated Escherichia coli B, a filament-forming strain, showed improved survival of X-rays after treatment with its own extract, either in liquid or on solid media. Extract-promoted recovery was not shown by a non-filamentforming strain E. coli B,-~. We failed to observe liquid holding recovery of X-irradiated bacteria in the absence of extract. Most experiments were performed in liquid medium where the increased number of survivors depended on the incubation period and the extract concentration. In about 30 min. the increase reached a saturation value; the maximum number of survivors was obtained using a certain (optimal) concentration of extract which was then employed in all subsequent experiments. The activity of the extract was completely destroyed by heating at 52' for 40 min. No activity was detected in the DNA, RNA or ribosomal fractions of the extract. The soluble remainder, however, was fully active, suggesting that soluble protein was the active principle.