SUMMARYThe death-rate of washed Aerobucter aerogernes in aqueous suspension at 47" depended on the nature of the growth medium, the composition of the liquid used to wash and resuspend the bacteria, the bacterial growth phase, the bacterial concentration in heated suspensions, the pH value, the oxygen tension and the composition of the diluent in which bacteria were heated. The relative resistance of bacteria in different growth phases differed according to the growth medium and the washing fluid; stationary phase bacteria were not more resistant than exponential phase organisms under all conditions. Starvation increased the thermal resistance of exponential and stationary phase bacteria. High bacterial concentration favoured survival a t 47" under most conditions; cell-free filtrate from a heated dense suspension (1010 bacteria/ml.) protected a sparser population of fresh bacteria ( 107-10g/ml.) heated in it. Protective material in filtrate was heat-stable ( 100°/15 min.) and diffused through cellophan. The optimum pH value for survival at 47" was near pH 6-5. Aerobic conditions favoured survival in distilled water but not in salt solutions or phosphate saline (pH 6-5). The effects of various concentrations of NaCl and KC1 on the survival of bacteria at 47' under aerobic conditions were different, K+ concentrations above 0.1 M being more lethal than equivalent concentrations of Na+; the lethal effect of heating in mixtures of these salts (total M > 0.1) increased with K+ concentration. Growth medium, Mg2+ (0-01-5 mM) and, to a lesser extent, Mn*+ (0.5 mM) or Co2+ ( 5 mM) decreased the death-rate, whereas ethylenediamine tetraacetic acid (mM), or various sugars, increased it. Mga+ but not M n 2 + reversed the lethal effect of sugars.Generally, conditions which accelerated the death-rate of Aerobucter wogem?s at 47" also increased the rate of degradation of endogenous RNA. This was accompanied by an increase in the ultraviolet absorption of cold acid-extracts of bacteria and of the suspending fluid. Bacterial protein was degraded to a smaller extent. Depletion of RNA is probably not the primary cause of death a t 47" but the effect on bacterial metabolism of a rapid increase in endogenous pool constituents resulting from RNA degradation may contribute to the lethal effect.

HESS, GEORGE E. (U. S. Army Chemical Corps, Fort Detrick, Maryland) AND MILTON SHON. Selection of thermostable Serratia marcescens from logarithmic-phase cultures as a means for inducing synchronous growth. J. Bacteriol. 83:781-784. 1962.-A relatively thermostable, apparently nonmultiplying, population of Serratia marcescens was found in cultures in the exponential phase of growth. Thermal treatment of the whole culture resulted in the death of the multiplying cell fraction, although the nonmultiplying fraction remained viable. Subcultivation of the thermally treated suspensions yielded cultures in which most of the viable organisms multiplied synchronously for at least two generations. Synchronous growth of organisms can be induced through various techniques (Campbell, 1957) which utilize the response of the organism to variables such as illumination level (von Denffer, 1958), incubation temperature (Lark and