SUMMARY: A mesophilic strain of , grown at 30° and suspended to a concentration of equiv. 0.02 mg. dry wt/ml. in 30 mm-NaCl, showed rapid loss of viability when chilled rapidly from 30° to 0° or -2°. The viability of a psychrophilic pseudomonad, grown at 30°, did not decrease to the same extent when a dilute suspension (equiv. 0.02 mg. dry wt/ml.) was rapidly chilled from 30° to 0° or -2°. Concentrated suspensions (equiv. 3.0 mg. dry wt/ml.) of both the mesophile and the psychrophile released about the same proportion of total endogenous ultraviolet-absorbing compounds when rapidly chilled from 30° to 5°, 0° or -2°. Loss of viability following rapid chilling of a dilute suspension of the mesophile or the psychrophile was partly or completely prevented by 5 mm-Mg and by bacteria-free filtrates from chilled concentrated suspensions of either bacterium. The viability of the bacteria grown at 10° did not decrease when dilute suspensions were rapidly chilled from 10° to -2°. Bacteria grown at 10° contained a greater proportion of unsaturated fatty acids than bacteria grown at 30°. Fatty acid analyses showed that susceptibility of the mesophile and psychrophile to cold shock could be correlated with the contents of unsaturated fatty acids in their lipids.


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